The balanced atmosphere

Circulation of Water in the Atmosphere

Inventory of water at the Earth's surface (volume in 10,000,000 cubic kilometers).

Reservoir	Volume	Percent of total
Oceans	1370	97.2
Ice Caps/Glaciers	29	2.15
Deep Groundwater	5.3	0.38
Shallow Groundwater	4.2	0.30
Lakes	0.125	0.01
Soil Moisture	0.065	0.005
Atmosphere	0.013	0.001
Rivers	0.0017	0.0001
Biosphere	0.0006	0.00004

Within the atmosphere, there is an unending circulation of water. Since the oceans occupy over 70 percent of the earth's surface, we can think of this circulation as beginning over the oceans.

Over the oceans, the sun's energy transforms enormous quantities of liquid water into water vapor in a process called evaporation.

Winds then transport the moist air to other regions, where the water vapor changes back into liquid, forming clouds, in a process called condensation. Under certain conditions, the liquid (or solid) cloud particles may grow in size and fall to the surface as precipitation.

If the precipitation falls into the ocean, the water is ready to begin its cycle again. If, on the other hand, the precipitation falls on a continent, a great deal of the water returns to the ocean in a complex journey.

This cycle of moving and transforming water molecules from liquid to vapor and back to liquid again is called the hydrologic (water) cycle.

Before falling rain ever reaching the surface, a portion of it evaporates back into the air. Some of the precipitation may be intercepted by vegetation, where it evaporates or drips to the ground.

Once on the surface, a portion of the water soaks into the ground, forming groundwater that can be tapped by wells.

Figure 1: Distribution of water on the Earth. The amount of water present in various natural reservoirs is represented in terms of comparative spherical volumes. The numbers under the name of each reservoir denotes the contents of that reservoir in 10,000,000 cubic kilometers.

What does not soak in collects in puddles of standing water or runs off into streams and rives, which find their way back into de ocean. Even the underground water moves slowly and eventually surfaces to then be evaporated or carried seaward by rivers.

Over land, a considerable amount of vapor is added to the atmosphere through evaporation from the soil, lakes, and streams. Even plants give up moisture by a process called transpiration. The water absorbed by a plant's root system moves upward through the stem and and emerges from the plant through numerous small openings on the underside of the leaf.

In all, evaporation and transpiration from continental areas amount to only 15 % of the nearly 1.5 billion billion gallons of water vapor that annually evaporate into the atmosphere; the remainning 85% evaporates from the oceans.

The total mass of water vapor stored in the atmosphere at any moment adds up to only a little over a week's supply of the world's precipitation. Since this amount varies only slightly from day to day, the hydrologic cycle is exceedingly efficient in circulating water in the atmosphere.

Figure 2: The detailed hydrologic cycle emphasizing processes involved in the transfer of water. Numbers on arrows show relative water fluxes.

Plant transpiration is defined as plant's loss of water, mainly through the stomates of leaves. Stomates consist of two guard cells that form a small pore on the surfaces of leaves. The guard cells control the opening and closing of the stomates in response to various environmental stimuli. The stomate also called STOMA, plural STOMATA, OR STOMAS, are microscopic openings or pores in the epidermis of leaves and young stems. Stomates are generally more numerous on the underside of leaves. They provide for the exchange of gases between the outside air and the branched system of interconnecting air canals within the leaf.

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