Lesson 2: Twists and Turns

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.

An Otherwordly Greenhouse Effect

Venus is an example of the greenhouse effect run amuck. Surface temperatures on Venus reach nearly 900 degrees Fahrenheit. Some of the solar radiation that reaches the planet's surface is reflected and then absorbed by the carbon dioxide atmosphere, rather than escaping into space. Carbon dioxide is one of the so-called "greenhouse gasses."

Twists and Turns

Earth tilts on its axis and orbits the Sun like a wobbly top. But what would happen if the Earth "stood up straight" and didn't tilt on its axis at all?

• There would be no seasons.

• Hours of daylight wouldn't increase in the spring and decrease in the fall.

• The North and South Poles, instead of having six months of light and six months of darkness as they do now, would have 12 hours of light and 12 hours of dark every day -- as would the rest of the planet.

If you like autumn leaves, blustery November days, and the change of the seasons, be thankful that our planet does tilt. That's what brings us spring, summer, fall, and winter. Currently, the tilt is 23.5 degrees. For a more technical look at the reason for seasons -- including the precession of the equinoxes -- see Weather, Chapter four.

In summer, the Northern Hemisphere points toward the Sun and the Southern Hemisphere points away. The farther north you are, the more hours of daylight you experience. Alaska truly becomes "The Land of the Midnight Sun." At the North Pole, the Sun circles the horizon day after day, never setting for six months. Meanwhile, researchers at the South Pole shiver in six months of endless night. The Southern Hemisphere has its summer when we have our winter.