Sally Novotny, age 14, of Sioux City, Iowa, for her question:
What is a tropical depression?
This expression may be new to you. If it is, your mind may conjure up a fanciful picture of what a tropical depression is. You might think of an economic collapse in tropi¬cal America or perhaps a travel weary tourist suffering through a blue mood while sit¬ting a straddle the equator.
Tropical depression is a meteorological term, though nowadays this weather event more often is called a low. And, of course, a low is a weather pocket of low barometric pressure. The typical picture of a low on a weather map is a series of oval or circular lines, one inside another. The lines are isobars, pieced together from data gathered from thousands of farflung weather stations. One item on each station report is an exact reading of the local atmospheric pressure registered by a barometer. Stations reporting the same barometric pressure are linked together with isobar lines.
Suppose an isobar strings together a line of points all registering a barometric pressure of 1020 millibars. By itself, this isobar has little to tell about the weather system of which it is a part. However, if the line is a smallish rang, it may be near the center of a high pressure weather cell. But, if the isobar ring is 100 to 500 miles wide, it may mark the rim of a low. Other isobars are needed to complete the picture. In a low cell, the pressure decreases toward the center. The bull's eye isobars inside the outer rim may read 1018, 1016, 1014 and so on.
This progression of readings is a pressure gradient. In a low or depression, the att¬mospheric pressure drops toward the center forming a trough of low pressure. The heavier higher pressure air around the rim tends to blow into the lighter trough of low pressure in the middle. Lows of this type are carried by the prevailing westerlies, bringing spells of stormy weather across most of North America. The rotating earth twists their winds to blow counterclockwise.
Similar low pressure troughs sometimes form in the tropics. These tropical depressions breed at certain seasons above the mid Pacific and Atlantic. A tropical depression may hover for days, absorbing immense energy from the heat and moisture of the ocean. North of the equator the spinning earth twists its winds counterclockwise, south of the equator they swirl clockwise. This tropical depression is, of course, a hurricane. It may be up to several hundred miles wide. Its winds, spiraling faster toward the center, may reach more than 200 miles per hour.
The stormy depression forms north or south, but not exactly on the equator. Soon it begins to move westward with the prevailing trade winds. It may be lost at sea. Or the howling hurricane may be swerved by a coastline and plow a path of destruction over the land.
The millibars on an isobar are refined units of marometric pressure. The home type barometer usually measures the weight of the atmosphere in inches of mercury. The standard pressure of one atmosphere is taken to be 29.91 inches, which equals 760 millimeters. The more refined bar unit is commonly used by weathermen. One bar equals 1,000 millibars, or 29.53 inches, and standard air pressure is taken to be 1013.2 millibars. Heavy high pressure air registers more and lighter low pressure air registers fewer millibars.