Alex Paulson, age 13, of Monroe, La., for his question:
HOW DOES A BAROMETER WORK ON WEATHER PREDICTIONS?
A barometer is an instrument that is used to measure the pressure of air. With this information, it is possible to make predictions about the weather. A barometer works by measuring the weight of the atmosphere.
A mercury barometer works on a simple principle. Fill a glass tube a bit over 30 inches long with mercury and close one end. Put your thumb over the open end and turn the tube upside down, inserting the open end into a cup of mercury. The liquid in the tube will fall until its top reaches about 30 inches above the surface of the liquid in the cup.
The mercury stays in the tube because of the pressure of the air on the surface of the mercury in the cup.
We know from the 1643 experiments of Evangelists Torricelli, a pupil of Galileo, that the weight of all the air above any point on the Earth's surface is equal to the weight of a column of mercury about 30 inches long.
The mercury barometer has made it possible for experts to predict changes in the weather days before they happen. The experts know that when the mercury falls rapidly, a storm is almost sure to follow. What is called a rising barometer tells them that better weather is on the way.
A weather expert can draw lines on a map connecting places of equal atmospheric pressure. In thin way, he is able to tell the probable course of winds and storms. The lines are called isobars, from the Greek words "iso" and "barns," which mean "equal" and "weight."
The modern mercury barometer works on the same principle as the tubs and cup, but it has extra devices that help give accurate information. By adjusting a thumbscrew, the surface of the fluid in the. reservoir can be adjusted. Under average conditions, sea level air pressure is 30 inches.
The National Weather Service since 1939 has used a new unit of measurement called the bar. The bar is taken to be a pressure of 29.53 inches of mercury at sea level., A millibar represents one one thousandth of a bar.
Air pressure goes up or drops because the blanket of air that surrounds the Earth isn't always the same in density or temperature. The sun may heat some areas of the Earth very quickly, such as a desert. Other areas, such as the surface of an ocean, may be heated very slowly.
The unequal heating of the atmosphere by the sun causes inequalities that lead to balancing out the inflow of dense, heavier cool air from one area to another where the air 1s less dense. This type of air movement causes wind and also causes the barometer to "fall." A barometer shows this change.
Although the mercury barometer is the most accurate of all available, it isn't always easy to carry from place to place. Sometimes the aneroid barometer is used. "Aneroid" means "without liquid."
The aneroid barometer uses a round brass box called a bellows fitted inside a clamp in the form of a "C." The clamp is called a C spring. The metal box is sealed after all air is removed. Changes in air pressure are then indicated on the barometer's pointer.