Seasons, short and simple

I love this question:

Why is it warmer in the summer than in the winter (for the Northern hemisphere)?

Go ahead and ask your friends. I suppose they will give one of the following likely answers:

• The tilt of the Earth
• The tilt of the Earth makes us closer to the Sun
• We are closer to the Sun

Really, that should be at least 85% of the answers. To really answer the question, think of the following key points:

• When it is warm in the Northern Hemisphere, it is cold in the Southern Hemisphere
• The Earth's orbit around the Sun is very close to being circular
• The seasons depend on the time of daylight
• The seasons depend on the path of the Sun in the sky

I am sure you have notice that the Sun is in the sky longer in the summer than in the winter. You may also notice that the path of the Sun is different in different seasons. How do these two things influence the temperature? First, the path of the Sun. In the winter, the Sun does not go as high in the sky. This means that less solar energy is striking a given area of land.

Think of the following analogy. Suppose I have a flat sheet of paper in the rain. If I hold it in the two orientations shown (and the rain is coming straight down), which would get wetter?

You can even count the number of 'rain lines' crossing each area. The one on the left has more rain hitting it even though they are the same area. The same thing happens with the Sun. In the winter, the Sun is lower in the sky making an image similar to the diagram above on the right. This means that less solar energy per area is hitting the Northern Hemisphere in the winter. Why does this happen? It is because the axis of the Earth's rotation is tilted with respect to its motion around the Sun. Maybe this picture will help.

This orientation also makes the Sun to be visible for a longer time during the summer. Here is a sketch that shows the solar energy hitting the Earth as a function of time of day.

In the red line (during the summer) the Sun is up longer AND at a greater angle. This means that more total solar energy (the area under the curve) gets to the surface of the Earth. The blue line is in the winter. Notice in the morning and in the evening, the Sun is at a low angle making less solar energy hitting the Earth. Also, these are just approximations to get my point across.

If you would rather listen to me, here is a video presentation of this topic.

Seasons from Rhett Allain on Vimeo.