page_title: Low-pressure area
section_title: Associated weather
hierarchical_section_title: Low-pressure area / Associated weather
context_page_description: A low-pressure area, low area or low is a region where the air 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 atmosphere. 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 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.
  A low-pressure area is commonly associated with inclement weather, while a high-pressure area is associated with light winds and fair skies.
  Thermal lows form due to localized heating caused by greater sunshine over deserts and other land masses.
context_section_description: Wind is initially accelerated from areas of high pressure to areas of low pressure.  This is due to density (or temperature and moisture) differences between two air masses.  Since stronger high-pressure systems contain cooler or drier air, the air mass is denser and flows towards areas that are warm or moist, which are in the vicinity of low-pressure areas in advance of their associated cold fronts.  The stronger the pressure difference, or pressure gradient, between a high-pressure system and a low-pressure system, the stronger the wind.  Thus, stronger areas of low pressure are associated with stronger winds.
  The Coriolis force caused by the Earth's rotation is what gives winds around low-pressure areas (such as in hurricanes, cyclones, and typhoons) their counter-clockwise (anticlockwise) circulation in the northern hemisphere (as the wind moves inward and is deflected right from the center of high pressure) and clockwise circulation in the southern hemisphere (as the wind moves inward and is deflected left from the center of high pressure).  A tropical cyclone differs from a hurricane or typhoon based only on geographic location.  Note that a tropical cyclone is fundamentally different from a mid-latitude cyclone.  A hurricane is a storm that occurs in the Atlantic Ocean and northeastern Pacific Ocean, a typhoon occurs in the northwestern Pacific Ocean, and a tropical cyclone occurs in the south Pacific or Indian Ocean. Friction with land slows down the wind flowing into low-pressure systems and causes wind to flow more inward, or flowing more ageostrophically, toward their centers. Tornadoes are often too small, and of too short duration, to be influenced by the Coriolis force, but may be so-influenced when arising from a low-pressure system.
caption: Schematic representation of flow (represented in black) around a low-pressure area in the Northern hemisphere. The pressure-gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows.