and night in the summer, it is at a very low angle in the sky. This means that not much
solar radiation reaches the ground near the poles. Because of this, during a large part of the
year, the polar areas are covered with ice and snow. These brilliant white substances have a
high albedo and reflect solar energy back into the atmosphere. For all of these reasons, the
region around the equator is much warmer than the areas at the poles.
Figure 15.17: The average annual temperature of the Earth, showing that the equatorial
region is much warmer than the polar regions. There is a roughly gradual temperature
gradient from the low to the high latitudes. ( 4 )
The difference in the amount of solar energy that the planet receives at different latitudes
drives much of the activity that takes place at the Earth’s surface. This includes the wind,
water cycle, and ocean currents. The differences in solar energy around the globe drive the
way the atmosphere circulates.
The Greenhouse Effect
The remaining factor in the Earth’s heat budget s the role of greenhouse gases. Greenhouse
gases warm the atmosphere by trapping heat. Sunlight strikes the ground, is converted to
heat, andradiatesbackintotheloweratmosphere. Someoftheheatistrappedbygreenhouse
gases in the troposphere, and cannot exit into space. Like a blanket on a sleeping person,
greenhouse gases act asinsulationfor the planet. The warming of the atmosphere due to
insulation by greenhouse gases is called thegreenhouse effect(Figure15.18).
The greenhouse effect is very important, since without it the average temperature of the
atmospherewouldbeabout-18oC(0oF).Withthegreenhouseeffect,theaveragetemperature