Abstract: The device comprises an elongated container (2) which can be attached to the roof (1) or a wall of a vehicle in close engagement therewith and transversely of a direction of travel. The container carries a supply of evaporation liquid (9), and contains an absorbent mat (7a) which emerges from the container at the rear lower edge thereof. The container (2) and mat (7a) can be mounted detachably on the roof (1) or the wall. The container (2) may have a cover (3) and be profiled in the manner of a spoiler. The mat (7a) is adapted to be withdrawn from the container (2) and fixed in its withdrawn state. To produce an excess pressure in the interior of the container the container (2) may have one or more ventilation openings (3c).