chapter  4
41 Pages

Understanding Weather and Climate

The Global Distribution of Energy When energy arrives from the Sun it is not evenly distributed over Earth's surface. The latitude where the sun is directly overhead, and solar intensity greatest, varies seasonally with Earth's orbit between the Tropics of Cancer (23.4˚N) and Capricorn (23.4˚S) due to the tilt of Earth©s axis. The intensity of radiation also varies with the curvature of Earth's surface, because incoming solar energy is spread over an increasingly larger area at high (polar) latitudes than it is at lower latitudes. The net impact is that Earth gains more energy from the Sun than it emits back out into space at low latitudes up to 40° north and south

of the equator, but loses more energy than it receives at high latitudes from 40° to 90° toward the poles (Figure 4.2). This energy imbalance sets up a temperature gradient between the tropics and poles, which sets the atmosphere and oceans in motion to equalize the imbalance. If there were no motion in the atmosphere or oceans to redistribute the excess energy received at low latitudes, the tropics would simply continue to heat, and the poles would continue to cool, creating a hostile environment for any form of life.