Low pressure area
From Freepedia
A low pressure area, or a low for short, is a region where the atmospheric pressure is lowest with relation to the surrounding area. Tropical storms, extratropical cyclones, subpolar cyclones, and subarctic cyclones are called low-pressure cells in English-speaking nations such as the United States and Canada.
Lows are frequently associated with stronger winds and atmospheric lift. This lift will generally produce cloud cover, due to adiabatic cooling, once the air becomes saturated. Thus, low pressure typically brings cloudy or overcast skies, which may minimize diurnal temperature extremes in both summer and winter, due to the significant cloud cover. This is due to less incoming shortwave solar radiation and lower temperatures, since the clouds reflect sunlight. At night, the absorbtive effect of clouds on outgoing longwave radiation, such as heat energy from the surface, allows for warmer diurnal low temperatures in all seasons.
Climatologically, low pressure forms at the Intertropical convergence zone (ITCZ), as part of the Hadley Cell circulation. Many of the world's rainforests are associated with these climatological low pressure systems. Frontal lows are temperate zone phenomena, and develop along the polar front as a result of the interaction between cold and warm surface air masses. Thermal lows also form over areas such as Death Valley as the result of intense ground heating; they are much smaller in geographic extent than either convergence lows or frontal lows.
Surface low pressure systems will tend to be smaller in area and have stronger surface winds than a given high pressure system, due to the addition of surface friction to the pressure gradient force and coriolis effect that drive the circulation.
In deserts, lack of ground and plant moisture that would normally provide evaporative cooling can lead to intense, rapid solar heating of the lower layers of air. The hot air is less dense than surrounding cooler air. This, combined with the rising of the hot air, results in an isolated low pressure area called a thermal low.
