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

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?

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.

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).

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).

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).

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.

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.

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.

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.