For example, around the area of 20° latitude, the air aloft is often sinking, due to the presence of the sub-tropical high—a semi-permanent high pressure system in the subtropical regions that facilitates sinking air motions (subsidence). The sinking air warms and creates a temperature inversion (an extremely stable air layer in which temperature increases with altitude, the inverse of the usual temperature profile in the lower atmosphere) known as the trade wind inversion. This warm layer is very stable, making it difficult for air currents to rise and form thunderstorms and (eventually) hurricanes. In addition, strong upper-level winds tend to decapitate developing thunderstorms by dispersing the latent heat and cutting off the storm’s source of fuel. At the surface, hurricanes can diminish rather quickly given the right conditions. These conditions include: (1) the storm moving over cooler water that can't supply warm, moist tropical air; (2) the storm moving over land, again cutting of the source of warm, moist air; and finally (3) moving into an area where the large-scale flow aloft is not favorable for continued development or sustainment. .