Low-pressure area most active tropical cyclone basin on Earth. 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
High-pressure area neighboring regions. These Highs change character and weaken once they move further over relatively warmer water bodies. Somewhat weaker but more common are high-pressure areas caused by atmospheric subsidence, that is, areas where large masses of cooler drier air descend from an elevation of 8 to 15 km after the lower temperatures have precipitated out the water vapor. Many of the features of Highs may be understood in context of middle- or meso-scale and relatively enduring dynamics of a planet's atmospheric circulation. For example, massive atmospheric subsidences occur as part of the descending branches of Ferrel cells and Hadley cells.