Gas molecules in the atmosphere are constantly moving. The collisions of these molecules with each other and other objects cause air pressure. Barometers measure air pressure in the atmosphere, so air pressure is sometimes called barometric pressure. Air pressure varies with elevation and latitude. Air pressure is almost constantly changing. Changes in air pressure have a direct effect on changes in the weather.
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One type of barometer uses mercury to indicate the level of air pressure. In mercury barometers, mercury in a 30-inch-tall glass tube rises and falls in response to changes in air pressure. Air pressure supports the mercury in the tube. As air pressure rises, the column of mercury also rises. The height of the mercury is typically measured in inches or millibars. Barometric pressure can also be measured in pascals, hectopascals or bars. Meteorologists typically use millibars to measure air pressure.
An aneroid barometer is an airtight box metal box with an indicator needle and a calibrated scale. Most of the air is removed from the box. Changes in air pressure cause the box to expand or contract. The needle moves to show the change in air pressure. The movement of the needle is compared with the calibrated scale. Aneroid barometers do not measure the level of air pressure, only changes in air pressure.
Effects of Elevation
Air pressure is lower at high elevations and higher at low elevations. Because air pressure fluctuates depending on elevation, weather station observers convert air pressure measurements to what they would be at sea level. This conversion makes it possible to compare air pressure at different locations, regardless of elevation. At sea level, standard air pressure is 1013.25 millibars, or 29.92 inches.
Density and Temperature
Air pressure is related to density and temperature. Lower density air contains fewer molecules, which results in low air pressure, such as on a mountain top. The opposite is true for high air pressure. Cool air is denser than warm air because the molecules are closer together. Within the constant fluctuations in air pressure, there is a cyclical trend related to heating from the sun. Air pressure is lowest around 4 a.m. and p.m. and highest around 10 a.m. and p.m.
Changes in air pressure correlate with changes in weather. A drop in air pressure usually results in warm, wet weather. A large, sudden drop may indicate a severe storm, such as a hurricane or tornado, is approaching. A rise in air pressure usually denotes cool, clear weather is on the way. These generalisation are not necessarily true for all locations. Near the equator, day-to-day changes in air pressure are small. In areas affected by air flow from the Gulf Stream, storms can cause dramatic changes in air pressure.
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