The first law of thermodynamics explains adiabatic cooling and the formation of contrails behind jet planes. During adiabatic processes, where there is no exchange of heat with the surroundings, gases rapidly expand and do work on the surroundings, converting their internal energy into mechanical energy. This rapid expansion leads to a decrease in temperature, resulting in adiabatic cooling. In the case of jet planes, the hot exhaust gases are released into the cold upper atmosphere, causing rapid expansion and adiabatic cooling. This cooling effect leads to the condensation of water vapour, forming contrails behind the planes. The first law of thermodynamics highlights the conversion of thermal energy into mechanical work and the subsequent cooling effect in these processes.