Why Does Winter Happen?

Today, listening to Science Friday on NPR, I learned the answer to a question about which I had long wondered. My question: The winter solstice is on December 22; that is the day on which the sun reaches its most southerly position in the sky. After that, the sun moves gradually farther north every day. My question is, if the sun is moving north, why do the days get colder and colder throughout our winter in the northern hemisphere?

Most people think that the reason our winters occur is because we move farther from the sun during that season. That is not the truth. The earth’s orbit describes an almost perfect circle around the sun, maintaining an almost consistent distance from the sun—there is a slight variation, but it is insignificant.

The earth’s north/south axis is tipped 23° from its horizontal orbit. In the winter, we, in the northern hemisphere are located on the part of the tilt that is away from the sun. This tip causes radiation from the sun to be more diffused than it is in the summer, when the sun’s rays strike the earth in a more perpendicular direction. Therefore, heat from the sun is spread over a larger area, and that heats the surface of the planet less than in the summer.

The reason for the lag in the onset of cold weather after the winter solstice is that the earth soaks up and radiates heat slowly in response to the abundance or the paucity of the sun’s heat radiation. Water, which covers 71% of the earth’s surface, is especially adapted to slow heat transfer. Liquid water is a mitigating influence on rapid changes in weather as the tilted earth rotates around the sun.

We are indebted to the changing seasons for much of life, as we know it. If the earth did not have the 23° tilt, there would be no seasonal changes. This would plunge much of northern Europe, Australia, the southern tips of Africa and South America, and northern Asia into a perpetual ice-crusted no-man’s land. Many crops are dependent upon a changing season system; and that would impair the growth of human populations.