water vapour of essay

How rising water vapour in the atmosphere is amplifying warming and making extreme weather worse

Distinguished Scholar, NCAR; Affiliate Faculty, University of Auckland, Waipapa Taumata Rau

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This year’s string of record-breaking disasters – from deadly wildfires and catastrophic floods to record-high ocean temperatures and record-low sea ice in Antarctica – seems like an acceleration of human-induced climate change.

And it is. But not only because greenhouse gas emissions continue to rise. What we are also observing is the long-predicted water vapour feedback within the climate system.

Since the late 1800s, global average surface temperatures have increased by about 1.1°C, driven by human activities, most notably the burning of fossil fuels which adds greenhouse gases (carbon dioxide and methane) to the atmosphere.

As the atmosphere warms, it can hold more moisture in the form of water vapour, which is also a greenhouse gas. This in turn amplifies the warming caused by our emissions of other greenhouse gases.

Some people mistakenly believe water vapour is a driver of Earth’s current warming. But as I explain below, water vapour is part of Earth’s hydrological cycle and plays an important role in the natural greenhouse effect. Its rise is a consequence of the atmospheric warming caused by our emissions arising especially from burning fossil fuels.

Water vapour: the other greenhouse gas

For every degree Celsius in warming, the water-holding capacity of the atmosphere increases by about 7% . Record-high sea temperatures ensure there is more moisture (in the form of water vapour) in the atmosphere, by an estimated 5-15% compared to before the 1970s, when global temperature rise began in earnest.

Water vapour is a powerful greenhouse gas. Since the 1970s, its rise likely increased global heating by an amount comparable to that from rising carbon dioxide . We are now seeing the consequences.

Read more: Global average sea and air temperatures are spiking in 2023, before El Niño has fully arrived. We should be very concerned

In many ways, water vapour is the most important greenhouse gas as it makes Earth habitable. But human-induced climate change is primarily caused by increases in the long-lived greenhouse gases carbon dioxide, nitrous oxide, methane and chlorofluorocarbons (CFCs).

As a general rule, any molecule with three or more atoms is a greenhouse gas, owing to the way the atoms can vibrate and rotate within the molecule. A greenhouse gas absorbs and re-emits thermal (infrared) radiation and has a blanketing effect.

Clouds have a blanketing effect similar to that of greenhouse gases but they are also bright reflectors of solar radiation and act to cool the surface by day. In the current climate, for average all-sky conditions, water vapour is estimated to account for 50% of the total greenhouse effect , carbon dioxide 19%, ozone 4% and other gases 3%. Clouds make up about a quarter of the greenhouse effect.

Why is water vapour different?

The main greenhouse gases – carbon dioxide, methane, nitrous oxide and ozone – don’t condense and precipitate. Water vapour does, which means its lifetime in the atmosphere is much shorter, by orders of magnitude, compared to other greenhouse gases.

On average, water vapour only lasts nine days, while carbon dioxide stays in the atmosphere for centuries or even millennia, methane lasts for a decade or two and nitrous oxide a century. These gases serve as the backbone of atmospheric heating, and the resulting rise in temperature is what enables the observed increase in water vapour levels.

Read more: Extreme precipitation events have always occurred, but are they changing?

The rise in carbon dioxide doesn’t depend on weather. It comes primarily from the burning of fossil fuels. Atmospheric carbon dioxide has increased from pre-industrial levels of 280ppmv to 420ppmv (an increase of 50%) and about half of that increase has happened since 1985.

This accounts for about 75% of the anthropogenic heating from long-lived greenhouse gases. The rest of human-induced atmospheric warming mainly comes from methane and nitrous oxide, with offsets from pollution aerosols.

The extra heating from water vapour has been on a par with that from increased carbon dioxide since the 1970s.

Water vapour and the water cycle

Water vapour is the gaseous form of water and it exists naturally in the atmosphere. It is invisible to the naked eye, unlike clouds, which are composed of tiny water droplets or ice crystals large enough to scatter light and become visible.

The most common measure of water vapour in the atmosphere is relative humidity.

During heatwaves and warm conditions, this is what affects human comfort. When we sweat, the evaporation of moisture from our skin has a cooling effect. But if the environment is too humid, then this no longer works and the body becomes sticky and uncomfortable.

This process is important for our planet, too, because about 70% of Earth’s surface is water, predominantly ocean. Extra heat generally goes into evaporating water. Plants also release water vapour through a process called transpiration (releasing it through tiny stomata in leaves as part of photosynthesis). The combined process is called evapotranspiration.

The moisture rises into the atmosphere as water vapour. Storms gather and concentrate the water vapour so that it can precipitate. As water vapour has an exponential dependence on temperature, it is highest in warm regions, such as the tropics and near the ground. Levels drop off at cold higher latitudes and altitudes.

The expansion and cooling of air as it rises creates clouds, rain and snow. This vigorous hydrological cycle means water vapour molecules only last a few days in the atmosphere.

Water is the air conditioner of the planet. It not only keeps the surface cooler (albeit at the expense of making it moister) but rain also washes a lot of pollution out of the atmosphere to everyone’s benefit.

Precipitation is vitally important. It nourishes vegetation and supports various ecosystems as long as the rate is moderate. But as the climate warms, higher moisture levels increase the potential for heavier rainfall and the risk of flooding.

Moreover, the latent energy that went into evaporation is returned to the atmosphere, adding to heating and causing air to rise, invigorating storms and making weather extremes greater and less manageable.

These changes mean that where it is not raining, drought and wildfire risk increase, but where it is raining, it pours.

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ENCYCLOPEDIC ENTRY

Water cycle.

The water cycle is the endless process that connects all of the water on Earth.

Conservation, Earth Science, Meteorology

Deer Streams National Park Mist

A misty cloud rises over Deer Streams National Park. The water cycle contains more steps than just rain and evaporation, fog and mist are other ways for water to be returned to the ground.

Photograph by Redline96

Water is one of the key ingredients to life on Earth. About 75 percent of our planet is covered by water or ice. The water cycle is the endless process that connects all of that water. It joins the Earth’s oceans, land, and atmosphere.

The Earth’s water cycle began about 3.8 billion years ago when rain fell on a cooling Earth, forming the oceans. The rain came from water vapor that escaped the magma in the Earth’s molten core into the atmosphere. Energy from the sun helped power the water cycle and Earth’s gravity kept water in the atmosphere from leaving the planet.

The oceans hold about 97 percent of the water on Earth. About 1.7 percent of Earth’s water is stored in polar ice caps and glaciers. Rivers, lakes, and soil hold approximately 1.7 percent. A tiny fraction—just 0.001 percent—exists in the Earth’s atmosphere as water vapor.

When molecules of water vapor return to liquid or solid form, they create cloud droplets that can fall back to Earth as rain or snow—a process called condensation . Most precipitation lands in the oceans. Precipitation that falls onto land flows into rivers, streams, and lakes. Some of it seeps into the soil where it is held underground as groundwater.

When warmed by the sun, water on the surface of oceans and freshwater bodies evaporates, forming a vapor. Water vapor rises into the atmosphere, where it condenses, forming clouds. It then falls back to the ground as precipitation. Moisture can also enter the atmosphere directly from ice or snow. In a process called sublimation , solid water, such as ice or snow, can transform directly into water vapor without first becoming a liquid.

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Related Resources

Steamy Relationships: How Atmospheric Water Vapor Amplifies Earth’s Greenhouse Effect

Water vapor is Earth’s most abundant greenhouse gas. It’s responsible for about half of Earth’s greenhouse effect — the process that occurs when gases in Earth’s atmosphere trap the Sun’s heat. Greenhouse gases keep our planet livable. Without them, Earth’s surface temperature would be about 59 degrees Fahrenheit (33 degrees Celsius) colder. Water vapor is also a key part of Earth’s water cycle: the path that all water follows as it moves around Earth’s atmosphere, land, and ocean as liquid water, solid ice, and gaseous water vapor.

Since the late 1800s, global average surface temperatures have increased by about 2 degrees Fahrenheit (1.1 degrees Celsius). Data from satellites, weather balloons, and ground measurements confirm the amount of atmospheric water vapor is increasing as the climate warms. (The United Nations’ Intergovernmental Panel on Climate Change Sixth Assessment Report states total atmospheric water vapor is increasing 1 to 2% per decade.) For every degree Celsius that Earth’s atmospheric temperature rises, the amount of water vapor in the atmosphere can increase by about 7%, according to the laws of thermodynamics.

