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The amount of heat energy received at any location on the globe is a direct effect of sun angle on climate, as the angle at which sunlight strikes the Earth varies by location, time of day, and season due to the Earth's orbit around the Sun and the Earth's rotation around its tilted axis. Seasonal change in the angle of sunlight, caused by the tilt of the Earth's axis, is the basic mechanism that results in warmer weather in summer than in winter.〔Windows to the Universe. (Earth's Tilt Is the Reason for the Seasons! ) Retrieved on 2008-06-28.〕 Change in day length is another factor.〔〔 (See also season.) == Geometry of sun angle == When sunlight shines on the earth at a lower angle (sun closer to the horizon), the energy of the sunlight is spread over a larger area, and is therefore weaker than if the sun is higher overhead and the energy is concentrated on a smaller area. (See Figure 1.) Figure 2 depicts a sunbeam one mile (1.6 km) wide falling on the ground from directly overhead, and another hitting the ground at a 30° angle. Trigonometry tells us that the sine of a 30° angle is 1/2, whereas the sine of a 90° angle is 1. Therefore, the sunbeam hitting the ground at a 30° angle spreads the same amount of light over twice as much area (if we imagine the Sun shining from the south at noon, the north-south width doubles; the east-west width does not). Consequently, the amount of light falling on each square mile is only half as much. The Sunbeam entering at the shallower angle must also travel twice as far through the Earth's atmosphere, which reflects some of the energy back into space. Figure 3 shows the angle of sunlight striking the earth in the Northern and Southern hemispheres when the Earth's northern axis is tilted away from the Sun, when it is winter in the north and summer in the south. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Effect of sun angle on climate」の詳細全文を読む スポンサード リンク
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