People who have grown up in the United States know that the wind and
weather tend to come from the west, out over the Pacific, and one often
forgets that it isn't like this everywhere. In temperate latitudes
(between 30 and 60 North) the winds tend to blow from west. In tropical
latitudes (between the equator and 20 north or south) they tend to blow
from the northeast in the northern hemisphere, and from the southeast in
the southern hemisphere. Here's a decent diagram of the world wind
patterns:
The second thing you need to know to understand the Atacama desert is that when moist air rises, it cools, and as it cools, the moisture it carries falls out as rain. Cold air from high in the atmosphere cannot have much water vapor, while warm air from the surface can have lots.
In the United States, moisture is carried from the Pacific eastward over the continent. As it rises over the western mountain ranges, it releases its moisture as rain over California, Oregon, and Washington. The area behind the mountains is in their "rain shadow", and is dry and desert-like.
If the same thing was happening in South America, we would expect Chile and Peru to be wet and rainy, and Brazil and Argentina to be dry. But over most of the northern part of the continent, the trade winds blow from the southeast, bringing moist air from the Atlantic (not the Pacific) over the Amazon Basin, where it falls as rain. The coasts of Peru and northern Chile actually lie in the rain shadow of the Andes mountains. This is familiar to me: I grew up in Hawaii, which is in the northern tradewind zone (see the diagram above). The northeast side of my island was wet and rainy; the southwest side was hot and dry.
The Atacama desert, however, is a special case. It lies at 25 South, almost exactly at the switchover point between the trade winds and the westerlies. Here, the surface winds are rushing away both to the north and south, and so air from higher up must descend to fill the gap. As I said earlier, ascending air causes rain: descending air tends to prevent rain. This phenomenon is not unique to the Atacama: almost all the major deserts of the world lie between 20 and 30 degrees north or south latitude: the Sahara, the American Desert Southwest, the Kalahari desert in southern Africa, and the Arabian desert.
Finally, the Atacama desert is very, very high. Its average elevation is about 4 kilometers, or 13,000 feet. Even when unusual weather brings moist air toward northern Chile from the Pacific, that air must climb 13,000 feet to reach the Atacama. By the time it does so, it has rained out almost all of its water -- there's none left to fall on the Atacama. I have seen a similar thing from the top of Mauna Kea in Hawaii, at 14,000 feet. There, looking upwind you can see an ocean of clouds below you, but the clouds are unable to reach up to the summit, which is almost always clear. You can see a live picture of Mauna Kea (taken from Mauna Loa, a nearby mountain almost as high) at Mauna Loa Observatory's site. (When I loaded it today, it showed the mountain sticking high above the clouds -- it might be different when you try.) The Atacama Plateau is much, much bigger than Mauna Kea, and so it's much, much more effective at keeping rainfall away.
Atmospheric phenomena, especially rainfall, are very difficult to demonstrate with simple classroom experiments. You might ask your class to name the large deserts of the world and find their latitude on a map, to notice the "desert band" which surrounds the Earth. You might use world weather information to plot the average wind direction for cities in South America (but be aware that since weather changes from day to day, you'll have to do this many times around the year.) You might try to estimate the height of clouds where you live (you need trigonometry to do this properly -- you could ask a nearby airport for cloud height information) and compare this to the height of the Atacama Plateau. Or you could go to Intellicast's website and download satellite pictures of clouds over the course of a few days, and watch how the cloud patterns move with respect to the Atacama Desert.
Try the links in the MadSci Library for more information on Earth Sciences.
