The wet bulb temperature is the temperature most relevant to snow generation. It is a combination of the air temperature in °Celsius or °Fahrenheit and the relative atmospheric humidity in %. It corresponds to the temperature reading on a thermometer when its mercury bulb is moistened with water. If the atmospheric humidity is very low, the moisture evaporates quickly and draws a great deal of heat from the mercury column. The thermometer reading is then lower than the actual air temperature. The atmospheric humidity has a much greater effect on the wet bulb temperature. The damper the air, the less moisture it can absorb and the colder it must be to form snow crystals from the fine droplets of water. The lower the atmospheric humidity, therefore, the easier it is for water to evaporate from the surface of the atomized water and cool down the drop.