11. Atmospheric Moisture and the Water Balance

The Hydrologic Cycle

Photo 21: Dried-up run-off river, Drumheller, Alberta. 2011-05-12

The Hydrologic Cycle is the cycle of water on Earth; the cycle by which water passes through water’s solid, liquid, and gas stages; as well as passing through the Hydrosphere, Atmosphere, Lithosphere, Cryosphere, and Biosphere. It is one of the most important cycles on our planet.

The main process is evaporation, in which water vaporizes above the oceans, which takes out the salt of the water and creates clouds; water then falls on the oceans and land in the form of precipitation. A second process is called transpiration by which water transpires into the Atmosphere through leafs. Water also evaporates from the land. These last two processes have the combined name: evapotranspiration. Evaporation and evapotranspiration together cause precipitation. Water from precipitation is absorbed by and transported on and in the surface of the earth. Transportation of water on the surface via rivers and streams is called runoff. Alberta has many seasonal runoff streams that are active in the spring but dry up in summer (photo 21).

Photo 22: Altocumulus Cloud, Edmonton, Alberta. 2011-05-10

Clouds

Clouds are an important part of the hydrologic cycle, they hold and transport water. Cloud-type classification is an important part of meteorology and climatology. There are 3 categories of clouds. Status clouds are layer like, thin, and cover a large area. Stratus clouds below 3km are called stratus clouds, between 3-6 km altostratus, and above 6 km cirrostratus. The second cloud type is the cumulus cloud, which looks like a puffy, thick, billowing mass that often develops at great heights. Depending on height, these clouds are called cumulus or stratocumulus (below 3 km), altocumulus (3-6 km), and cirrocumulus (above 6 km). Very large cumulus clouds are cumulonimbus clouds, extending from 500 m at the base to 12 km at the top. The third type is the cirrus cloud, which is very thing and wispy, streak-like clouds. These clouds occur at heights above 6 km. Often the term cirrocumulus and cirrostratus is used as well, to imply a combination.

Cloud classification is still a form of interpretation as clouds are never really the same. The image above (photo 22) is a cumulus cloud; however, cumulus clouds are often have more crisp edges and defined curves, this cloud has some of the streaky aspects of the stratus cloud. Estimating the height of the cloud at about 5km, I would guess this cloud can also be called altocumulus.