On the horizontal movement of thermal updrafts and their drag coefficient in windy weather

Abstract

In meteorology and in numerous publications on gliding, thermals are described as a phenomenon where air near the ground heats up, and this warm air drifts horizontally over the ground before ascending at a lifting edge. The evolving updraft column then is tilted by the wind. However, the analysis of GPS data from gliders and data from boundary layer measurement masts shows a completely different scenario. Thermal updrafts within the convective layer are vertically oriented even in the presence of wind. They move horizontally forward at the speed at which they detach from the ground. Updraft columns must be regarded as predominantly frictionless and have a drag coefficient of zero or nearly zero.