Originally published in The Technology Teacher, May/June 2002, by the International Technology Education Association
ity, just like everything else that doesn’t float off into space. For every square centimeter of Earth’s surface, the atmosphere above it, all the way up to space, weighs about 1.03 kilograms, which exerts a normal sea level pressure on you of 14.7 pounds per square inch! Earth’s atmosphere extends up more than 150 kilometers (more than 100 miles), but half of it is in the lowest 5.5 kilometers (3.3 miles).
So, all these interesting statistics aside, the
point is, air exerts pressure because of its weight. But it also exerts pressure because of its tempera- ture. Molecules of air are in constant motion, bumping against each other and bouncing off in all directions. The warmer the air, the more active its molecules and the more pressure it exerts (pro- vided its container doesn’t expand).
Barometric pressure means the same thing as atmospheric pressure, but it is an actual measure- ment taken with an instrument called a barometer. Barometers may measure pressure in atmospheres (atm), inches of Mercury (“Hg), millibars (mb), or other units. Weather forecasters on TV usually use “Hg, while meteorologists usually use mb. Nor- mal atmospheric pressure at sea level is defined as 1 atm, which corresponds to 29.92 “Hg or 1013.25 mb. Barometric pressure readings are taken at many locations by the National Weather Service and combined to produce maps showing high and low pressure areas, thus helping to predict what the weather will do over large regions in the immedi- ate future.
Thus, because of gravity, the atmosphere near the surface is “heaviest,” especially when it is cold, and would seem to exert the most pressure. But because of heat energy, the molecules of warmer air push out in all directions, serving to increase pressure. The important thing is, air does not exert the exact same amount of pressure everywhere. And from this fact, comes a lot of weather!
Remember this: Air tends to move from high pressure areas to low pressure areas.
If many barometric pressure readings are taken and recorded on a map, and then the read- ings that match are connected by a line, you have an isobar. Maps showing isobars are very useful in locating areas of high and low pressure, which, in turn, help predict which way the air masses are and will be moving. This moving air is also what we call wind, and wind drives the major weather surface features, such as highs, lows, and fronts, which, in turn affect weather.
The pressure difference between two points is called a pressure gradient. The force that moves the air from high to low pressure areas is called the pressure gradient force.
It is common for high pressure areas to have fair weather.