<P> A low - pressure area, low, or depression is a region on the topographic map where the atmospheric pressure is lower than that of surrounding locations . Low - pressure systems form under areas of wind divergence that occur in the upper levels of the troposphere . The formation process of a low - pressure area is known as cyclogenesis . Within the field of meteorology, atmospheric divergence aloft occurs in two areas . The first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies (a trough with large wavelength that extends through the troposphere). A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength . Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity . </P> <P> Thermal lows form due to localized heating caused by greater sunshine over deserts and other land masses . Since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of the Earth's surface . Large - scale thermal lows over continents help drive monsoon circulations . Low - pressure areas can also form due to organized thunderstorm activity over warm water . When this occurs over the tropics in concert with the Intertropical Convergence Zone, it is known as a monsoon trough . Monsoon troughs reach their northerly extent in August and their southerly extent in February . When a convective low acquires a well - hot circulation in the tropics it is termed a tropical cyclone . Tropical cyclones can form during any month of the year globally, but can occur in either the northern or southern hemisphere during November . </P> <P> Atmospheric lift will also generally produce cloud cover through adiabatic cooling once the air becomes saturated as it rises, although the low - pressure area typically brings cloudy skies, which act to minimize diurnal temperature extremes . Since clouds reflect sunlight, incoming shortwave solar radiation decreases, 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 experienced in its vicinity . Globally, low - pressure systems are most frequently located over the Tibetan Plateau and in the lee of the Rocky mountains . In Europe (particularly in the British Isles), recurring low - pressure weather systems are typically known as "depressions". </P>

Which type of air pressure is associated with warm air rising