FIGURE 4.6 The Earth, the Sun, and the seasons. The primary cause of the seasons is the tilt of the Earth's rotational axis, which affects the flow of solar energy to each hemisphere as the Earth revolves around the Sun. The designations of summer and winter in this figure apply to the northern hemisphere. When it is summer in the northern hemisphere, it is winter in the southern hemisphere, and vice versa.

the Sun while the other half is close to it. If you look again at Figure 4.6, you will see that, in the northern hemisphere, the Earth is closest to the Sun during the winter and farthest from the Sun during the summer. This may seem counterintuitive, but keep in mind that the average distance between the Earth and the Sun is 150 million kilometers (93 million miles), so that variations of even several thousand kilometers are negligible.

Although most people characterize the seasons mainly on the basis of temperature — that is, winter is cold and summer is hot — day length and precipitation also follow an annual cycle. The waveform of the precipitation cycle may be somewhat irregular, but day length and temperature have sinusoidal waveforms when averaged over many years (Figure 4.8). Note that the day-length curve is smoother than the temperature curve. In fact, the variation in day length progresses smoothly even on a day-to-day basis. Temperature, on the other hand, may show considerable day-to-day variation. For example, Figure 4.9 shows the curves for average highs and record highs in New York City. Although both curves rise in the summer,

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