 |  Environmental Science: A Global Concern, 7/e William P. Cunningham,
University of Minnesota
Mary Ann Cunningham,
Vassar College
Barbara Woodworth Saigo,
St. Cloud State University
Air, Weather, and Climate
BE ALERT FOR: Air Moves Water and Energy
Understanding the role of the atmosphere as the great weather engine is much easier if you learn a few guidelines first. Air currents move around much more than air. They also transport significant amounts of both water and energy.
Let's deal with water first. Air, particularly warm air, can contain substantial amounts of water vapor. In fact, much of the solar energy that is absorbed by the earth is used to evaporate water, transferring it into air masses as vapor. So, as air masses move around the earth, huge quantities of water often are transported as well. As air masses cool, the water-holding capacity of the air is reduced and the excess water condenses and falls to earth, contributing to local weather.
Because water has a great capacity to absorb energy, it serves as a powerful energy sponge. So, moisture-laden air currents carry along this energy, as well as the moisture, as they move. The energy is released as heat when the water vapor later condenses.
It will also help to remember that warm air rises and that it cools when it does so. These vertical currents of air, called convection currents, effectively mix air and help transport both energy and water within the atmosphere.
BE ALERT FOR: Milankovitch Cycles
A promising theory to explain the periodic patterns of weather cycles involves the Milankovitch cycles. To understand this theory, you need to first understand these cycles.
First, the earth's axis is tipped. When the axis is tipped toward the sun, we experience summer in the northern hemisphere. When tipped away from the sun, we experience winter. (If the earth's axis were continuously perpendicular to the sun, we would not experience the different seasons.) But the axis does not remain in a constant angle of tip. The angle changes back and forth in a cycle of about 44,000 years. The earth's axis also wobbles in a 22,000-year cycle.
Finally, during the course of a year, the earth is not always at the same distance from the sun. Currently, when the earth is closest to the sun, the northern hemisphere is tipped away from the sun and we experience winter. When we are farthest from the sun, the northern hemisphere is tipped toward the sun, giving us summer. This so-called eccentricity of our orbit also changes in a cyclic pattern.
Each of the three cycles can influence the intensity of solar radiation experienced at a given location.
BE ALERT FOR: Energy Spectrum
Understanding the greenhouse effect requires a quick review of an energy concept first encountered back in Chapter 3. To maintain a temperature equilibrium on earth, the total amount of energy coming in from space must be balanced by the loss of an equal amount of energy reradiated by the earth's surface back into space. Climatologists believe that by increasing the amounts of certain greenhouse gases in the atmosphere, this balance will be destroyed. The anticipated problem is that these gases will reduce the amount of energy leaving for space, keeping it here to make the earth progressively warmer.
The puzzler is this: Since energy must pass through the same atmosphere, regardless of whether it's coming in or going out, how could the atmosphere reduce the amount going out without having reduced the amount coming in as well?
The explanation is that electromagnetic energy exists in different forms. Sunlight, ultraviolet, infrared, and microwaves are all energy forms, but each has different properties. Energy comes in principally as visible sunlight but leaves as invisible infrared. The greenhouse gases do not affect inbound sunlight. They do, however, prevent the escape of infrared.
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2003 McGraw-Hill Higher Education