A hot tower is a tropical cumulonimbus cloud that reaches out of the lowest layer of the atmosphere, the troposphere, and into the stratosphere. These formations are called "hot" because of the large amount of latent heat released as water vapor condenses into liquid and freezes into ice within the cloud. Hot towers in regions of sufficient vorticity may acquire rotating updrafts; these are known as vortical hot towers In some instances, hot towers appear to develop characteristics of a supercell, with deep and persistent rotation present in the updraft. The role of hot towers in tropical weather was first formulated by Joanne Simpson in 1958. Hot towers dominated discussions in tropical meteorology in the 1960s and are now considered the main drivers of rising air within tropical cyclones and a major component of the Hadley circulation. Although the prevalence of hot towers in scientific literature decreased in the 1970s, hot towers remain an active area of research. The presence of hot towers in tropical cyclones is correlated with an increase in the tropical cyclones's intensities. Observation Hot towers were first detected by radar in the 1950s. Aerial reconnaissance was used to probe hot towers, though planes avoided the most dangerous cores of hot towers due to safety concerns. The launch of the Tropical Rainfall Measuring Mission (TRMM) in 1997 provided the resolution and coverage necessary to systematically catalog hot towers and precisely assess their structure globally. Prior to 1997, the small size and short duration of hot towers limited studies of hot towers to aerial observations as the resolutions of satellite sensors at microwave and infrared wavelengths were too coarse to properly resolve details within hot towers. Structure The term hot tower has been applied to both rapidly rising parcels of air and the tall cumulonimbus clouds that accompany them. The regions of rising air are horizontally small and span about across. Their greatest extent is in the vertical, reaching altitudes as high as and exhibiting high reflectivity. Hot towers are effectively undilute; as they ascend, the surrounding air does not mix with the rising parcels of air. As a result, the equivalent potential temperature within a hot tower remains nearly constant throughout their entire vertical extent. This allows for efficient transport of heat from the lower troposphere to the stratosphere. Hot towers forming within areas of rotation may feature rotating updrafts; these are known as vortical hot towers