A barometer is a meteorological instrument that measures atmospheric pressure. Meteorologists (scientists who study the weather) use barometers to predict changes in the weather using the fact that a change in air pressure is a change in weather. A rising barometer is usually an indication of improving weather and a falling barometer usually means a storm is on the way. If it is rising, it means that the air pressure is increasing. If it is falling, it means the air pressure is decreasing. If the barometer is used to measure heights, a decreasing barometer can be lined up with increasing heights measured in feet above sea level.
One type of barometer is the aneroid barometer. It is portable and can measure small changes in air pressure. It is the type of barometer that is used in the home. A diaphragm at the top moves with every change in air pressure which is connected to levers which move a dial that is lined up with a scale that gives the air pressure readings.
Another type of barometer that is more accurate than the aneroid barometer is the mercury barometer. It is filled with mercury. It was invented in 1643 by the Italian physicist Evangelista Torricelli. A glass tube about three feet in height filled with mercury with one open end is used. The weight of the mercury in the glass tube balances with the atmospheric pressure outside the barometer.
Meteorologists today use weather stations to forecast the weather. They use digital barometers to determine if the air pressure is rising or falling. Digital barometers use electrical charges to read whether it is rising or falling.
When more air is leaving the area than can be replaced by surrounding areas, it is low air pressure. This causes the weight of the mercury to fall. If more air is coming into the area than is leaving from surrounding areas, then it is high pressure. The weight of the mercury rises when it is high pressure.
Since changes in air pressure are a good tool to use to forecast the weather, barometers are very useful. Isobars show where the locations of areas of equal pressure are. They are placed on weather maps. Local weather can be forecast using several guidelines: 1. A falling barometer means the air pressure is decreasing and is an indication that there will be wind, storms, and rain. 2. A rising barometer means good weather is likely. 3. If the mercury in the barometer is falling slowly, change in weather is not likely. 4. If there is a rapid decrease in the height of the mercury, then a storm is likely to happen. 5. If there is a slow, sustained falling of the mercury in the barometer, then there will probably be a long period of poor weather. 6. If the mercury in the barometer is rising quickly, then an improvement of the weather is likely. 7. A slow and sustained rise in the mercury in the barometer indicates long lasting good weather is ahead.