Rapid intensification is a meteorological situation where a tropical cyclone intensifies dramatically in a short period of time. The United States National Hurricane Center defines rapid intensification as an increase in the maximum sustained winds of a tropical cyclone of at least 30 knots (35 mph; 55 km/h) in a 24-hour period.
In order for rapid intensification to occur, several conditions must be in place. Water temperatures must be extremely warm (near or above 30 °C, 86 °F), and water of this temperature must be sufficiently deep such that waves do not churn deeper cooler waters up to the surface. Wind shear must be low; when wind shear is high, the convection and circulation in the cyclone will be disrupted. Dry air can also limit the strengthening of tropical cyclones.
Usually, an anticyclone in the upper layers of the troposphere above the storm must be present as well--for extremely low surface pressures to develop, air must be rising very rapidly in the eyewall of the storm, and an upper-level anticyclone helps channel this air away from the cyclone efficiently.[further explanation needed]Hot towers have been implicated in tropical cyclone rapid intensification, though they have diagnostically seen varied impacts across basins.
The United States National Hurricane Center previously defined rapid deepening of a tropical cyclone, when the minimum central pressure decreased by 42 millibars (1.240 inHg) over a 24-hour period. However it is now defined as an increase in the maximum sustained winds of a tropical cyclone of at least 30 knots (35 mph; 55 km/h) in a 24-hour period.