Some people mistakenly believe water vapor is the main driver of Earth’s current warming. But increased water vapor doesn’t cause global warming. Instead, it’s a consequence of it. Increased water vapor in the atmosphere amplifies the warming caused by other greenhouse gases.

It works like this: As greenhouse gases like carbon dioxide and methane increase, Earth’s temperature rises in response. This increases evaporation from both water and land areas. Because warmer air holds more moisture, its concentration of water vapor increases. Specifically, this happens because water vapor does not condense and precipitate out of the atmosphere as easily at higher temperatures. The water vapor then absorbs heat radiated from Earth and prevents it from escaping out to space. This further warms the atmosphere, resulting in even more water vapor in the atmosphere. This is what scientists call a "positive feedback loop." Scientists estimate this effect more than doubles the warming that would happen due to increasing carbon dioxide alone.

A Different Breed of Greenhouse Gas

The greenhouse gases in the dry air in Earth’s atmosphere include carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons. While making up around 0.05% of Earth’s total atmosphere, they play major roles in trapping Earth’s radiant heat from the Sun and keeping it from escaping into space. Each is driven directly by human activities.

All five of these greenhouse gases are non-condensable . Non-condensable gases can’t be changed into liquid at the very cold temperatures present at the top of Earth’s troposphere, where it meets the stratosphere. As atmospheric temperatures change, the concentration of non-condensable gases remains stable.

But water vapor is a different animal. It’s condensable – it can be changed from a gas into a liquid. Its concentration depends on the temperature of the atmosphere. This makes water vapor the only greenhouse gas whose concentration increases because the atmosphere is warming, and causes it to warm even more.

If non-condensable gases weren’t increasing, the amount of atmospheric water vapor would be unchanged from its pre-industrial revolution levels.

Carbon Dioxide Is Still King

Carbon dioxide is responsible for a third of the total warming of Earth’s climate due to human-produced greenhouse gases. Small increases in its concentration have major effects. A key reason is the length of time carbon dioxide remains in the atmosphere.

Methane, carbon dioxide, and chlorofluorocarbons don’t condense, and they aren’t particularly chemically reactive or easily broken down by light in the troposphere. For these reasons, they remain in the atmosphere for anywhere from years to centuries or even longer, depending on the gas.

In contrast, a molecule of water vapor stays in the atmosphere just nine days, on average. It then gets recycled as rain or snow. Its amounts don’t accumulate, despite its much larger relative quantities.

“Carbon dioxide and other non-condensable greenhouse gases act as control knobs for the climate,” said Andrew Dessler, a professor of Atmospheric Sciences at Texas A&M University in College Station. “As humans add carbon dioxide to the atmosphere, small changes in climate are amplified by changes in water vapor. This makes carbon dioxide a much more potent greenhouse gas than it would be on a planet without water vapor.”

Wreaking Havoc on the Global Water Cycle

Increases in atmospheric water vapor also amplify the global water cycle. They contribute to making wet regions wetter and dry regions drier. The more water vapor that air contains, the more energy it holds. This energy fuels intense storms, particularly over land. This results in more extreme weather events.

But more evaporation from the land also dries soils out. When water from intense storms falls on hard, dry ground, it runs off into rivers and streams instead of dampening soils. This increases the risk of drought.

In short, when atmospheric water vapor meets increased levels of other greenhouse gases, its impacts on Earth’s climate are substantial.

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• Climate Change
• Greenhouse Effect
• Greenhouse Gases

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Have a question?

Why do we blame climate change on carbon dioxide, when water vapor is a much more common greenhouse gas, extra water vapor we put in the atmosphere doesn’t last long enough to change the long-term temperature of our planet. but water does play a major supporting role in climate change..

November 3, 2023

With all the attention given to humans’ climate-warming carbon dioxide (CO 2 ) emissions, you might be surprised to learn that CO 2 is not the most important greenhouse gas affecting the Earth’s temperature. That distinction belongs to water.

We can thank water vapor for about half of the “greenhouse effect” keeping heat from the sun inside our atmosphere. 1 “It’s the most important greenhouse gas in our climate system, because of its relatively high concentrations,” says Kerry Emanuel, professor emeritus of atmospheric science at MIT. “It can vary from almost nothing to as much as 3% of a volume of air.”

Compare that to CO 2 , which today makes up about 420 parts per million of our atmosphere—0.04%—and you can see immediately why water vapor is such a linchpin of our climate system.

So why do we never hear climate scientists raising the alarm about our “water emissions”? It’s not because humans don’t put water into the atmosphere. Even the exhaust coming from a coal power plant—the classic example of a climate-warming greenhouse gas emission—contains almost as much water vapor as CO 2 . 2 It’s why that exhaust forms a visible cloud.

But water vapor differs in one crucial way from other greenhouse gases like CO 2 , methane, and nitrous oxide. Those greenhouse gases are always gases (at least when they’re in our atmosphere). Water isn’t. It can turn from a gas to a liquid at temperatures and pressures very common in our atmosphere, and so it frequently does. When it’s colder it falls from the air as rain or snow; when it’s hotter it evaporates and rises up as a gas again.

“This process is so rapid that, on average, a molecule of water resides in the atmosphere for only about two weeks,” says Emanuel.

This means extra water we put into the atmosphere simply doesn’t stick around long enough to alter the climate; you don’t have to worry about warming the Earth every time you boil a kettle. And there’s really no amount of water vapor we could emit that would change this. “If we were to magically double the amount of water vapor in the atmosphere, in roughly two weeks the excess water would rain and snow back into oceans, ice sheets, rivers, lakes, and groundwater,” Emanuel says.

Nonetheless, water vapor is an important part of the climate change story—just in a slightly roundabout way.

At any given temperature, this is a theoretical upper limit to the amount of water vapor the air can hold. The warmer the air, the higher that upper limit. And while the air rarely holds as much water as it could —thanks to rain and snow—Emanuel says that over the long term, rising temperatures steadily raise the average amount of water vapor in the atmosphere at any given time.

And of course, temperatures today are rising, thanks to humans’ emissions of longer-lasting greenhouse gases like CO 2 . Water vapor amplifies that effect. “If the temperature rises, the amount of water vapor rises with it,” says Emanuel. “But since water vapor is itself a greenhouse gas, rising water vapor causes yet higher temperatures. We refer to this process as a positive feedback, and it is thought to be the most important positive feedback in the climate system.”

In short, it’s true that water vapor is in some sense the “biggest” greenhouse gas involved in climate change, but it’s not in the driver’s seat. CO 2 is still the main culprit of the global warming we’re experiencing today. Water vapor is just one of the features of our climate that our CO 2 emissions are pushing out of balance—well beyond the stable levels humanity has enjoyed for thousands of years.

Thank you to several readers for sending in related questions, including Arthur Donavan of Reno, Nevada, Jen-shih Lee of Rancho Santa Fe, California, and John Mitchell of Palm Bay, Florida. You can submit your own question to Ask MIT Climate here .

Read more Ask MIT Climate

1 NASA Global Climate Change: " Steamy Relationships: How Atmospheric Water Vapor Amplifies Earth's Greenhouse Effect ." February 8, 2022.

2 Song, Chunshan, et al., " Tri-reforming of Methane over Ni Catalysts for CO2 Conversion to Syngas With Desired H2/CO Ratios Using Flue Gas of Power Plants Without CO2 Separation ." Studies in Surface Science and Catalysis , Volume 153, 2004, doi:10.1016/S0167-2991(04)80270-2.

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Listen to this episode of MIT's "Today I Learned: Climate" podcast breaking down in detail how the greenhouse effect works.

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What Is the Water Cycle?

Water can be found all over Earth in the ocean, on land and in the atmosphere. The water cycle is the path that all water follows as it moves around our planet.

Credit: NASA/JPL-Caltech Data source: NASA's Earth Observatory

On Earth, you can find water in all three states of matter: solid , liquid and gas . Liquid water is found in Earth’s oceans, rivers, lakes, streams—and even in the soil and underground. Solid ice is found in glaciers , snow, and at the North and South Poles . Water vapor—a gas—is found in Earth’s atmosphere.

How does water travel from a glacier to the ocean to a cloud? That’s where the water cycle comes in.

