Barometers come in several different shapes. One of the most usual 'types has a straight glass tube, partly filled with silvery mercury. Naturally a falling barometer is not supposed to fall off the wall. But the mercury in the glass tube does rise higher and fall lower with chang¬ing weather conditions. To the weatherman, a falling barometer means a change in the weather usually a change for the worse.
The business of a barometer is to weigh the air. Maybe you thought that the filmy air has no weight at all. But it does. The air in your class room weighs several pounds, and there is enough air in an empty drinking glass to weigh as much as an aspirin. But these are small samples and the barometer is interested in a sample of the total atmos¬phere. The great shell of filmy air that surrounds the whole earth weighs about five quadrillion tons. This figure is five plus 15 zeros.
It's enormous weight presses down upon the surface of the planet Earth. However, the global atmosphere is in a constant state of weathery turmoil. And its filmy air is a mixture of flowing and blowing gases. Masses of cool air tend to be thicker and heavier and masses of warm air tend to be thinner and lighter.
The barometer measures the changes in the weight of the air, the differences between masses of light air and masses of heavy air. This is called air pressure, or atmospheric pressure. The barometer measures a slim column of air that reaches from the ground way up to the top of the atmosphere.
The usual barometer has a glass tube fixed on top of a small glass bulb. The bulb and part of the tube are filled with silvery, liquid mercury. The weight or pressure of the atmosphere pushes the mercury higher up the tube or lets it fall lower. The glass is fixed firmly onto a wooden or maybe a plastic frame. The frame is marked with a ladder of lines to show just how much the barometer rises and falls.
When the changing weather brings a mass of extra heavy air, it pushes down on the barometer with extra weight. This sends the mercury higher up the glass tube. The barometer is rising. Usually this sort of air mass brings a spell of calm, steady weather. So usually a rising barometer is a sign that a few pleasant, sunny days are on the way.
But this nice mass of air is sure to be followed by one that is not so nice. It may be a cold front of light weight, blustery air. In this case, the air pressing down on the barometer is much less. With less pressure, more of the mercury falls down the glass tube. The barometer is falling.
We can tell when the weather is damp or dry, warm or cool. But we need a barometer to show us the changes in air pressure. As a spell of calm weather approaches, the mercury usually climbs higher and we say that the barometer is rising. But as a howling hurricane approaches, the mercury in the glass tube falls lower and still lower.