Without much difficulty the equation for I( a) can be modified to take into account the angle of the path of light. Temperatures are higher because the sun's rays are striking the earth more directly at these latitudes even though the earth is actually farther away from the sun during the months of June, July, and August. In the northern hemisphere, for example, the zenith angle is least during the summer months. It is not due to distance changes between the earth and the sun. The fact that the more northern and southern latitudes of the earth have clearly distinguishable seasons is primarily due to the angle of the radiation striking a particular latitude. The zenith angle has a very significant effect on the intensity of the radiation we receive from the sun. The decrease of sunlight intensity with increasing latitude is reflected in studies of the incidence of skin cancer relative to geographical location. Because the majority of the earth's surface receives solar radiation at large zenith angles, it is worthwhile to consider the effect of the zenith angle on the intensity of sunlight. Only near the equator does sunlight reach the earth at close to perpendicular (i.e., z = 0). We see right away that z varies greatly from latitude to latitude. The zenith angle z is defined to be the angle subtended by the sun at the center of the earth and perpendicular to the surface of the earth (see the following figure). The intensity of solar UV light falling on the atmosphere at an angle z from the zenith will be attenuated as it penetrates into the atmosphere. So far in this project we have assumed that the sun is directly overhead, i.e., solar radiation is striking the earth's surface at a right angle.
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