How does the wind work, and why does it blow from certain directions at certain times of the year?
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tl;dr-
The sun is hot, and the earth keeps spinning
You know how when you open a door in winter, you let the hot air out, and the cold air comes rushing in? On a planetary scale, the sun heats our planet, but it can only heat 50% of the planet at a time, with the rest of the planet being night. As the earth rotates, different parts of the earth get heated by the sun. Additionally, water and land retain heat differently, and the seas get currents of their own, which work more or less like wind, but in the water.
As the earth rotates, cold air takes the place of hot air. If the surface of the earth was uniform, and the rotation of the earth was uniform, then wind would also be uniform. However, as previously mentioned, water and land retain heat differently. Urban areas in particular retain heat from the sun due to the amount of pavement and other similar activity, and that’s recognized as the “urban heat island effect.” Land retains heat more easily than water. Land also retains cold more easily than water, in part because it does not have currents like water does.
While the land doesn’t move fast enough to affect its own heat, and the seas have currents of their own, one thing remains the same- the direction of the rotation of the earth. The sun rises in the east and sets in the west. In other words, our cosmic heat sources moves from east to west. The equator is the midpoint between the poles, but because we have seasons, the sun heats the northern and southern hemispheres back and forth. The areas of earth that face the sun most directly are called the Tropic of Cancer and the Tropic of Capricorn, and they are lines of latitude offset from the equator.
If you look at a map of earth’s wind currents, you’ll see two major areas called “Hadley cells”. These represent the intense amount of pressure caused by heating the tropics, with the equator between them. Hadley cells move air from the tropics to the equator, because the equator is actually not as hot as the tropics, and they meet at an intertropical convergence zone. However, you should also note that while the Hadley cells converge between the tropics around the equator, they *also* move from east to west.
Hot rises above cold, and cold rushes in to replace it. This results in east to west “trade winds” between the tropics because they follow the sun, becoming increasingly west to east in the mid-latitude “westerlies,” there are more “polar easterlies,” and so on.
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