Monday, March 16, 2009 | 11:56 AM PT
Question submitted by Tim Hoang
Wow, Tim, this is a huge question!
Basically the atmosphere can be thought of as a fluid. As this fluid ebbs and flows around the world, we get swells and troughs forming, and these swells and troughs bring certain types of weather regimes with them. That's the "Coles Notes" version, here's the science:
A low pressure area, or "low", is a region where the atmospheric pressure is lower in relation to the surrounding area. Low pressure systems form under areas of upper level divergence on the east side of upper troughs, or due to localized heating caused by greater insolation or active thunderstorm activity. Those that form due to organized thunderstorm activity over the water which acquire a well-defined circulation are called tropical cyclones.
Lows are frequently associated with atmospheric lift. This lift will generally produce cloud cover through adiabatic cooling, once the air becomes saturated as it rises. Thus, low pressure typically brings cloudy or overcast skies, which may minimize diurnal temperature extremes in both summer and winter. Since the clouds reflect sunlight, incoming shortwave solar radiation is less which causes lower temperatures during the day. At night, the absorptive effect of clouds on outgoing longwave radiation, such as heat energy from the surface, allows for warmer diurnal low temperatures in all seasons. The stronger the area of low pressure, the stronger the winds that are experienced in its vicinity.
A red letter "L" is used to denote an area of low pressure on a weather map.
Conversely, a high-pressure area (also called a "high" and denoted with a blue "H" on weather maps) is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment. Winds within high-pressure areas flow outward due to the higher density air near their center and friction with land. Due to the coriolis force, winds flow clockwise around high-pressure systems in the northern hemisphere and counter-clockwise in the southern hemisphere. Regions of high-pressure are alternatively referred to as anticyclones. High-pressure areas are generally associated with cooler, drier air as well as clearing skies due to their formation within areas of atmospheric subsidence, or areas of large scale air descent. The strongest high-pressure areas are associated with arctic air masses during the winter, which modify and weaken once they move over relatively warmer water bodies. The area of high pressure associated with the descending branch of the Hadley cell, known as the subtropical ridge, steer tropical waves and tropical cyclones across the ocean and is strongest during the summer. The subtropical ridge also helps form most the world's deserts. Arctic high-pressure systems weaken with height, while subtropical ridges strengthen with height.
Hope this gives you an idea of why we track these areas on weather maps.