The Water Cycle

Credit: NASA/JPL-Caltech

The Sun’s heat causes glaciers and snow to melt into liquid water. This water goes into oceans, lakes and streams. Water from melting snow and ice also goes into the soil. There, it supplies water for plants and the groundwater that we drink.

Snow falling on a glacier during winter months usually replaces any water that melts away in the summer. However, due to Earth’s overall warming , most glaciers today are losing more ice than they regain, causing them to shrink over time.

How does water get into the atmosphere? There are two main ways this happens:

• Heat from the Sun causes water to evaporate from oceans, lakes and streams. Evaporation occurs when liquid water on Earth’s surface turns into water vapor in our atmosphere.
• Water from plants and trees also enters the atmosphere. This is called transpiration .

Warm water vapor rises up through Earth’s atmosphere. As the water vapor rises higher and higher, the cool air of the atmosphere causes the water vapor to turn back into liquid water, creating clouds. This process is called condensation .

When a cloud becomes full of liquid water, it falls from the sky as rain or snow—also known as precipitation . Rain and snow then fill lakes and streams, and the process starts all over again.

Clouds, like these over the savannah in Nairobi, Kenya, form when water vapor in the atmosphere condenses back into liquid water. Credit: Department of State

Why Do We Care About the Water Cycle?

We care about the water cycle because water is necessary for all living things. NASA satellites orbiting Earth right now are helping us to understand what is happening with water on our planet.

Water in the Soil

Humans need water to drink, and to water the plants that grow our food. NASA has a satellite called SMAP —short for Soil Moisture Active Passive —that measures how much water is in the top 2 inches (5 cm) of Earth’s soil . This can help us understand the relationship between water in the soil and severe weather conditions, such as droughts.

Water in the Atmosphere

NASA’s CloudSat mission studies water in our atmosphere in the form of clouds. CloudSat gathers information about clouds and how they play a role in Earth’s climate. Also, the international satellite called the Global Precipitation Measurement Mission (GPM) observes when, where and how much it rains and snows on Earth.

Water in the Oceans

As Earth’s climate becomes warmer, land ice at the North and South Poles starts melting. The water then flows into the ocean, causing sea level to rise. NASA’s Jason-3 mission—short for Joint Altimetry Satellite Oceanography Network-3 —orbits Earth collecting information about sea level and ocean temperature. This helps track how the ocean responds to Earth’s changing climate.

NASA is also tracking how Earth’s water moves all around our planet. This is the work of the GRACE-FO —or Gravity Recovery and Climate Experiment-Follow On —mission. It tracks the movement of water from one month to the next, and can even measure changes in deep groundwater hundreds of feet below Earth’s surface.

NASA’s Aqua satellite also collects a large amount of information about Earth’s water cycle, including water in the oceans, clouds, sea ice, land ice and snow cover.

Related NASA Missions

Water in the Atmosphere: The factors that influence evaporation and condensation

by Anne E. Egger, Ph.D., Ulyana Horodyskyj Pena, Ph.D.

Have you ever noticed your hair becomes frizzy when it's humid outside? Did you know this occurrence was the basis for one scientist inventing a tool to measure humidity? Though it was invented hundreds of years ago, the device is still used by scientists today. It's just one of the many ways we measure water in the atmosphere.

The rate of evaporation from Earth’s surface is determined by the air temperature, water temperature, wind, and the amount of water vapor already in the air. These factors vary regionally and change over a wide range of timescales on Earth.

As warm, saturated air rises and cools, it leads to the condensation of water vapor into liquid droplets and the formation of clouds. This process determines the amount of precipitation in different regions of the planet.

Using observations from weather balloons and satellites, scientists can map the amount of water vapor in the atmosphere and show that it circulates in predictable patterns, contributing to the characteristics of Earth’s climatic zones.

• Introduction

The Hawaiian Islands lie in the middle of the Pacific Ocean at about 19° N latitude . Hawai’i, the “Big Island” furthest to the southeast, is home to the small cities of Hilo on the east coast and Kailua-Kona on the west coast (Figure 1). The cities are roughly 100 kilometers (60 miles) apart and connected by Saddle Road, which winds between Mauna Kea and Mauna Loa, two major volcanic mountains that form the island. Despite the proximity of these cities, one thing about them is very different: While a rain gauge at the Hilo airport collects approximately 3.3 meters (3300 millimeters) of precipitation annually, the one at Kailua-Kona collects 0.3 meters (300 millimeters) in the same time period (Figure 1). Why do two cities less than an hour’s drive from each other have a ten-fold difference in the amount of rain they receive?

Figure 1: Annual average rainfall in mm/year on the Big Island of Hawai’i. Rainfall data are from The Online Rainfall Atlas of Hawaii.

This is a big difference on a small island, and we see even bigger differences as we look at other locations around the world. Think about precipitation where you live. How often does it rain? How much rain or snow falls in a year? When it’s not raining, is the air where you live dry, or does it feel damp? How often is it cloudy? You might think about precipitation, clouds, and humidity as you prepare to go to school or to work for the day—will you be comfortable going outside? Should you bring a raincoat or umbrella?

Precipitation, clouds, and humidity are all examples of water in the atmosphere . The amount and nature of water in the atmosphere vary based on latitude , geographic setting, and regional climate: some parts of the world are wetter and cloudier than others. The amount and nature (or “phase”) of water in the atmosphere also change over time as water moves through the atmosphere. Learning more about these processes will help us explain why the eastern side of the Big Island is so much wetter than the western side.

• History of measuring water in the atmosphere

Human societies have long been concerned with understanding precipitation—how much will fall and when—because rain is critical for growing food. For example, on the Big Island of Hawai’i, historic sugarcane plantations were all on the eastern (Hilo) side of the island. Sugarcane depends on large amounts of rainfall to thrive, and the plantations maintained rigorous observational records (Giambelluca et al., 2013).

Rain gauges provide a way to collect and measure fallen precipitation . The first known rainfall records date back to the ancient Greeks. But the earliest rain gauge is credited to King Sejong of Korea in the early 1400s, who mounted bronze canisters of uniform size on posts at observatories across the region (Figure 2). Officials at the observatories would observe the precipitation in these canisters and report back to the King. Based on these observations , court officials would determine the potential harvest and, thus, how much the farmers in the region should be taxed.

Figure 2: A modern installation of a rain gauge from the era of King Sejong in Korea, displayed in the Jang Yeong Sil Science Garden in Busan.

Later versions of the rain gauge included features like funnels that channeled water into a bucket which tipped when it was full, allowing for more accurate measurements of the amount of precipitation . Modifications allow for measurement of snow and hail in addition to rain.

Rain gauges are still widely used today as one component of modern weather stations and remain relatively simple, with a funnel directing the water into a calibrated canister (Figure 3). However, the purposes of measuring precipitation today are about something other than crop taxation. Instead, measurements from networks of rain gauges are used for weather forecasting, as long-term records of precipitation help us understand how climate is changing over time. Because precipitation can vary so much even in a small geographic area, these forecasts and predictions benefit from many rain gauges, more than any one group or agency can install and maintain. To address this need, a grassroots organization called CoCoRaHS (pronounced KO-ko-rozz and stands for the Community Collaborative Rain, Hail and Snow Network) began in 1998 at the Colorado Climate Center, hosted by Colorado State University. A flood in Fort Collins, where the university is based, prompted the development of the organization. Currently, CoCoRaHS has more than 25,000 active observers. Anyone can participate in making precipitation observations that are important to the network, and many thousands of people report their observations on any given day (Figure 4).

Figure 3: A 4” plastic rain gauge, typical of those used at CoCoRaHS observing stations.

Figure 4: An example of reports from CoCoRAHs users on 2 July 2023. A total of nearly 11,700 observations were submitted on this day.

Precipitation may be the most obvious form of water in the atmosphere: we can see liquid water actively falling from the sky and feel ourselves get wet. Precipitation is also relatively easy to measure, but it is only a small proportion of the water that exists in the atmosphere . Instead, most water is in the vapor phase in the atmosphere (see our Composition of Earth’s Atmosphere module).

Comprehension Checkpoint

• Measuring water vapor using hygrometers

Water in the vapor phase is invisible. It is also more difficult to measure than precipitation and more important for weather forecasting. Accurate measurements of the amount of water vapor in the air, called “humidity,” can help forecast rapid phase changes of water in the atmosphere , including cloud formation and precipitation.

