Open Site - Science: Earth Sciences: Hydrology: Water Cycle: Atmostphere

Top: Science: Earth Sciences: Hydrology: Water Cycle: Atmostphere

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Overview

The water cycle is all about storing water and moving water on, in, and above the Earth. Although the atmosphere may not be a great storehouse of water, it is the superhighway used to move water around the globe. Evaporation and transpiration change liquid water into vapor, which ascends into the atmosphere due to rising air currents. Cooler temperatures aloft allow the vapor to condense into clouds and strong winds move the clouds around the world until the water falls as precipitation to replenish the earthbound parts of the water cycle. About 90 percent of water in the atmosphere is produced by evaporation from water bodies, while the other 10 percent comes from transpiration from plants.

There is always water in the atmosphere. Clouds are, of course, the most visible manifestation of atmospheric water, but even clear air contains water—water in particles that are too small to be seen. One estimate of the volume of water in the atmosphere at any one time is about 3,100 cubic miles (mi3) or 12,900 cubic kilometers (km3). That may sound like a lot, but it is only about 0.001 percent of the total Earth's water volume of about 332,500,000 mi3 (1,385,000,000 km3), as shown in the table below. If all of the water in the atmosphere rained down at once, it would only cover the ground to a depth of 2.5 centimeters, about 1 inch.

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Weight of Clouds

At sea level, the weight (pressure) of air is about 14 1/2 pounds per square inch (1 kilogram per square centimeter). Since air has weight it must also have density, which is the weight for a chosen volume, such as a cubic inch or cubic meter. If clouds are made up of particles, then they must have weight and density. The key to why clouds float is that the density of the same volume of cloud material is less than the density of the same amount of dry air. Just as oil floats on water because it is less dense, clouds float on air because the moist air in clouds is less dense than dry air.

A basic "everyday" cloud—the cumulus cloud with a volume of about 1 cubic kilometer (km) located about 2 km above the ground is a cube about 1 km on each side. The National Oceanic and Atmospheric Administration (NOAA) provides some estimates of air and cloud density and weight. NOAA found that dry air has a density of about 1.007 kilograms/cubic meter (kg/m3), moist air comes in at about 0.627 kg/m3, and the density of the actual cloud droplets is about 0.0005 kg/m3. The density of the cloud is thus about 62 percent of dry air. In the final calculations, the 1 km3 cumulus cloud weighs a whopping 1.4 billion pounds (635 million kilograms)! But the cloud floats because the weight of the same volume of dry air is even more, about 2.2 billion pounds (1 billion kilograms). Still, remember that it is the lesser density of the cloud that allows it to float on the dryer and more-dense air.