Diane DeVore, age 11, of Des Moines, Iowa, for her question:
WHAT IS BAROMETRIC PRESSURE?
A barometer is an instrument that is used primarily in weather forecasting, although it is also used to measure the heights of mountains. The instrument measures the pressure of the air. When mercury in a barometer falls, a storm is very likely to follow. When the mercury starts to rise, it foretells that clearing weather is on the way.
Barometric pressure is what makes a barometer either fall or rise. Another name for this power is atmospheric pressure the pressure exerted by the earth's atmosphere at any given place. It is also a value of standard or normal atmospheric pressure equivalent to the pressure exerted by a column of mercury 29.92 inches long.
A pupil of Galileo named Evangeligta Torricelli made an experiment in 1643 that established the principles of barometric pressure. He took a glass tube about 30 inches long and filled it with mercury. One end was sealed and the other end was put upside down into a cup full of mercury. The mercury stayed in the tube because of the pressure of the air on the surface of the mercury in the cup.
Torricelli's experiment proved this: 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 high.
In 1939 the National Weather Service, then called the United States Weather Bureau, adopted a new unit of measure called the bar. The bar is taken to be the normal pressure of mercury at sea level. Scientists then recorded the air pressure in millibars which represent one one thousandth of a bar. This exceptionally small unit of measure assured great accuracy.
A barometer can be made without liquid. This type is called aneroid, which means dry. It operates by recording the pressure on an airtight box from which part of the air has been exhausted. The surface of the box moves in or out with changes in air pressure and these changes are then transferred to a pointer which moves across a scale on the surface of a dial. Aneroid baromers, while not as accurate as mercury barometers, are able to show much slighter changes in the atmosphere.
A barograph is a barometer which records atmospheric pressure on a revolving drum. Scientists use this type of equipment for keeping records of changes in atmospheric pressure.
Barometric pressure can allow some people with rheumatism and similar ailments a chance to forecast changes in weather. When a storm is approaching, their aches increase.
Scientists believe that the decreased pressure of the air causes the air in the cells of the human body to exert an increase of outward pressure. This pressure causes pain in the sensitive tissues of rheumatic persons, and like barometers, they can predict the approaching storm.