Chapter 7
Atmospheric Moisture

Cumulus clouds over the Atlantic Ocean
(Courtesy NOAA)

"It is better to read the weather forecast before we pray for rain."
- Notebook; More Maxims of Mark, Johnson, 1927

Water in its various forms sustains life, transports energy and erodes the surface beneath our feet. Water is needed for cell growth, photosynthesis, the formation of soil, and to absorb and transport nutrients in plants and animals. Without water, living things could not survive. Energy is transported between the various spheres of the Earth system via phase changes of water. Nearly every portion of the Earth has been sculpted by the movement of water across the surface at some point in geologic history. Here  we'll look at water in its various forms, as a gas, liquid, and solid. We'll investigate how it moves through and the vital role it plays in the Earth system. You will become familiar with the geographic distribution of precipitation and its impact on the environment.

The animation below shows the global pattern of cloud cover measured as the fraction of sky covered by clouds from January 2005 to April 2011. Cloud fraction ranges from 0 for cloudless skies (dark blue) to 1 for total cloud cover (white). Watch the animation by clicking the play button and describe what you see.

Seasonal changes in cloud fraction
Courtesy NASA Earth Observatory (Source)

Tropical regions appear to uniformly cloudy through out the year. The noticeable band of clouds circling the tropics shifts slightly to the north and south through the year. Much of subtropical Africa and Antarctica have relatively low cloud cover throughout the year. The midlatitudes have more variable cloud conditions. Why does this spatial and temporal pattern in cloud cover occur and what are the implications? Will this pattern change in the future?

Learning Outcomes:

By the end of the chapter you should be able to:

• Explain the phase changes of water.
• Determine the stability of air and its likelihood for cloud development
• Identify common cloud types.
• Describe how precipitation forms.
• Explain the global pattern of precipitation.

Assess if you are ready for this chapter by Getting Ready for Chapter 7: Atmospheric Moisture.

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