Seasonal Temperatures

Now, you may have noticed that the winter temperatures are colder than the summer temperatures, and perhaps you have even wondered why this is. Well, there are a few reasons, and this last lesson will briefly explain what those reasons are, and why they interact with our temperatures here on the surface.

First, you must consider that the sunlight in the winter is spread out over a larger area, and second, the sunlight also passes through the atmosphere at a lower angle which makes it pass through a thicker amount of atmosphere. Remember: Sunlight is energy, and energy is heat, so the heat that a location receives is directly related to the amount of energy that reaches the Earth's surface. Here, you will see these concepts illustrated in depth.

Light Spread (NOT butter...!)

The analogy often used to describe this phenomena has to do with a flashlight being shined at a wall. If you shine it straight at the wall, you get a shape like Figure 1, below. If you shine it at an angle, though, say with the flashlight pointed partially toward the ceiling, you would get a spread-out shape like Figure 2 shows.

Now, consider this:

Still don't get it? That's all right. Think of it this way. Energy per square inch is, if E is energy and A is area,

E / A

If the area of the circle in Figure 1 is 4 square inches, and the area of Figure 2 is 7 square inches, and if the flashlight puts out 28 Watts, it becomes

(28 Watts) / (4 Square Inches)

for Figure 1, and

(28 Watts) / (7 Square Inches)

for Figure 2. You can see that the answer for Figure 1 is 7 Watts per Square Inch, and only 4 Watts per Square Inch for figure 2, proving that Figure 1 gets more energy per square inch than Figure 2.

This same thing happens on Earth, to a far bigger extent. Think of the Sun as a giant flashlight shooting out single beams toward Earth. I know it seems a little silly, but the concept is still the same. See?

Atmospheric Scattering

As the sun passes through the atmosphere, there are little particles of gas and even dust that will scatter light, much like the sunbeams that can be seen in a room that has just been dusted. At the equator, the sun, mainly, passes through little atmosphere, and is therefore subjected to less scattering. On the other hand, however, the light reaching the poles passes through up to 45 atmospheres' worth of air!

If we look again at single beams hitting the earth at the equator and poles, you can see that x, the depth of atmosphere through which the light must travel near the pole, is much greater than y, the depth of atmosphere through which sunlight passes near the equator.

Add them together...


And there... you have it!