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Why is the sun cold in the winter?
Category: Earth Science
Published: July 26, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
The sun does not get cold in the winter. The sun is a giant ball of fire driven by nuclear reactions. The surface of the sun stays at an incredibly hot temperature of about 5800 Kelvin all year long. The high temperature of the sun causes it to constantly emit prodigious amounts of thermal radiation in all directions, mostly in the form of infrared waves, visible light, and ultraviolet waves. This radiation spreads throughout our solar system and warms everything it hits.
Regions of earth are colder in the winter because the tilt of the earth causes the sunlight to be spread over a larger area and therefore be weaker per unit area in these regions. Some people think the earth gets colder in the winter because the earth is farther away from the sun, but this idea is wrong. In fact, the earth is farthest from the sun in the northern hemisphere's summer, and not winter. But distance to the sun does not have much effect on the amount of sunlight the earth receives because the distance to the sun does not really change that much. The earth is about 150,000,000 kilometers away from the sun. Over the course of a year, the distance from the earth to the sun changes by at most 5 million kilometers. To put this in perspective, the orbit of the earth around the sun throughout the year is plotted below to scale. You can't tell, but at this scale the earth is 302 pixels from the sun when at the leftmost point in its orbit and 298 pixels from the sun when at the rightmost point.
The axis of earth's rotation is tilted about 23 degrees with respect to its orbit around the sun. This mean's that in the north's summer time, the northern hemisphere is tilted towards the sun and in the winter time, the northern hemisphere is tilted away from the sun. When a region of land is tilted towards the sun in the summer, that means that the sun spends more time higher in the sky and the sunlight received by that spot of land is more direct. In contrast, when that spot on the earth is tilted away from the sun in the winter, the sun is in general lower in the sky, and it receives less direct sunlight. Less direct sunlight means that the sunlight is coming in at a low angle, so that it is smeared across a greater area when it hits the surface. Because it is smeared over a greater area, winter sunlight has less power per unit area, and therefore heats the earth less, leading to cold winters.
Interestingly, when the northern hemisphere is tilted towards the sun, the southern hemisphere is tilted away. This means that when the north is having summer, the south is having winter and when the north is having winter, the south is having summer. The effect of sunlight angle on temperature can be most easily seen at the extremes. Near the south pole and the north pole, the sun is always very low in the sky, and the sunlight always comes in at a very low angle. As a result, the poles are generally very cold. In contrast, near the equator, the sun is always high in the sky around noon and the sunlight is very direct. As a result, the regions near the equator are generally very hot.