One of the earliest descriptions of an instrument for measuring humidity (also known as a “hygrometer”) is in a Chinese Han Dynasty manuscript from 120 BCE . Unlike a rain gauge , which measures liquid water directly, hygrometers measure water vapor indirectly, making use of the fact that some substances absorb water vapor more readily than others. The Han Dynasty instrument consisted of equal masses of feathers and charcoal hanging in balance. Charcoal readily absorbs water vapor, whereas feathers do not. When the amount of water vapor in the air increased, the charcoal would absorb more and become heavier, hanging below the feathers. When the humidity decreased, the water absorbed by the charcoal would evaporate, and the charcoal would become lighter, regaining balance with the feathers.

Later inventors and cultures used the same principle (that some materials absorb water more readily than others) to develop more precise hygrometers, allowing observers to quantify the amount of water vapor rather than just visualize it. In 1480, Leonardo da Vinci placed beeswax, a waterproof material that doesn’t change due to moisture, in one balance pan and a cotton ball that could absorb moisture in the other pan. If the cotton absorbed moisture from the air, its weight increased, tipping the balance. An observer could measure the difference with a measuring stick.

A few hundred years later, Swiss physicist and geologist Horace Benedict de Saussure observed that strands of human hair lengthen by as much as 2% as the humidity increases. You may have noticed this effect, especially if you have long hair: On a humid day, your hair might be curlier or “frizzy” as water molecules are incorporated into its structure. Saussure used this knowledge to invent the hair hygrometer, which is still in use today as it can react quickly to changes in humidity. These are the only mechanical hygrometers still usable at temperatures below freezing, as all other hygrometers respond much too slowly at low temperatures to be useful.

However, the amount of water vapor in the air is not an independent variable . If you live in a place with cold winters, you might have noticed that your hair and skin are drier in the winter. That’s partly because the air is drier: The amount of moisture that air can contain is directly related to the air temperature, and warmer air can contain more water vapor than colder air.

In meteorology , saturation is the state of the atmosphere in which air contains the maximum amount of water vapor that it can exist at a specific temperature and air pressure. Saturation is the principle that underlies the concept of relative humidity , which measures how close the air is to saturation with water vapor at a specific temperature and pressure. Whereas absolute humidity is a measure of the actual amount of water vapor in the air, regardless of the air’s temperature, relative humidity is expressed as a percentage and changes as air temperature changes. For example, on a typical spring day in the northern part of the United States, when a nighttime low of 45° F is reached, the relative humidity is close to 100% and there may be dew or frost on the ground by morning. As the day warms to a high of 70° F, the relative humidity decreases to around 45%.

Relative humidity is measured by taking advantage of the process of evaporation with a pair of calibrated thermometers (Figure 5). The first is a regular thermometer with a dry bulb that measures the air temperature. The second thermometer has a wet cloth on the bulb, which stays damp through a wicking mechanism. In an environment like a desert, most moisture is quickly evaporated due to the warm, dry air, few clouds and low humidity, which encourages evaporation. Water evaporating from the wick into the air uses energy and lowers the thermometer's temperature. With greater evaporation, the difference in temperature between these two thermometers is greater, and the relative humidity is lower. As the air nears saturation, evaporation decreases. If the air is fully saturated, no water can evaporate from the wet bulb, and the temperature readings will be the same. In this case, the relative humidity is 100%. Modern wet-dry bulb hygrometers typically have tables associated with them that allow for easy relative humidity calculations (Figure 5).

Figure 5: A wet-dry bulb hygrometer with a table.

Understanding and measuring relative humidity was an important step in weather forecasting. Modern versions of these early instruments are still used in weather stations around the world. In the United States, many stations are operated by the National Weather Service, and the data they collect are used as input for local and regional forecasting models .

• How water gets into the atmosphere

Although water vapor is everywhere in Earth’s atmosphere , the amount varies over time and space. One important factor is the evaporation rate or the extent to which water vapor gets into the atmosphere through the evaporation of liquid water from Earth’s surface . In evaporation, water undergoes a phase change from the lower-energy liquid phase to the higher-energy gas phase, requiring an input of energy . On most of Earth’s surface, the energy source for evaporation is the sun: When sunlight warms the water, the heat energy excites the water molecules , and they move faster and faster until they move so fast that they escape as a gas. The warmer the water is, the greater the evaporation rate.

The sun is also warming the air. As the air temperature increases, its saturation point increases and more water vapor can exist in the air. So, the warmer the air above the water, the greater the evaporation rate. If the air and water on Earth did not move, the evaporation process would be controlled almost entirely by daily temperature changes. However, both the atmosphere and the ocean are characterized by movement. Circulation in the atmosphere produces winds on the ocean’s surface (see our Factors that Control Regional Climate module), and circulation in the ocean produces currents (see our Ocean Currents module). The presence of wind means that a parcel of air (defined as a local mass of air with temperature and/or moisture characteristics that are different from the surrounding air) saturated with water vapor will be moved out, leaving “space” for unsaturated air to move in, thus allowing more evaporation to occur. Ocean currents mean that waters of different temperatures can be brought in and change the evaporation rates accordingly.

Putting together all these factors, we can predict where evaporation rates are very high and very low. Evaporation is greatest where there is a large expanse of warm water with warm air temperatures and a constant wind that brings in unsaturated air. A good example of this is the lower latitudes of the Pacific Ocean, where trade winds , or, winds coming from the northeast and flowing towards the equator blow steadily over thousands of miles of open ocean warmed by the tropical sun (Figure 6). For that reason, the air that arrives on the east coast of the Big Island of Hawaii, where Hilo is located, is warm and fully saturated. In contrast, evaporation rates are low where there are cold water and cold air temperatures, like the Bering Sea in the northern Pacific Ocean (Figure 6). Since the air at high latitudes is so cold, it will have very little water vapor in it, so less evaporation can occur there.

Figure 6: Map of the Pacific Ocean. The colors represent sea-surface temperature, with warmer temperatures in reds and cooler temperatures in blue. Curved white lines represent wind speed and direction.

• How water vapor condenses in the atmosphere

That warm, fully saturated air that arrives at Hilo with a relative humidity of 100% does not yet contain liquid water. As long as the air temperature remains the same, water vapor will not condense to a liquid. For clouds to form and rain to start falling in Hilo, these factors—air temperature and capacity—must change.

If you look up at a cloud-free sky, it may appear empty. However, near-invisible sub-microscopic water droplets abound. The drops may start to clump together due to random collisions in the air, but if evaporation outpaces condensation , the droplets will not survive long. If the air cools, the molecules become less energetic, and the evaporation rate decreases. When there is more condensation than evaporation, liquid water droplets or ice crystals can persist and can start to form clouds. The temperature at which this happens varies based on the relative humidity and is called the dew point .

How do air masses cool and reach their dew point? One way is through lifting due to the terrain. As the sun warms the air at the surface , the air expands and becomes lighter, rising through the atmosphere . The expansion causes the air temperature to decrease (see the Ideal Gas Law in our Properties of Gases module).

Still, water droplets and ice crystals don’t spontaneously form, even as the air cools. They need seed particles to collect on—those particles could be dust, salt spray from the ocean, or aerosols, and, collectively, are called cloud condensation nuclei (CCN). When warm, saturated air rises and cools below its dew point with CCNs present, water vapor will condense, and clouds will form. So: How does the air near Hilo reach its dew point temperature and start to form clouds?

On Hawai’i, air masses coming off the Pacific are saturated with water vapor and laden with CCN, typically salt particles. Those air masses encounter two mountains that rise gradually to over 4000 m (over 13,000’) tall, and the air masses also rise as they move up the slopes (Figure 7). As they do so, the air expands and cools, and water vapor condenses and creates clouds and precipitation . In Figure 7, you can see that the zone with the highest annual precipitation is slightly inland (and uphill) from Hilo, where the air has risen and cooled enough to reach its dew point.

Figure 7: Map of Hawai’i showing annual rainfall. Direction of the trade winds are noted. Graphs show annual rainfall and elevation along the profile line shown on the map.

This cooling and expansion with rising air happens at a predictable rate, called the lapse rate, which varies based on the moisture content of the air. When unsaturated air rises, it cools at a rate of 1° C per 100 m of elevation. The lapse rate decreases as the amount of moisture increases to a low of 0.6° C per 100 m of elevation for saturated air. Since the air approaching Hilo is coming from over the Pacific Ocean and will be saturated, the lapse rate near Hilo is closer to the lower rate. The graph in Figure 7 has gray lines every 1000 m. As an air parcel rises past each line, its temperature decreases by approximately 6° C (0.6°/100 m * 1000 m).

