###### Atmosphere - Energy and Latitude

This is Antarctica. What season is this? Which is night and which is day?

The sun is always up, even in the middle of the night in Antarctica during the summer. The photo on the left is Antarctica during the night in the summer. The photo on the right is Antarctica during the day in summer. Similarly, in the winter Antarctica is mostly dark all day.

Different parts of Earth’s surface receive different amounts of sunlight.  The sun’s rays strike Earth’s surface most directly at the equator. This focuses the rays on a small area. Because the rays hit more directly, the area is heated more.    You can see this in the figure below. Notice that the Earth is tilted on its axis.   Because of the Earth’s tilt the sun’s rays strike the surface at a slant at the poles; they are less focused.  This spreads the rays over a wide area. Because the sun’s rays hit at a slant, the area is heated less.   The more focused the rays are, the more energy an area receives and the warmer it is; the less focused the rays are, the less energy an area receives and the cooler it is.

The lowest latitudes get the most energy from the sun. The highest latitudes get the least.

The tilt of the Earth causes different areas to receive different amounts of solar energy.  The difference in solar energy received at different latitudes drives atmospheric circulation. Places that get more solar energy have more heat. Places that get less solar energy have less heat. The air above the areas receiving more heat warms up and rises.  The air over the areas receiving less heat is cooler and so it sinks.  The rising warm air and sinking cool air create wind which moves air and heat around the planet.

Source: Open Education Group Textbooks - Earth Science

In order to help you understand this more clearly, refer to the photo. In this photo, a grid of equal dimensions has been placed on a projector screen, superimposed upon a globe. Because of the curvature of the globe which represents Earth, the grids are not uniform. Notice at the equator, the grids are proportionally smaller than they are at the poles. This represents the solar radiation being scattered over a greater distance at the poles. Notice that at the equator, the grids are not distorted, meaning solar radiation is not scattered near as much. Because of this, the air, ocean, and Earth's surface gets heated at the equator more than at the poles.

If we were to compare these two regions, we would find that there is a net loss of solar radiation in the higher latitudes and a net gain in the tropics. This difference in solar radiation creates a movement of air (wind) from the warmer tropical regions toward the cooler northern and southern regions, the poles. See Figure 2.

This moving air creates oceanic surface currents that moves the water in the same direction as the wind, that is, currents from the equator and tropics move warm water northward toward higher latitudes, and currents from the poles move cooler water southward toward lower latitudes. Now think about this, do winds within the area where you live blow from the north toward the south or south toward the north? If you analyze wind patterns where you live, you probably will discern that the winds generally have a westerly or an easterly component to the north-south directions, i.e. northwest, southeast, southwest, west southwest, etc. What is the reason for this? Why don't winds blow just north and south, moving the ocean currents north and south according to Figure 2? Great question. Research will help provide the answer.

Summary

• A lot of the solar energy that reaches Earth hits the equator.
• Much less solar energy gets to the poles.
• The difference in the amount of solar energy drives atmospheric circulation.

Practice
Use this resource to answer the questions that follow.

1. What is latitude?
2. What does latitude means to the heating of the Earth?
3. Why do high latitudes receive less sunlight?
4. What is the angle of incidence?

 Latitude is the measurement of the distance of a location on the Earth from the equator. The further away from the equator that a location resides, the less sunlight that this location receives. It all has to do with the shape of the Earth. Because the Earth is round, only the front most portion receives direct sunlight.

Review

1. The North Pole receives sunlight 24 hours a day in the summer. Why does it receive less solar radiation than the equator?
2. What part of Earth receives the most solar radiation in a year?
3. What makes the atmosphere move the way it does?