Reason for the Seasons
Why is the summer warmer than the winter?
There are three reasons why summer is warmer than winter, and all three have to do with the greater altitude of the sun in the sky during the day in the summer.
The fourth effect many people expect will be important is the distance from the earth to the sun. But that effect turns out not to matter much at all. Click here for details.
Reason 1: Area Affect
The altitude of the sun in the Ames, Iowa sky throughout the day at the solstices and the equinoxes are displayed in a chart at the end of this section for reference. From that graph you can verify that in the summer, not only is the sun up longer than 12 hours, but it is also higher in the sky at every time of the day.
Longer days mean the earth is soaking up the suns energy for more hours. When the sun is higher in the sky, the energy is concentrated on a smaller area. This is called the "Area Affect." In other words, when the sun is lower in the sky, the energy is spread out over a larger area as demonstrated in the illustration below.
"Longer days allow more hours to soak up energy from the sunshine. When the sun is higher in the sky, the amount of its light energy gets spread out over a smaller area than when the sun is low in the sky. This is the area effect."
Reason 2: Air Mass Effect
In addition to being spread out over a larger area when the sun is lower in the sky, the sunlight must also pass through a greater amount of the earth's atmosphere. The atmosphere appears to the sunlight to be "thicker." If we approximate a local area as a flat plane (ignoring the curvature of the earth), we can use the following diagram to illustrate the point. This is called the Plane Parallel approximation.
Click "Continue" at the bottom of the graphic when you are ready to go to the next step.
When the sun is lower in the sky, its light must pass through more of the atmosphere before reaching us and therefore the light is more likely to bounce into space or be scattered to the ground somewhere else. This is the "air mass effect."
Using facts that we already know, we can tell which of these effects - length of day, area effect, or air mass effect - is most important to us. This evaluation can be done by calculations of how much heat reaches the ground depending on the altitude of the sun, but it gets rather complicated.
The Extra to the right is an optional exercise that we encourage you to think about.
Reason 3: Length of Day (Based on Latitude)
At the equator in the summer, the sun is up 12 hours every day. At the latitude of Minneapolis, 45N, the longest day is more than 12 hours. Further north, at 66.5N, the longest day has 24 hours with the sun above the horizon. This is also a big effect, and in fact calculations show it is slightly more important than the area effect. Why, then, is Winnipeg cooler than Des Moines in July? Go to the next sub-unit and find out!