The Development of Thermals from Rest

Conventional techniques for releasing a thermal in laboratory experiments induce enough initial motion to affect seriously the thermal's subsequent evolution. We have invented a mechanism for releasing thermals from very close to a state of rest. This allows the examination of the transient behavior of thermals previous to the development of self-similarity. A thermal starting from rest exhibits a much smaller entrainment rate than a self-similar thermal for a distance from its starting point of at least six initial diameters. Since we find that thermals typically penetrate four initial diameters or less in a stably stratified environment, this has potentially great significance for atmospheric convection. Numerical simulations using an axisymmetric, two fluid model aid in the interpretation of these results.