Temperature decreases fairly rapidly between the surface and the 500 millibar level. In fact, it is rare to have a 500 millibar temperature above freezing. It is usually well below freezing at the 500 millibar level. In the developing stages of a warm season thunderstorm, the entire storm will be composed of liquid water droplets. Even when the environmental temperature is below freezing, the water droplets will remain supercooled until the temperature falls significantly below freezing.
The sudden freezing of the top of a thunderstorm is termed glaciation. Within a matter of minutes, the entire top of the storm freezes over. It is fairly easy to differentiate the frozen from unfrozen cloud components. The unfrozen component of the cumulonimbus has a sharp well defined contrast while the frozen component appears wispy, smooth and brighter.
The vapor pressure is lower over ice than over water. As soon as the first ice particles form, water vapor rushes toward the ice particles and deposition occurs. This release of latent heat helps the thunderstorm remain warmer than the surrounding environmental air.
Thunderstorm glaciation is especially impressive if strong upper level winds are in place. Right before your eyes, the top of the storm will flash into the frozen state and the wind shear will blow an anvil curtain downwind from the storm. When the top of the storm flashes to the frozen state, the storm is beginning to become mature and lightning will likely follow shortly.