The earth?s atmosphere is put into motion by differential heating. As we all know, the sun has different strength in different latitudes. Alaska can be pretty chili even in the summer whereas Hawaii or Florida and regions around the equator are almost always hot! Now, when the sun is very hot on the equator it heats the air on the ground which makes it less dense and causes it to rise because it is much lighter than the air above. Looking at the air temperatures in Alaska at the same time, we notice that it is very cold, which results in a high pressure region. A lot of cold and sinking air results in high pressure whereas warm rising air gives a low pressure region. The general movement of air masses is from high to low pressure, similar to a ball that rolls down the hill by itself and not uphill. For a short summary, we have rising air above the equator which moves northward in great heights and sinks back to the ground in higher latitudes, where it returns to the equator as a near-ground wind. This pattern can be looked at as a circulation cell due to the different strength of the sun at different latitudes. Unfortunately, we do not only have one of these circulation cells on each hemisphere, that would be too easy to understand. We have to account for the rotation of the earth, which disturbs our simple picture. Moving air masses on a rotating earth leads to a deflection of the air so that instead of one significant wind pattern per hemisphere we have three, with changing conditions every 30 degrees of latitude. Around the equator are the predominant Northeast and Southeast Trade Winds, winds coming from the northeast (northern hemisphere) and the southeast (southern hemisphere) respectively. The air is carried back towards the east with the ?Westerlies? between 30 and 60 degrees north. The regions on the equator (Doldrums) and on 30 degrees north (Horse Latitudes) are notorious for weak and sluggish winds and caused many problems for sailing ships before the invention of the engine. Many ships who got stranded in the Doldrums or Horse Latitudes suffered hunger and water shortage because there was no wind to push them forward on their travels and the tale says that ships had to throw animals (like horses) they carried along overboard, because they couldn?t afford to feed them. Hence the name ?Horse Latitudes?.
The general idea is that if you want to cross the Atlantic Ocean in your little sailboat, you leave North America at New York and take advantage of the ?Westerlies? that take you straight to the northern European coast. On the way back you want to go to the Canary Islands off the coast of northern Africa so you can sail with the Northeast Trade Winds to arrive in the Caribbean from where you can sail up along the coast back to New York. And remember not to get caught in the Doldrums or the Horse Latitudes.
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