 A low pressure system represents an area of lower pressure at the surface where winds blow counterclockwise around its center. Unsettled weather is often associated with low pressure because air rises, cools, and eventually becomes saturated. At sea level air pressure is normally around 1013.25mb (29.92 inches of mercury).
A rapidly falling barometer usually indicates strong winds and storms. Wind is the natural result of having a low pressure area next to a higher pressure area since the air molecules will move from the higher pressure zone to the low pressure area, and the greater the pressure difference between a low pressure area and a high pressure area, the stronger the winds will be. For example, extremely strong hurricanes are presaged by air pressure drops of between 30 and 70mb.
A common point of confusion is why air laden with water vapor should result in lower pressure than dry air. It is counter-intuitive for many people that adding water vapor to air actually makes air less dense; however, this is in fact the case. The reason for this is based in the relative weights of the molecules involved. Since air is approximately 80% nitrogen and 20% oxygen, a liter of air weighs about 1.28 grams. However, water vapor weighs 0.8 grams, significantly less than air by itself. Because of this, when air becomes humid, with water vapor replacing some of the air, the total weight becomes less, resulting in lower air pressure. |
