A simple theoretical analysis of heat-and-mass transfer to a spherical water-droplet moving in humid air has been made, in connection with the possible use of spray or mist cooling in railway tunnels and other systems. In particular, the histories of the temperature and diameter of the droplet, and the dependence of the time of complete evaporation (or life-time) on the initial size have been studied. The relationships between life-time and the maximum time the droplet may be air borne in the particular environment, (such as a tunnel), has been investigated using the settling time of a freely-falling droplet as a basis of measurement. (c) 2006 Elsevier Ltd. All rights reserved.