• The distribution of precipitation

Warm, saturated air full of CCN, rising and cooling, explains the high annual rainfall in Hilo but does not answer why it is so dry in Kona. Look back again at the rainfall map and the graph in Figure 7. Notice that the region of low precipitation includes the two mountains, Mauna Kea and Mauna Loa. The air mass rises more than 4000 m from sea level to pass over the mountains, meaning its temperature cools by 25° C (80° F) or more. This is a large enough change to condense most of the water vapor and precipitate water out of the clouds on the side facing the wind. But the air mass keeps moving southwestward. As It descends and sinks along the western slope of the mountains, now at the higher lapse rate of 1° C per 100 m, the air will compress and its temperature will increase. Thus, the air mass reaches Kona as a warm, relatively dry air mass (Figure 7), with a high capacity to absorb water vapor, promoting evaporation and preventing condensation .

This pattern of precipitation is called the rainshadow effect. The rainshadow effect occurs in many places around the world where a mountain range is aligned perpendicular to the prevailing winds coming off the ocean. Kona sits in the rainshadow of Mauna Kea; the Atacama Desert of Chile is in the rainshadow of the Andes; and Death Valley in California is in the rainshadow of the Sierra Nevada Mountains. The rainshadow effect is often visible in satellite imagery because the amount of rain strongly influences vegetation that can grow. Figure 8 shows a satellite image of the Big Island. The deep green vegetation on the island’s east side indicates high rainfall, while a mostly unvegetated, brown and black landscape on the west side indicates the rainshadow.

Figure 8: Satellite image of Hawaii from Google Earth.

• Measuring and influencing water in the atmosphere

Scientists still use thermometers and hygrometers to make measurements at tens of thousands of weather observation stations on the ground. Other ground-based instruments provide a bigger picture. For example, Doppler radar is a ground-based system that can detect most precipitation within 145 km (90 miles) of the radar antenna, with heavy rain or snow detection within approximately 250 km (155 miles). However, other technologies are needed to get measurements vertically through the atmosphere and across the globe.

Since around 1900, scientists have used weather balloons to record temperature, pressure, and humidity vertically through the atmosphere. The National Weather Service launches modern weather balloons daily from 120 sites in the US; globally, there are 900 locations. The balloons are equipped with radiosondes , instruments that transmit data back to ground-based stations. They rise at approximately 300 m per minute (1,000 feet/minute), transmitting their position, temperature, relative humidity , and air pressure every second or two. From this information, the wind speed and direction can also be calculated. Once the balloons reach an altitude of about 35 km (>20 miles), they burst, and the radiosondes fall back to the surface , slowed by small parachutes.

The launch of weather satellites, starting in the 1960s, expanded our ability to collect global data. Satellites can collect data to determine moisture and clouds in the atmosphere globally, enabling meteorologists to predict storm systems better. The data also allows for long-term studies showing how weather systems change over time under various influences.

Large-scale features of the planet primarily control the patterns we observe in relative humidity, evaporation , and precipitation (like the rainshadow effect). Those large-scale features are the incoming energy from the sun, circulation in the atmosphere, and the distribution of oceans and mountain ranges. However, humans also influence the distribution of water in the atmosphere.

• The human influence

Around 2000, meteorologist J. Marshall Shepherd was a research scientist at the NASA Goddard Space Flight Center. During this time, he became interested in using space-based methods to demonstrate the impact of urban environments on precipitation . Shepherd knew that ground-based measurements revealed an “urban heat-island effect,” in which replacing soil and vegetation with the asphalt and concrete of cities leads to an increase in air temperature and evaporation that influences precipitation downwind of the city. But these studies relied on volunteers and ground-based measurements, so their findings were limited to a few urban areas.

Shepherd used the precipitation radar collected with the Tropical Rainfall Measuring Mission (TRMM) satellite from 1998 to 2000 to expand the observations of precipitation around major cities in the southeastern United States. These cities included Atlanta, GA, Nashville, TN, and three cities in Texas: Dallas, San Antonio, and Waco. In analyzing the satellite data , he determined that the region downwind of these cities experienced an average 28% increase in precipitation compared to upwind areas (Shepherd et al., 2002). The warm air generated by the urban environment caused greater evaporation, rising and cooling, and condensation as it moved away from the city. In Atlanta, Shepherd even determined that the number of rain delays had increased at the baseball stadium, thanks to this phenomenon!

Shepherd then sought to correlate the satellite data with ground-based measurements to extend his findings back in time. He examined a rain gauge record that began in the late 1890s in Phoenix, AZ, which is located in a very dry climatic zone. The record revealed that increases in the amount and frequency of precipitation only appeared after urbanization, starting in about 1950 (Shepherd, 2006). As the evidence for the influence of urban centers on precipitation accumulated, Shepherd began to address the implications and potential hazards, including flooding (KC et al., 2015). Many communities are unprepared for these relatively rapid changes in climatic conditions. For instance, a city like Hilo, Hawaii, which experiences high yearly rainfall, is prepared to deal with runoff. However, cities downwind of Phoenix, AZ, have not had time to adapt to high rainfall and are more likely to be impacted by flooding.

Likewise, Earth’s warming climate is leading to global-scale changes in water distribution in the atmosphere . Temperature and precipitation characteristics of a region change as regional climatic zones shift in response to global temperature changes (see our Factors that Influence Regional Climate module). Places like Hawaii, in the middle of the Pacific Ocean and the zone of trade winds , will experience change more slowly than places on the margins of climatic zones and on large landmasses, like Phoenix and other inland cities.

In all cases, the regular measurements we take at the surface , throughout the atmosphere, and from satellites can help us forecast the impacts of water in the atmosphere in the short term and help us identify long-term trends to make predictions about the future. Short-term forecasts and long-term trends help decide everything from whether to carry an umbrella to which crops to plant to how to build structures to withstand extreme floods.

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• Review Article
• Published: 13 July 2021

The residence time of water vapour in the atmosphere

• Luis Gimeno   ORCID: orcid.org/0000-0002-0778-3605 1 ,
• Jorge Eiras-Barca 1 , 2 ,
• Ana María Durán-Quesada 3 , 4 ,
• Francina Dominguez 5 ,
• Ruud van der Ent 6 ,
• Harald Sodemann   ORCID: orcid.org/0000-0002-8167-0860 7 , 8 ,
• Ricardo Sánchez-Murillo 9 ,
• Raquel Nieto   ORCID: orcid.org/0000-0002-8984-0959 1 &
• James W. Kirchner   ORCID: orcid.org/0000-0001-6577-3619 10  

Nature Reviews Earth & Environment volume  2 ,  pages 558–569 ( 2021 ) Cite this article

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• Atmospheric dynamics
• Climate change

Atmospheric water vapour residence time (WVRT) is an essential indicator of how atmospheric dynamics and thermodynamics mediate hydrological cycle responses to climate change. WVRT is also important in estimating moisture sources and sinks, linking evaporation and precipitation across spatial scales. In this Review, we outline how WVRT is shaped by the interaction between evaporation and precipitation, and, thus, reflects anthropogenic changes in the hydrological cycle. Estimates of WVRT differ owing to contrasting definitions, but these differences can be reconciled by framing WVRT as a probability density function with a mean of 8–10 days and a median of 4–5 days. WVRT varies spatially and temporally in response to regional, seasonal and synoptic-scale differences in evaporation, precipitation, long-range moisture transport and atmospheric mixing. Theory predicts, and observations confirm, that in most (but not all) regions, anthropogenic warming is increasing atmospheric humidity faster than it is speeding up rates of evaporation and precipitation. Warming is, thus, projected to increase global WVRT by 3–6% K −1 , lengthening the distance travelled between evaporation sources and precipitation sinks. Future efforts should focus on data integration, joint measurement initiatives and intercomparisons, and dynamic simulations to provide a formal resolution of WVRT from both Lagrangian and Eulerian perspectives.

