What factors determine moderate thermal development, release, rate of ascent, and decay?
Development – The surface areas that produce thermals best are parking lots, ploughed fields, dried grass areas or bare rocks (to mention but a few). Release – A general surface wind serves to release thermally as it swirls around ground obstructions. Rate of ascent – On clear, hot days it is common to get a layer very close to the ground that is superheated. This layer is called the superadiabatic layer. This layer has a lapse rate greater than the cooling rate of thermals. As a result the difference between the thermal temperature and that of the surrounding air is continually getting greater as the thermal rises through this layer. Thus the thermal accelerates upwards. This aspect influences the rate of ascent of a thermal. (Temperature difference between thermal cooling rate and lapse rate increases – thermal rate of ascent increases). Decay – When a thermal reaches an inversion layer its climb is stopped. When the thermal reaches dew point level and forms a cumulus cloud mixing oc
