A low, or a low pressure area, is a region where the atmospheric pressure is lowest with relation to the surrounding area. 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 radiaion and lower temperatures, since the clouds reflect sunlight. At night, the absorbtive effect of clouds on outgoing longwave radiaion (i.e. 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 circulations. Many of the worlds 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 and coriolis effect that drive the circulation.
See also