We hear a lot about high and low pressure areas on the daily weather forecast. And while we might not totally understand what this means, or the significance, most of us recognize that low pressure usually means some sort of weather event in the near future. And we know that when air masses come together there will be a change.
Low pressure areas are produced when the weight of the atmosphere above the area is reduced by actually removing part of the atmosphere. This is common where air masses converge. Cold and warm air masses that meet, cause a condition where the colder air flows beneath the warmer air, and the warmer air flows over the colder air in an attempt to reduce the difference in temperature. These air masses, with their extremely different temperature variations, naturally mix, and in the process, some of the atmosphere is displaced, causing an area of low pressure.
Low pressure areas can be formed when cooler air flows over warmer land masses, such as islands, or over large bodies of warmer water such as the Great Lakes. Lake effect storms are examples of this development of low pressure. They can also be formed by the addition of water vapor produced by snow or rain storms and taken up into the atmosphere. The eye of the hurricane or a typhoon is one of the most obvious examples of this. In this low pressure center of the storm, over 10% of the atmosphere may be removed.
Tropical cyclones hold the record for producing the lowest air pressure in the world. Tropical heated water vapor and precipitation releases the heat and warms the eye in the center of the storm. In fact, the lowest pressure ever recorded occurred during Typhoon Tip in 1979, when, in the column of air that made up the eye of the storm, 14% of the atmospheric mass was removed.
On a weather map, high and low pressure areas are marked with isobars, which indicate the amount of pressure. Lines around the area, also indicate the area which each individual pressure extends. These low pressure systems are characterized by a counterclockwise rotation. By determining the atmospheric pressure in these converging masses, more accurate forecasting of potential bad weather can be accomplished, and storm warnings issued.
Over all, when the weather forecaster announces that the barometer is dropping, this lower pressure is a fairly good indicator that there will be a change in the weather, and usually not for the better.