Distribution of incoming solar radiation

The distribution of solar radiation is obviously not uniform over the globe, as figure 1.1 illustrates. Plainly, low latitudes receive, on average, much more solar radiation than high latitudes, which is the reason why temperatures, on average, decrease with latitude.

The situation is made more complex by the nonzero obliquity of Earth's axis of rotation; that is, the rotation axis of Earth is not perpendicular to its orbital plane, as illustrated in figure 1.2. The axis of Earth's rotation is fixed in space and does not vary as Earth rotates around the sun; that is, relative to the distant galaxies, the line from the South Pole to the North Pole always has the same orientation. But because Earth rotates around the sun, the orientation varies relative to the sun. One day a year, the North Pole is most inclined toward the sun, and this day is known as the Northern Hemisphere's summer solstice. These days, it usually occurs on June 20 or 21. The Northern Hemisphere receives much more radiation than the Southern Hemisphere at this time of year, so this corresponds to the Northern Hemisphere's summer and the Southern Hemisphere's winter. In fact, not

Equinox, March 20-21

Equinox, March 20-21

Figure 1.2. Earth's orbit around the sun and the march of the seasons. Earth's axis of rotation is at an angle with respect to the axis of rotation of earth around the sun. The Northern Hemisphere's summer and the Southern Hemisphere's winter result when the north Pole points toward the sun, and the opposite season occurs six months later. The eccentricity is much exaggerated in the figure.

Figure 1.2. Earth's orbit around the sun and the march of the seasons. Earth's axis of rotation is at an angle with respect to the axis of rotation of earth around the sun. The Northern Hemisphere's summer and the Southern Hemisphere's winter result when the north Pole points toward the sun, and the opposite season occurs six months later. The eccentricity is much exaggerated in the figure.

only is the sun higher in the sky during summer, but the day is also much longer, and at latitudes above the Arctic circle, the sun does not set for about two weeks on either side of the summer solstice.

Progressing from June on, as earth moves around the sun, the distribution of solar radiation becomes more equal between the hemispheres, and the length of day evens out. Then we enter autumn in the northern Hemisphere and spring in the Southern Hemisphere. At the equinoxes (about March 20 and September 22), the hemispheres receive equal amounts of radiation from the sun, which is directly above the equator. The northern Hemisphere's winter solstice occurs on December

21 or 22, when the South Pole is most inclined toward the sun. It might seem from this description that in the Northern Hemisphere the coldest day should be on December 21 and the warmest day on or about June 21. In fact, the coldest and warmest times of year occur a few weeks after these dates; the main reason is thermal inertia in the oceans, which delays the onset of the warmest and coldest days. We'll discuss this effect more in chapter 5. Finally, we note that because of the eccentricity of Earth's orbit, Earth's distance from the sun varies throughout the year. However, this variation is a minor factor in seasonality, and for most intents and purposes we can regard Earth's orbit as circular.

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  • Sofia
    How is incoming solar radiation distributed?
    8 months ago
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    How is incoming solar radiation distribution?
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