Key idea: Atmospheric moisture makes a significant contribution to the weather on Earth. The other terrestrial planets have little atmospheric water content. Moisture enters our atmosphere by evaporation, and returns to the surface of the Earth by condensation and precipitation. Atmospheric moisture helps to redistribute the uneven heat energy from the Sun.

Strait of Gibraltar from Space Shuttle Astronomical tidbit: A large fraction of the Earth's surface is covered with liquid water. It is the only planet with liquid water on its surface. Water constantly cycles between the surface and the atmosphere of Earth, and it is essential for supporting life as we know it. Moisture enters the atmosphere when surface water evaporates, and returns to the surface when the atmospheric water vapor condenses and precipitates. The photograph at the left was taken from aboard NASA's Space Shuttle. The Atlantic Ocean is in the foreground and the Mediterranean Sea is in the background. Linking the two is the Strait of Gibraltar. In addition to these vast bodies of surface water, atmospheric moisture is visible in the photograph in the form of clouds of water droplets.

Earth's atmosphere contains approximately 1% water vapor. The atmospheres of the other terrestrial planets have very little water content, but water vapor is measurable on Venus and Mars. Water is constantly cycled between the atmosphere and surface of Earth. Most water vapor enters our atmosphere by evaporation from surface water and wet soil. Some water vapor also enters our atmosphere from the leaves of plants by a process called transpiration. Water returns to the surface by condensation in the atmosphere and precipitation. Clouds of water droplets form when water vapor condenses in the atmosphere. These water droplets may then precipitate to the surface in the form of rain or snow. You are all well aware, however, that clouds to do necessarily lead to precipitation.

Evaporation removes heat energy from the source of the water. That is why you cool off when your sweat evaporates!!! When the water vapor condenses in the atmosphere, the heat energy in the vapor is released. The evaporation of water, the movement of moist air masses, and the subsequent release of heat energy through condensation and precipitation help to redistribute the uneven heat energy from the Sun. You have already learned in a previous lesson why the Sun does not heat the Earth in a uniform way. Movement of moist air masses also transports water from the oceans to the land areas across our entire planet.

Activity/demonstration: Investigate the evaporation process and the factors which affect the rate of evaporation.

At any temperature, there is a limit to the amount of water vapor that the atmosphere can contain. Once the water vapor limit is reached, the air is said to be saturated. An attempt to add more water vapor to saturated air will produce condensation. As the temperature of the atmosphere increases, the saturation limit also increases. Warm air can therefore contain more water vapor than cool air without condensation.

Activity/demonstration: Add ice cubes to a glass of water and observe the formation of condensation. Prove to yourself that there is water vapor in our atmosphere even if it cannot be seen.

At a particular temperature, the relative humidity is the ratio (usually expressed as a percentage) of the actual moisture content of the air to the moisture content of saturated air at that temperature. Usually the humidity is highest at night when the temperature is cool and the water vapor saturation limit is low, and the humidity is lowest in the afternoon when the temperatures and corresponding saturation limits are high. In desert areas where there is very little water vapor in the atmosphere, the humidity is generally low. In tropical areas where there is much water vapor in the air, the humidity is typically high.

Question: Why is it so uncomfortable in the summer time in certain parts of the US where the temperature and relative humidity are both often very high???

Activity/demonstration: Measure the relative humidity with a psychrometer and a relative humidity chart.

Suppose that there is some moisture in the air, but that the air is not saturated. If the air is then cooled, the saturation limit of the air decreases. Eventually a temperature is reached when the air becomes saturated. Below this point, the water vapor begins to condense. The temperature at which this condensation occurs is called the dew point. The dew point depends on the amount of water vapor in the air and on the atmospheric pressure.

As do the other gases in the atmosphere, the water vapor in the atmosphere contributes to the pressure which the atmosphere exerts on itself and on the surface of the Earth. The fraction of the total atmospheric pressure contributed by water vapor, however, is small and is at most a few tens of millibars at the Earth's surface. Remember that you learned in an earlier lesson that 1000 millibars is equal to 1 bar which is approximately the average atmospheric pressure at the surface of the Earth.

There may be a substantial amount of water vapor in the atmospheres of the Gas Giant planets. Scientists believe that it is possible that water vapor rises in the atmospheres of these planets, until it is cooled sufficiently to condense and precipitate. The precipitation then falls deep into the Gas Giant atmospheres until it is heated enough to vaporize. The water vapor then rises and the cycle continues. Lightning has been observed in the atmosphere of Jupiter and it is even possible that thunderstorms occur on that planet!!!

Last updated: November 17, 1999
Joe Twicken /
Rob Wigand