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Acknowledgements

L.G., R.N. and J.E.-B. were funded by the Spanish government within the LAGRIMA (RTI2018-095772-B-I00) project, funded by Ministerio de Ciencia, Innovación y Universidades, Spain, which are also funded by FEDER (European Regional Development Fund, ERDF). J.E.-B. was also supported by the Xunta de Galicia (Galician Regional Government) under grant ED481B 2018/069 and by the Fulbright Program (US Department of State). L.G., R.N. and J.E.-B. were partially supported by Xunta de Galicia, Spain under project ED413C 2017/64 ‘Programa de Consolidacion e Estructuracion de Unidades de Investigacion Competitivas (Grupos de Referencia Competitiva)’ co-funded by the European Regional Development Fund, European Union (FEDER). J.E.-B. thanks the Defense University Center at the Spanish Naval Academy (CUD-ENM) for all the support provided for this research. R.V.d.E. acknowledges funding from the Netherlands Organization for Scientific Research (NWO), project number 016.Veni.181.015. A.M.D.-Q. acknowledges support from IAEA CRP F31006 (UCR project number B9519). F.D. is supported by National Science Foundation (NSF) CAREER Award AGS 1454089. H.S. acknowledges support by the Norwegian Research Council (Project SNOWPACE, grant no. 262710) and by the European Research Council (Consolidator Grant ISLAS, project no. 773245).

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Gimeno, L., Eiras-Barca, J., Durán-Quesada, A.M. et al. The residence time of water vapour in the atmosphere. Nat Rev Earth Environ 2 , 558–569 (2021). https://doi.org/10.1038/s43017-021-00181-9

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Studying Earth’s Stratospheric Water Vapor

Home » Blog » Studying Earth’s Stratospheric Water Vapor

What does water vapor have in common with Sisyphus, the mythological Greek character cursed to roll a rock uphill only to have it roll back down again? Water is continuously cycling on Earth between bodies of water such as oceans, lakes and rivers, land surfaces, and in the atmosphere. When water warms and evaporates from the Earth’s surface it becomes gaseous in the form of water vapor, H 2 O. As water vapor rises into the atmosphere, it cools and can condense into clouds which can produce rain or snow bringing water back to the Earth’s surface. And the cycle begins again.

Water vapor is also an important component in Earth’s evolving climate system. As a major greenhouse gas – a gas that traps heat – water vapor absorbs heat produced by Earth’s surface and the shining Sun. The water molecules then emit that heat back to Earth’s surface which can increase the temperature. This relationship between an increase in water vapor in the atmosphere contributing to warming temperatures, and warmer temperatures causing an increase in water vapor is called a positive feedback loop.

Although water vapor in the stratosphere is only a few molecules per million air molecules, this positive feedback relationship between water vapor and temperature is important as scientists study to better understand how much this impacts Earth’s changing climate.

In addition to measuring stratospheric ozone and aerosols, the Stratospheric Aerosol and Gas Experiment (SAGE) III instrument on the International Space Station (ISS) measures trace gases including water vapor. Unlike many other science data instruments, SAGE III provides a very precise and highly accurate measurement of water vapor in the upper troposphere and throughout the stratosphere.

Other satellite-based instruments, such as the Microwave Limb Sounder (MLS) on NASA’s Aura and the High-Altitude Lidar Observatory (HALO), measure atmospheric water vapor in the upper troposphere and stratosphere. SAGE III uses the solar occultation technique, which is unique, in that it can take more precise measurements covering vertical layers of atmosphere.

“Because SAGE III provides such a high accuracy data set, we can look at different levels of the atmosphere in more detail than ever before. We can see every kilometer in the vertical profiles of data,” said Mijeong Park, Project Scientist at the National Center for Atmospheric Research in Boulder, CO.

In partnership with the National Center for Atmospheric Research (NCAR), the National Oceanic and Atmospheric Administration (NOAA), and the Jet Propulsion Laboratory (JPL), the SAGE III team at NASA’s Langley Research Center in Hampton, Virginia released initial analyses of the SAGE III water vapor data version 5.1 in the paper “Near-Global Variability of Stratospheric Water Vapor Observed by SAGE III/ISS.”

Throughout the paper, the SAGE III version 5.1 water vapor data are validated against MLS version 5 retrievals and show overall first-rate agreement between the two data sets. The relatively young SAGE III/ISS dataset is recording water vapor seasonal variability that agrees well with MLS from the tropopause through the middle stratosphere (∼16–30 km).

By looking at SAGE III data between 2017 and 2020, scientists were given some insight into the year-to-year variability of H 2 O during boreal summer monsoon season. A monsoon is a seasonal change in wind and rain patterns observed in certain parts of the world, including North America.

“By looking at multiple years of data, we can understand how much water vapor is going into the stratosphere through the summer monsoon circulation each year,” said Park.

In the figure above, SAGE III (a and c) is compared to MLS (b and d) for August 2017 (top) and January 2018 (bottom). In August of 2017, SAGE III H2O showed that water vapor over the North American monsoon region was relatively higher than over the Asian monsoon region. While the SAGE III instrument takes about one month to cover the latitude range ∼60N–60S, scientists have found that this monthly sampling captures more localized values of water vapor in the lower stratosphere.

Although the summer monsoon season varies year by year, SAGE’s ability to detect the interannual variability of stratospheric water vapor during monsoon season helps scientists better understand how changes in water vapor are contributing to Earth’s climate.

Scientists are also able to study relative humidity (RH) with SAGE III’s water vapor data. Relative humidity tells us how much water vapor is in the air, relative to how much water vapor the air could hold at a given temperature. As air temperatures rise, warmer air can hold more water vapor increasing the saturation point. Cold air can hold less water vapor.

The RH-temperature relationships captured by SAGE III agree with the near-tropopause data derived from high-resolution Upper Troposphere/Lower Stratosphere (UTLS) aircraft measurements, which enhances the science community’s confidence in the quality of the SAGE III data set.

“The SAGE III data can be used for more detailed studies of relative humidity distribution and its variability because of the accuracy. It will also help scientists to better simulate our climate using global climate models,” said Park.

While SAGE III will continue to measure water vapor from ISS over the coming years, a longer record of water vapor data is needed.

“It is very important to have a continuous measurement of water vapor anywhere on Earth. There are many ways to measure water vapor, by satellite, like SAGE, by airplane, or by ground-based instruments. There is only one continuous water vapor record of 30-plus years from balloon measurements in Boulder, Colorado. Satellite missions have limited lifetimes. We need continuous measurements of water vapor to really understand how water vapor affects our climate,” said Park.

Science General Overview How can we predict climate? Is the climate changing? Arctic Global climate Manitoba’s climate Other observed changes What about lag time? What about water vapour? What causes climate change? Feedback processes Greenhouse effect Greenhouse gases (GHG) Natural processes The Sun’s effect on climate What might happen in the future? What was climate like in the past? Why must we prevent a 2ºC rise? Are humans the cause?

Water vapour is the most abundant greenhouse gas in the atmosphere, yet other greenhouse gases (such as carbon dioxide and methane) are often portrayed as the main drivers of climate change. Why is that?

When compared to other greenhouse gases, water vapour stays in the atmosphere for a much shorter period of time . Water vapour will generally stay in the atmosphere for days (before precipitating out) while other greenhouse gases, such as carbon dioxide or methane, will stay in the atmosphere for a much longer period of time (ranging from years to centuries ) thus contributing to warming for an extended period of time.

The addition of water vapour to the atmosphere, for the most part, cannot be directly attributed to human generated activities. Increased water vapor content in the atmosphere is referred to as a feedback process. Warmer air is able to hold more moisture. As the climate warms, air temperatures rise, more evaporation from water sources and land occurs, thus increasing the atmospheric moisture content. The increase in water vapour in the atmosphere, because water vapour is an effective greenhouse gas, thus contributes to even more warming: it enhances the greenhouse effect.

Water vapour is often discussed and recognized as being an important part of the global warming process. The water vapour feedback process is most likely responsible for a doubling of the greenhouse effect when compared to the addition of carbon dioxide on its own (3) .

• Intergovernmental Panel on Climate Change (IPCC)
• NSIDC  Arctic Sea Ice News & Analysis
• National Oceanic and Atmospheric Administration (NOAA) 
• NASA Goddard Institute for Space Studies (GISS)
• NOAA Climate.gov
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1) RealClimate. 2005. Water vapour: feedback or forcing? http://www.realclimate.org/index.php/archives/2005/04/water-vapour-feedback-or-forcing/

2) IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.  https://www.ipcc.ch/report/ar4/syr/

3) Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland, 2007: Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.  http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521705967

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You are here, gg 140: the atmosphere, the ocean, and environmental change,  - water in the atmosphere ii.

