What is air pressure and how is it used to predict the weather?
• Air Pressure – Air pressure is also called barometric or atmospheric pressure. By definition, it is the weight of the atmosphere pushing down on the earth. • Barometer – A barometer is the instrument that measures air pressure and it uses units of millibars (mb) or inches of mercury (inches). Factors that affect air pressure: • Temperature – As the temperature of the air increases, the air pressure decreases, which is an indirect relationship. • Humidity – As the humidity of the air increases, the air pressure decreases, which is an indirect relationship. You might ask “WHY?”: Humid air is lighter than dry air. When water vapor enters dry air, it does not squeeze in between the molecules of air. Instead, the water vapor pushes out many of the air’s heaviest molecules like Nitrogen and Oxygen. In other words, lighter molecules of water vapor replace heavier molecules and the result is the air weighs less and therefore has less air pressure. • High air pressure – means that the atmosph
