As a result, the portion of the Earth that receives the most direct Sun rays varies as the Earth rotates around the Sun. The Sun's rays shine most directly on the equator at the equinox, and the Northern and Southern Hemispheres receive the same quantity of sunshine. But during other parts of the year, more or less of the Earth's surface is covered by clouds or water.
At the equator, there are 24 hours of daylight and 24 hours of darkness. At the poles, there is only one hour of daylight and one hour of darkness. So the polar regions experience dark days and light nights regardless of the season.
The amount of sunlight at the equator varies by about 15 percent from summer to winter. During spring and fall, the sun is at its highest point in the sky and experiences no change in altitude. However, during the summer months it goes down farther south than it does during the winter months.
At the poles, there is no variation in altitude for the sun. It is always either up or down depending on whether you are at the North Pole or the South Pole.
But because the angle between the sun and the horizon changes at both the north and south poles, the amount of daylight and darkness there varies throughout the year. In fact, at the South Pole, the sun is below the horizon for half of each day and rises every morning above the horizon.
The quantity of sunlight received by different sections of the Earth's surface varies. At the equator, the sun's rays reach the Earth's surface the most directly. The rays are focused on a narrow region as a result of this. The sun's rays reach the Earth's surface at a slant near the poles due to the Earth's tilt; they are less focussed. This difference in concentration and direction causes differences in temperature throughout the year.
At the equator, the average intensity of sunlight is about 1,360 watts per square meter (W/m2). At the North Pole, the intensity is only 730 W/m2, while at the South Pole it is only 250 W/m2.
These figures should be compared with the solar constant, which is approximately 600 W/m2 everywhere on the Earth except where shadowed by a mountain peak or similar obstruction. Thus, the average intensity of solar radiation at the Earth's surface is more than three times that of what reaches the top of the atmosphere. However, since clouds and other atmospheric effects reduce that amount further, total solar irradiance actually ranges from 270 W/m2 below 500 m altitude to 320 W/m2 above that level.
At the poles, there is no direct path for sunlight to reach the ground, so it must pass through the atmosphere before reaching it. As mentioned earlier, at high latitudes the angle at which sunlight enters the atmosphere is greater than at low latitudes, so more of it is reflected back into space.