Air is able to hold a limited amount of water vapor, and that amount depends on the temperature of the air. When this saturation vapor pressure is exceeded, liquid water begins to condense and clouds form. There are several different types of clouds, some which rain and others which do not, and each with characteristics specific to it. Vortices are a particular type of cloud phenomenon in which there is a low pressure anomaly in the center of the cloud with rotating air around it, forming funnel clouds as seen in tornados. The low pressure allows liquid water to condense and form the funnel shaped cloud. Haze is another specific type of cloud in which liquid water condenses onto pollution particles in the air.

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Water vapour.

Water vapour is water in gaseous instead of liquid form. It can be formed either through a process of evaporation or sublimation . Unlike clouds , fog, or mist which are simply suspended particles of liquid water in the air , water vapour itself cannot be seen because it is in gaseous form. [2]

Water vapour in the atmosphere is often below its boiling point . When water is boiled the water evaporates much faster and makes steam . Steam often has droplets of water, which is what is seen water is boiling. Since both water droplets and water vapour are present, this is called wet steam (also called wet vapour ). As the mixture gets hotter, the water droplets go away and it becomes dry steam (also called dry vapour ). [3] Power plants use water vapour in the form of steam (dry is better, but wet is used too) as a working fluid to turn their turbines to make electricity .

Atmospheric Water Vapour

Water vapour is important for a number of different reasons, but its presence in the atmosphere is one of the most important. Water vapour is present within the atmosphere in varying amounts but is a vital component of the hydrologic cycle . In the atmosphere, water vapour can exist in trace amounts or even make up as much as 4% of the atmosphere. This concentration depends largely on where the water vapour levels are measured. On average, the value of water vapour in the atmosphere is 2-3%. In arid or very cold locations - such as polar regions - the amount of water vapour in the air is much lower. [4]

Even on a clear day, water vapour exists in the atmosphere as an invisible gas - unlike clouds which are droplets of liquid water that can be seen. If the conditions are right, water vapour in the air can collect on small particles of dust, salt, or smoke in the air to form small droplets. These droplets gradually increase in size and over time become various forms of precipitation . Since water vapour is so prominent in the atmosphere and forms precipitation, water vapour is a major component of the hydrologic cycle. When water holding areas are heated by the Sun , some of the water being held evaporates and becomes vapour, powering the cycle. [5]

In addition to being created by evaporating water, plants are capable of producing water vapour through a process of transpiration .

Product of Combustion

When hydrocarbons undergo combustion , water is created by the chemical reactions. However, the high temperatures mean that the water is in its gaseous form: water vapour. An example of a hydrocarbon combustion reaction is shown below to illustrate how water is created during this process.

Since water vapour is released when hydrocarbons are burned, water vapour is a fairly large component of the flue gases released from coal fired power plants or natural gas power plants . A surprisingly large amount of water is released when coal is burned. For example, when burning bituminous coal roughly 0.4 kilograms of water are produced for every kilogram of coal burned. This means that tonnes of water are be produced per hour at a large-scale coal fired power plant.

Climate Impacts

Water vapour moves across the Earth's surface, cycling through a variety of storage areas. It is this cycling that shapes the climate of different regions across the globe, providing precipitation and supporting life. As the climate changes, the distribution and cycling of water vapour changes as well. [7]

Water vapour is actually the most abundant greenhouse gas in the atmosphere, and the most potent of all the greenhouse gases as a result of its chemical structure. Its presence accounts for around two thirds of the natural greenhouse effect . [2] In the past there has been debate over how severe an impact water vapour has on global warming , but recently studies have confirmed that the amplification that water vapour has on heat is strong enough to double climate warming caused by increased CO 2 levels. [8]

As a greenhouse gas, water vapour serves creates a positive feedback cycle for global warming. This means that the warmer the world gets, the more water vapour will exist in the air as evaporation rates from oceans , lakes, and streams increase. [7] Since there is then more water vapour in the air, the water vapour itself contributes to more warming. Human activities do not increase the overall water vapour content in the atmosphere, but human activities can cause more water to evaporate as a result of increased temperature of the atmosphere. Thus anthropogenic warming is enhanced by a greater water vapour content in the atmosphere. [2]

• ↑ Pixabay. (September 3, 2015). Kettle Steam [Online]. Available: https://pixabay.com/p-653673/?no_redirect
• ↑ 2.0 2.1 2.2 Climap. (September 3, 2015). Water Vapour [Online]. Available: http://climap.net/water-vapor
• ↑ Bhattacharjee, Thermodynamics: an interactive Approach . Pearson, 2015.
• ↑ The Weather Prediction. (September 3, 2015). Atmospheric Water Vapour [Online]. Available: http://www.theweatherprediction.com/habyhints/40/
• ↑ USGS. (September 10, 2015). Precipitation - The Water Cycle [Online]. Available: http://water.usgs.gov/edu/watercycleprecipitation.html
• ↑ American Chemical Society. (September 3, 2015). Methane and oxygen react [Online]. Available: http://www.middleschoolchemistry.com/multimedia/chapter6/lesson1 , [October 25,2013]
• ↑ 7.0 7.1 QUEST. (September 10, 2015). Water Vapour - Positive Feedback Cycle [Online]. Available: http://science.kqed.org/quest/2014/12/12/water-vapors-role-in-climate-change/
• ↑ Kathryn Hansen, NASA. (September 3, 2015). Water Vapor Confirmed as Major Player in Climate Change [Online]. Available: http://www.nasa.gov/topics/earth/features/vapor_warming.html

The state of water vapour

Water can exist in a liquid, gas (water vapour) or solid (ice) state in the atmosphere, with individual molecules continuously evaporating (going from the liquid to gas state), condensing (going from the gas to liquid state), subliming (going from the solid to the gas state) and depositing (going from the gas to the solid state). 

But what governs which of those processes occurs where and when?

Basically, they are all occurring all the time, it is just the rate at which they are occurring that varies. 

Evaporation, for example, mostly depends on temperature — the warmer it is, the faster the rate of evaporation from lakes, rivers, puddles, vegetation, our skin, and cloud droplets. 

Condensation, on the other hand, mostly depends on the concentration of water vapour already in the air. The more humid the air, the fast the rate of condensation.

If there is more evaporation going on than condensation, the sky is clear. If there is more condensation occurring than evaporation, cloud droplets form — the temperature at which this happens is called the dew point temperature. At the dew point temperature, the relative humidity of the air is 100% and it is said to be saturated.

So, what do we mean by ‘relative humidity’? Relative humidity is the amount of water vapour present in air expressed as a percentage of the amount needed to achieve saturation at the same temperature.

Cloud can form either when the amount of water vapour in the air increases without changing the temperature of the air — this increases the rate of condensation, or by cooling the air down without changing the amount of water vapour in the air — this decreases the rate of evaporation. 

Have you ever watched a puffy cumulus cloud on a warm, sunny day? You can literally see parts of the cloud evaporating whilst other parts grow. Have you also noticed that, unlike most primary school drawings, cumulus clouds have flat bases? Cloud base marks the level in the atmosphere which is at the dew point temperature — below cloud base there is more evaporation going on than condensation, above cloud base there is more condensation. 

Have you ever heard the expression ‘it’s too cold for snow’? What it really means is ‘it’s too dry for snow’ — in the centre of large continents such as N America and Antarctica, in the very cold winter, where you are a long way away from large bodies of liquid water, the rate of sublimation or evaporation is so low that the air is very, very dry and no snowflakes can form. 

It’s also worth noting that, when we talk about evaporation rates, it’s the rate at which water would evaporate from the flat surface of a lake that’s meant. In reality, it’s much easier for a water molecule to escape from a curved surface — the more curved the surface, the fewer nearby molecules holding it the water molecule place. So, the smaller a raindrop, the easier it is for it to evaporate. 

In practice, this means that cloud droplets tend to form on a small particle — soot, salt, pollen, even bacteria — this makes them instantly bigger and therefore makes it harder for water molecules to escape. At times, there aren’t enough of these small particles, known as Cloud Condensation Nuclei, in the air — and, if this is the case, the relative humidity can be over 100%; it is super-saturated. This is one example of why defining saturation (100% relative humidity) as ‘the amount of water vapour the air can hold’ is really unhelpful.

