Very simple: cold air always flows downwards. A roof-top (or cab-top) air conditioner takes advantage of this physical principle. The cold air currents can be distributed inside the vehicle while saving energy without loss of performance. In addition, mounting the unit on the roof requires a lot less effort and outlay than the installation of a fully integrated air conditioner. While the latter only works with the motor running, Dometic offers air conditioning while stationary and - depending on the version - additional air conditioning while driving.