Twists and Turns

Our neighbors in the solar system -- Mercury, Mars, Venus, the Moon, and Mars -- don't have weather the way we do. Mercury and the Moon have virtually no atmospheres and no weather. Venus, by contrast, has too much of an atmosphere. Venus is a suffocating, 900-degree inferno with an atmosphere composed mostly of carbon dioxide. Air pressure on Venus is 90 times greater than the surface air pressure of Earth. That's a crushing 1300 pounds of pressure per square inch. Mars, meanwhile, is a cold desert with a very thin carbon dioxide atmosphere.

Figure 2-1: Venus and Mars (photo courtesy of http://www.nasa.gov)

What's so different about Earth? Water! Abundant water is one of the key ingredients in our weather -- and it's what our neighbors in space mostly lack. (See Upgren and Stock's book Weather, Chapter 5, for more about comparative planetology.)

The three main ingredients in our weather are the Sun, water, and dirt.

• The Sun provides heat and energy, stirring the wind.
• Evaporation and condensation of water regulate temperature; and without water we would have no clouds or precipitation.
• By "dirt," I mean specks of sea salt, volcanic ash, and other impurities in the air. They act as condensation nuclei -- tiny surfaces upon which water vapor can condense -- and exist at the core of every cloud droplet, raindrop, and snowflake.