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Water Vapor in the Earth's Atmosphere

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Have you ever wondered how much water vapor is in the Earth 's atmosphere or what the maximum amount is that air can hold?

How Much Water Vapor Is in the Earth's Atmosphere?

Water vapor exists as an invisible gas in the air. The amount of water vapor in air varies according to the temperature and density of air. The amount of water vapor ranges from a trace amount up to 4% of the mass of air. Hot air can hold more water vapor than cold air, so the amount of water vapor is highest in hot, tropical areas and lowest in cold, polar regions.

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How light can vaporize water without the need for heat

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It’s the most fundamental of processes — the evaporation of water from the surfaces of oceans and lakes, the burning off of fog in the morning sun, and the drying of briny ponds that leaves solid salt behind. Evaporation is all around us, and humans have been observing it and making use of it for as long as we have existed.

And yet, it turns out, we’ve been missing a major part of the picture all along.

In a series of painstakingly precise experiments, a team of researchers at MIT has demonstrated that heat isn’t alone in causing water to evaporate. Light, striking the water’s surface where air and water meet, can break water molecules away and float them into the air, causing evaporation in the absence of any source of heat.

The astonishing new discovery could have a wide range of significant implications. It could help explain mysterious measurements over the years of how sunlight affects clouds, and therefore affect calculations of the effects of climate change on cloud cover and precipitation. It could also lead to new ways of designing industrial processes such as solar-powered desalination or drying of materials.

The findings, and the many different lines of evidence that demonstrate the reality of the phenomenon and the details of how it works, are described today in the journal PNAS, in a paper by Carl Richard Soderberg Professor of Power Engineering Gang Chen, postdocs Guangxin Lv and Yaodong Tu, and graduate student James Zhang.

The authors say their study suggests that the effect should happen widely in nature— everywhere from clouds to fogs to the surfaces of oceans, soils, and plants — and that it could also lead to new practical applications, including in energy and clean water production. “I think this has a lot of applications,” Chen says. “We’re exploring all these different directions. And of course, it also affects the basic science, like the effects of clouds on climate, because clouds are the most uncertain aspect of climate models.”

A newfound phenomenon

The new work builds on research reported last year , which described this new “photomolecular effect” but only under very specialized conditions: on the surface of specially prepared hydrogels soaked with water. In the new study, the researchers demonstrate that the hydrogel is not necessary for the process; it occurs at any water surface exposed to light, whether it’s a flat surface like a body of water or a curved surface like a droplet of cloud vapor.

Because the effect was so unexpected, the team worked to prove its existence with as many different lines of evidence as possible. In this study, they report 14 different kinds of tests and measurements they carried out to establish that water was indeed evaporating — that is, molecules of water were being knocked loose from the water’s surface and wafted into the air — due to the light alone, not by heat, which was long assumed to be the only mechanism involved.

One key indicator, which showed up consistently in four different kinds of experiments under different conditions, was that as the water began to evaporate from a test container under visible light, the air temperature measured above the water’s surface cooled down and then leveled off, showing that thermal energy was not the driving force behind the effect.

Other key indicators that showed up included the way the evaporation effect varied depending on the angle of the light, the exact color of the light, and its polarization. None of these varying characteristics should happen because at these wavelengths, water hardly absorbs light at all — and yet the researchers observed them.

The effect is strongest when light hits the water surface at an angle of 45 degrees. It is also strongest with a certain type of polarization, called transverse magnetic polarization. And it peaks in green light — which, oddly, is the color for which water is most transparent and thus interacts the least.

Chen and his co-researchers have proposed a physical mechanism that can explain the angle and polarization dependence of the effect, showing that the photons of light can impart a net force on water molecules at the water surface that is sufficient to knock them loose from the body of water. But they cannot yet account for the color dependence, which they say will require further study.

They have named this the photomolecular effect, by analogy with the photoelectric effect that was discovered by Heinrich Hertz in 1887 and finally explained by Albert Einstein in 1905. That effect was one of the first demonstrations that light also has particle characteristics, which had major implications in physics and led to a wide variety of applications, including LEDs. Just as the photoelectric effect liberates electrons from atoms in a material in response to being hit by a photon of light, the photomolecular effect shows that photons can liberate entire molecules from a liquid surface, the researchers say.

“The finding of evaporation caused by light instead of heat provides new disruptive knowledge of light-water interaction,” says Xiulin Ruan, professor of mechanical engineering at Purdue University, who was not involved in the study. “It could help us gain new understanding of how sunlight interacts with cloud, fog, oceans, and other natural water bodies to affect weather and climate. It has significant potential practical applications such as high-performance water desalination driven by solar energy. This research is among the rare group of truly revolutionary discoveries which are not widely accepted by the community right away but take time, sometimes a long time, to be confirmed.”

Solving a cloud conundrum

The finding may solve an 80-year-old mystery in climate science. Measurements of how clouds absorb sunlight have often shown that they are absorbing more sunlight than conventional physics dictates possible. The additional evaporation caused by this effect could account for the longstanding discrepancy, which has been a subject of dispute since such measurements are difficult to make.

“Those experiments are based on satellite data and flight data,“ Chen explains. “They fly an airplane on top of and below the clouds, and there are also data based on the ocean temperature and radiation balance. And they all conclude that there is more absorption by clouds than theory could calculate. However, due to the complexity of clouds and the difficulties of making such measurements, researchers have been debating whether such discrepancies are real or not. And what we discovered suggests that hey, there’s another mechanism for cloud absorption, which was not accounted for, and this mechanism might explain the discrepancies.”

Chen says he recently spoke about the phenomenon at an American Physical Society conference, and one physicist there who studies clouds and climate said they had never thought about this possibility, which could affect calculations of the complex effects of clouds on climate. The team conducted experiments using LEDs shining on an artificial cloud chamber, and they observed heating of the fog, which was not supposed to happen since water does not absorb in the visible spectrum. “Such heating can be explained based on the photomolecular effect more easily,” he says.

Lv says that of the many lines of evidence, “the flat region in the air-side temperature distribution above hot water will be the easiest for people to reproduce.” That temperature profile “is a signature” that demonstrates the effect clearly, he says.

Zhang adds: “It is quite hard to explain how this kind of flat temperature profile comes about without invoking some other mechanism” beyond the accepted theories of thermal evaporation. “It ties together what a whole lot of people are reporting in their solar desalination devices,” which again show evaporation rates that cannot be explained by the thermal input.

The effect can be substantial. Under the optimum conditions of color, angle, and polarization, Lv says, “the evaporation rate is four times the thermal limit.”

Already, since publication of the first paper, the team has been approached by companies that hope to harness the effect, Chen says, including for evaporating syrup and drying paper in a paper mill. The likeliest first applications will come in the areas of solar desalinization systems or other industrial drying processes, he says. “Drying consumes 20 percent of all industrial energy usage,” he points out.

Because the effect is so new and unexpected, Chen says, “This phenomenon should be very general, and our experiment is really just the beginning.” The experiments needed to demonstrate and quantify the effect are very time-consuming. “There are many variables, from understanding water itself, to extending to other materials, other liquids and even solids,” he says.

“The observations in the manuscript points to a new physical mechanism that foundationally alters our thinking on the kinetics of evaporation,” says Shannon Yee, an associate professor of mechanical engineering at Georgia Tech, who was not associated with this work. He adds, “Who would have thought that we are still learning about something as quotidian as water evaporating?”

“I think this work is very significant scientifically because it presents a new mechanism,” says University of Alberta Distinguished Professor Janet A.W. Elliott, who also was not associated with this work. “It may also turn out to be practically important for technology and our understanding of nature, because evaporation of water is ubiquitous and the effect appears to deliver significantly higher evaporation rates than the known thermal mechanism. …  My overall impression is this work is outstanding. It appears to be carefully done with many precise experiments lending support for one another.”

The work was partly supported by an MIT Bose Award. The authors are currently working on ways to make use of this effect for water desalination, in a project funded by the Abdul Latif Jameel Water and Food Systems Lab and the MIT-UMRP program.

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Press mentions, interesting engineering.

Interesting Engineering reporter Rizwan Choudhury spotlights a new study by MIT researchers that finds light can cause evaporation of water from a surface without the need for heat. The photomolecular effect “presents exciting practical possibilities,” writes Choudhury. “Solar desalination systems and industrial drying processes are prime candidates for harnessing this effect. Since drying consumes significant industrial energy, optimizing this process using light holds immense promise.”

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