What causes the seasons? How do seasons occur? Information about earth movement and seasons.
Suppose someone asked you why the days are warmer in summer than in winter. Your first answer would probably be, “Because the days are longer in summer.” This answer would be partly correct. During the longer days in summer, our part of the earth gets more hours of sunshine to heat it. But there are other reasons why summer is warmer than winter.
You have surely noticed that the sun is almost straight overhead at noon in summer. In winter, the sun at noon is scarcely halfway up to that point. It appears much lower in the sky. In summer, the northern half of the earth where we live is tilted toward the sun. In winter, just the opposite is true. The northern half of the earth is tilted away from the sun. Because the earth’s axis is tilted, the sun s rays strike the earth at different angles during different seasons. In summer, its rays strike the earth nearly straight, or almost vertically. In winter, they strike the earth at much more of a slant.
Vertical rays spread over less surface than slanting rays. In our part of the earth, the rays that fail on more than 2 square feet at noon December 21 are concentrated on just 1 square foot at noon June 21. That is, each square foot gets about twice as much energy from the sun. This energy is ab-sorbed by the earth’s surface and changed into heat. So the nearly vertical rays in summer give much more heat to the earth than the slanting rays in winter.
There is still another reason for the greater heat in summer. The earth’s atmosphere always contains dust particles, water vapor, and other materials that absorb and scatter the sun’s rays. In winter, the slanting rays must pass through many miles of this atmosphere before reaching the earth’s surface. In summer, when the rays shine almost vertically, they have fewer miles of the atmosphere to pass through. As a result, the sun’s rays in summer actually have more energy left when they reach the earth’s surface.
You have now learned three reasons why summer is warmer than winter:
- (1) The days are longer in summer. The part of the earth where it is summer gets more hours of sunshine to heat it.
- (2) In summer, the sun’s rays strike the earth almost vertically. They give much more heat to the earth.
- (3) In summer, the sun’s rays have fewer miles of the atmosphere to pass through. So they have more energy left when they reach the earth’s surface.
Each of these three reasons is related to two facts that you have already learned about the earth. The length of the days and the angle of the sun’s rays both change because the axis of the earth is tilted as the earth revolves around the sun. Of course, changes in the seasons take place not only in the northern half of the earth where we live but also in the southern half. However, the seasons there are just the opposite of ours. When we are having summer, it is winter in the southern half of the earth. And when we are having winter, it is summer in the southern half of the earth.