Hot tower

A hot tower is a tropical cumulonimbus cloud that penetrates the tropopause, i.e. it reaches out of the lowest layer of the atmosphere, the troposphere, into the stratosphere. In the tropics, the tropopause typically lies at least 15 kilometres (9.3 mi) above sea level. These towers are called "hot" because they rise high due to the large amount of latent heat released as water vapor condenses into liquid and freezes into ice. The presence of hot towers with a tropical cyclone's eyewall can indicate strengthening is more likely during the next six hours.

The hot tower hypothesis was proposed in 1958 by Herbert Riehl and Joanne Simpson (then Joanne Malkus) after extensive study of moist static energy profiles in the tropics. Prior to 1958, the mechanism driving the global-scale circulation pattern called Hadley cells was poorly understood. Riehl and Simpson proposed that the energy feeding these convective cells was supplied by the release of latent heat during condensation and subsequent freezing of warm, moist air in areas of convection about 5 kilometres (3.1 mi) wide. The large horizontal extent of these convective cells provides a buffer from the dry air surrounding the convective region that allows the parcel to rise at nearly the moist adiabatic lapse rate. NOAA also defined the term Towering cumulonimbus in 2009.

In 2007, the National Aeronautics and Space Administration (NASA) hypothesized that the wind shear between the eye and the eyewall could enhance updraft and be a purely dynamic generator of convection. Hot towers may appear when the hurricane is about to intensify. A particularly tall hot tower rose above Hurricane Bonnie in August 1998 as the storm intensified before striking North Carolina, United States. Bonnie caused more than $1 billion in damage and three deaths, according to the National Oceanic and Atmospheric Administration and the National Hurricane Center. Hot towers are also known to form when a tropical cyclone is about to undergo an explosive intensification.

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