Deborah Paull, age 10, of Bossier City, Louisiana, for her question:
What exactly does barometric pressure measure?
When the weatherman mentions barometric pressure, he is talking about the weight of the air. Perhaps you thought of the air as a filmy nothing with no weight at all. But air does have weight. And the shifting weight of the airy atmosphere helps the weatherman to make his forecasts.
The Greek word "boros" means "weight" and the old word was borrowed to name the barom eter. This gadget is really a special weight meter, or weight measurer. Its job is to weigh a small section of the airy atmosphere that begins at ground level and reaches to hundreds of miles above our heads. Anything with weight presses down upon the things below it. The weight of the atmosphere presses down with millions of tons upon the surface of the globe. The barometer measures the barometric pressure or weight of a small, tall sample of the atmosphere above it.
If a solid brick weighs one pound today, we expect the same brick to weigh one pound tomorrow. Most substances do not get heavier or lighter from day to day. The airy atmosphere over any given spot, however, may change its weight from hour to hour and sometimes from one minute to the next. This is because air is a mixture of gases made of separate, free floating molecules. Heat makes its frisky molecules spread out and take more room for themselves.
Warm air tends to become thin and lose some of its weight. The molecules of cool air tend to close ranks and crowd into less space. Cool air tends to become heavier and gain weight. These changes in the weight of the atmosphere are linked to changing weather events, The barometric pressure of an air sample shows whether the atmosphere is getting lighter or heavier.
The global atmosphere struggles to keep its weight even around the globe. Masses of light and heavy air mix and merge with flowing and blowing winds. Amass of cool, heavy air tends to flow outward and mingle with the lighter air around it. Its spirally breezes often bring a warm spell. Heavy barometric pressure forecasts fair weather. Amass of light air tends to drag in blustery winds from the heavier air around it. When the barometric pressure shows the air, is losing weight quickly, we can expect a storm.
The most common barometer measures the changing air pressure with a glass tube of mercury standing in a pool of mercury inside a glass bulb. Air presses down on the surface of the pool and its weight pushes the mercury up in the tube. If the sample of air gains weight, the extra pressure on the pool pushes the mercury higher up the tube. This rising barometric pressure forecasts fair weather. If the air loses weight, the surface of the pool is free to rise and this makes the mercury in the tube fall lower. This falling barometric pressure is a stormy warning.
Our barometer measures a sample of the atmosphere above one square inch. At sea level, the average weight of this tall, airy column is about 14 1/2 pounds. A scale of degrees beside the column of mercury measures this pressure as 30 inches tall. Light air pressure makes the mercury fall below this point and heavier pressure pushes it higher.
The sample of air is about 1,000 miles high and its heaviest layers are at the bottom. As we go higher, the sample column gets shorter and the pressure gets lighter. Air pressure can show how high we are above sea level. We can use barometric pressure to measure the height of a mountain and pilots use a special barometer to show the altitudes of their high flying planes.