Moisture

Water in the atmosphere is a critical variable in any study of meteorology, and is a critical component in air quality modeling. Most readers will already be familiar with the concept of "acid rain", in which precipitation in the atmosphere (rain or snow) combines with nitric or sulfuric acids in the lower atmosphere to produce an acidic wet deposition, causing damage to buildings, forested areas, and bodies of water, to mention a few.

As a very basic introduction to moisture (more detailed descriptions will follow in later readings), the reader should, at a minimum, understand the hydrologic, or water, cycle. The graphic below shows the basic hydrologic cycle:

The cycle basically has five components:

1. Condensation: water, when cooled, condenses to form clouds containing water
2. Precipitation: water in the form of rain or snow is deposited to the land surface from clouds
3. Runoff: water moves from high elevations to lower elevations, typically ending up in lakes, rivers, streams, or oceans
4. Infiltration: some water is absorbed into the ground, ending up under the ground as groundwater. Some of this water is taken up by plants
5. Evapotranspiration surface water is transported to the lower atmosphere by evaporation. Water is also released by plants in a process known as "transpiration". Both evaporation and transpiration are typically combined to define water moving from the surface to the atmosphere.

A runnable model of the hydrologic cycle is available for your use. You should use this model to answer the "Quick Quiz" question below!

You are encouraged to visit the NASA animation of the hydrologic cycle.