Howard O'Neal, age 13, of Rock Island, Ill., for his question:
WHAT EXACTLY IS A TRADE WIND?
A trade wind is a strong wind that blows toward the equator from either the northeast or from the southeast. Sailors depended a great deal on the trade winds in the days of sailing ships.
The paths of the winds are so regular, especially over the oceans of the world, that in the olden days navigators named them trade winds, which, in the language of seafaring men of that day, meant course winds or track winds.
The trade winds are actually a part of a great system of winds that blow over the face of the earth. They blow toward the equator from about the 30th parallels of north and south latitude.
Differences between the temperature in low latitudes and the temperature in the polar regions cause trade winds. The heating of the air in low latitudes makes it expand and become light. Then it rises. This creates an area of low pressure near the earth's surface.
Cooler and heavier air from the polar regions then tends to flow in to fill the area of low pressure. These polar winds do not blow due north or due south, because of the eastward whirling of the earth. Instead, these winds blow from the northeast and from the southeast.
The belt of rising air between the trade winds is a region of mild winds and calms. This region is often called the doldrums because it is so calm. Sailing ships of early days were often stranded for many weeks at a time in the doldrums.
The name "doldrums" means "listlessness." Meteorologists actually call the doldrums the intertropic convergence zone. Early sailors were the first to use the doldrums name because their ships were often unable to sail there.
In the doldrums, the air moves upward, causing sudden thunderstorms and gusty winds. The region is one of the rainiest in the world. It is also dangerous to airplanes because the turbulent clouds build up higher than most aircraft can fly.
Trade winds have a great deal to do with rainfall on land. When trade winds blow against mountain ranges, they are forced upward. As the warm air rises, it cools. Its moisture condenses and falls as rain on the mountain slopes.
The horse latitudes are regions of calms that lie between the oceanic belts of winds called the prevailing westerlies and the trade winds. These areas are calm because the air above them is gently sinking from higher altitudes toward the earth's surface.
The sinking air results in areas of high barometric pressure. The horse latitudes coincide with the subtropical maritime anticyclones, or subtropical highs. These high pressure areas in the atmosphere center over the oceans at about 30 degrees latitudes north and south of the equator.
The high pressure areas move farther north when it is summer in the Northern Hemisphere, and farther south when it is summer in the Southern Hemisphere.