Looking
at weather maps, it seems as if our weather mostly comes from the west.
Why?
In the 48 mainland states of the US, weather indeed tends to move from
west to east. California gets rain from the Pacific Ocean, and rainstorms
over Ohio often migrate to the east coast states the next day.
But it is not always so in the rest of the world. In particular, closer to the equator weather moves east to west. If you watch weather forecasts during the hurricane season (peaking July-September), with satellite pictures of developing tropical storms and hurricanes, you will see them migrate westwards, from the shores of Africa to Caribbean islands such as Puerto Rico, south of the US. After that the typical track of a hurricane curves north and then slants eastward again, as it enters the region of westward-moving weather. It may hit the US and then blow out again to the Atlantic--or sometimes, just move off-shore in the Atlantic on an eastward slanting track. The coastline of the US also slants to the northeast, so many hurricanes follow that direction and drench one coastal state after the other.
In the era of sailing ships, it was well known that to sail from Europe to America (as Columbus did), it paid to stay well south, where the wind blew in your direction, but on the return trip it was better to sail further north, taking advantage of westward winds. Because that choice was important for merchant ships, the eastward winds became known as "trade winds." All sailors knew to avoid the region in-between the two air streams, "the doldrums" where sailing ships risked getting stuck with no wind at all.
When the Spanish ruled Mexico and Peru, they mined there (using native slaves) great amounts of silver and gold, sending them to Spain every year by a "treasure fleet." That fleet would sail north along the shore of Florida (which also belonged to Spain), then enter the region of favorable westward winds. Sometimes though the fleet encountered a hurricane and ships were sunk, which is why nowadays sunken treasure is still occasionally found by divers off Florida.
But what causes all those winds? What causes the weather, you may well ask. The Sun, of course! Sunlight heats the ground, and if heat were not removed at the same rate, the ground would get hotter and hotter. The atmosphere responds with air motions, which help remove that heat.
It is a complicated process, this removal business. Anything hot shines--a very hot object like the filament in a lightbulb shines strongly in visible light, while one that is just warm like the sun-heated ground shines (not nearly as strongly) in invisible infra-red light. (Infra-red light is also the radiation which warms your hand when you hold it above a hot stove.) However, air absorbs infra-red--that is the famous "greenhouse effect," greatly strengthened by small amounts of gases like carbon dioxide, methane, even water vapor. That heat is re-emitted, then re-absorbed, often several times, until it reaches the high atmosphere (8-10 miles--12-16 kilometers--up). Above that, not enough air (and water vapor) remains to absorb it again, and it may escape to space.
The
Sun's heating is most concentrated near the equator, where sunlight is
nearly vertical. This concentration gives rise of world-wide wind patterns,
which help spread the warmed air and allows heat to be returned from a
larger area. You would expect warm air to flow away from the equator and
return after it has cooled down, but the rotation of the earth deflects
such flows, instabilities complicate them, and the uneven distribution
of land and ocean (land heats up more) make the pattern even more complex.
All those factors lead to easterly winds near the equator and westerly
ones halfway between equator and poles, as well as other complications
and features of our weather.
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Dr. Stern has a large educational web collection on astronomy, physics
and space entitled "From Stargazers to Starships," at this web
address : http://www.phy6.org/stargaze/Sintro.htm
The
effects of sunlight on Earth, including the wind patterns described here,
are covered there in section S-1 "Sunlight and the Earth"
http://www.phy6.org/stargaze/Sun1lite.htm
and in the two sections following.
This week's question is provided by David P. Stern, a GSFC physicist, retired as an emeritus, who has created several large web courses on space, astronomy, physics, astronomy and more (even math). See http://www.phy6.org/prospect.htm.