Meaning of THERMAL in English
A rising bubble of warm air which forms at the surface where heating is large and dry convection is occurring under superadiabatic conditions. Clouds start out as thermals before they reach their convective (lifting) condensation level but not all thermals become clouds. This is because the surface characteristics (supplying the thermal) are either too dry or too cold for the thermal to reach saturation aloft.
Weather and meteorology English vocabulary. Английский словарь погоды и метеорологии . 2012