Evaporation is the process where molecules leave a condensed liquid state and go into a gaseous state. Each molecule has its own "temperature" (energy level). The temperature we measure is an average value of all the molecules. During the evaporation process, some of the fastest-moving molecules (hottest ones with highest energy) on the surface manage to escape from the condensed liquid resulting only the cooler molecules (the ones with less energy) left behind in the liquid. That is why evaporation lowers the liquid temperature and is a cooling process. The evaporation process gets slower and slower as the temperature goes down.