DELTA T

A simple representation of the mean lapse rate within a layer of the atmosphere, obtained by calculating the difference between observed temperatures at the bottom and top of the layer. Delta Ts often are computed operationally over the layer between pressure levels of 700 mb and 500 mb, in order to evaluate the amount of instability in midlevels of the atmosphere. Generally, values greater than about 18 indicate sufficient instability for severe thunderstorm development.