Influences on Barometric Pressure

Measuring Atmospheric Pressure

• When an air molecule strikes a surface, it exerts a small amount of pressure. The molecules in the atmosphere are in constant motion; atmospheric pressure comprises the total combined pressure of these molecules. When recorded at ground level, air pressure is usually measured in inches of mercury, based on its effect on the column of mercury used in barometers. When recorded from the air, atmospheric pressure is typically measured in millibars.

Elevation

• Elevation is one of the main factors that influence barometric pressure readings. At higher altitudes, air density decreases. Because fewer molecules are exerting pressure, the atmospheric pressure is lower. The decrease in atmospheric pressure with elevation is independent of pressure changes due to weather systems. In order to compare barometric readings from locations at different elevations, observation stations convert the direct pressure readings to the reading the barometer would have given if it were at sea level.

Temperature

• The molecules in cold air are slower and denser than the fast, widely spaced molecules in warm air. Because temperature affects air density, it also affects barometric pressure readings. In fact, air pressure rises and falls every day in a cycle driven by the sun's heat. Air pressure is lowest around 4 a.m. and 4 p.m., and highest near 10 a.m. and 10 p.m. The differences between the highest and lowest measurements in this daily cycle are greatest near the equator, and lowest near the North and South poles.

Weather Systems

• The movement of weather systems through a region can also change barometric pressure. These pressure changes are more dramatic than daily temperature-related fluctuations, and may indicate large storms. In England, for example, large pressure changes are associated with storms. In the tropics, weather-related pressure changes are minimal, except in the case of cyclones. The northernmost U.S. states see the greatest storm-related pressure changes, according to the National Weather Service.