The Sun

Because of the relative sizes of the earth and its sun (the sun being much bigger), the rays of the sun travel in parallel lines to the earth and through the earth's atmosphere. Only about half of the incoming rays get to the earth's surface because air molecules, clouds, and dust either absorb some of the rays or reflect them back into space. The sun's rays that do penetrate the earth's atmosphere warm the earth's surface but despite the fact that they all begin with the same potential energy, the incoming rays have different heating effects at different places on the earth.

The Rays of the Sun as They Travel Through the Earth’s Atmosphere at the Equator and at the North Pole (Winter in the Northern Hemisphere – Summer in the Southern Hemisphere)


If you look at figure above, you will see one of the reasons why. Because of the curvature of the earth, the sun's rays travel through more atmosphere and therefore through more air molecules and dust to reach the poles than they do at the equator. (And it hits a bigger area at the poles.) So although the energy coming from the sun is the same, less of the sun's energy reaches the poles than the Equator. In addition, the snow and ice at the poles reflects much of the sunlight back into the atmosphere.

Consequently, the Earth is most intensely heated by the sun at the equator and least at the poles. The exact same phenomenon of uneven heating occurs in the oceans as well. The oceans at the equator are warmer than that closer to the poles.

Once the land and oceans of the earth absorb the sun’s rays, the energy is transferred into heat and this heat is then reradiated back out into the atmosphere. Much of that heat is absorbed by the molecules in the air (such as carbon dioxide). Since there is less energy reaching the poles there is less heat radiating from at the poles and the air is cold. Conversely, the intense energy reaching the equator means that the air at the equator is hotter.

In addition, the energy from the sun is also used in the process of evaporation (water changing from a liquid to a gas). The hotter the air, the more evaporation can occur. Warm air evaporates more water than cold air and it can hold more water molecules in the form of vapor than cool air. Consequently not only is the air at the equator warmer it is also “wetter”.

These differences in the heat and water vapor in the air prime the global weather machine because weather is really nothing more than an attempt of air molecules to homogenize these differences.

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How Weather Works