Evaporative cooling is a cooling of the air due to latent heat absorption of water molecules. When water evaporates, the evaporation process requires taking heat from the environment in order for the evaporation to occur. With the removal of heat from the air, the air cools. The amount of water that is able to evaporate into a volume of air impacts the cooling. Evaporative cooling can occur until the relative humidity reaches 100% (saturated air). Thus, initially dry and warm air will produce the greatest amount of evaporate cooling when this air is saturated through the evaporation process. This is because dry air can evaporate a greater amount of moisture as compared to less dry air when both are initially at the same temperature and warm air can evaporate a greater amount of moisture as compared to cold air. One reason rainfall is associated with cooler air is because the rain cools the air through evaporative cooling. When rain falls into dry air it will dramatically change the weather of that air. This makes it a significant weather changer. Rain falling into dry air will increase the dewpoint and lower the temperature. The wet bulb temperature is the temperature that the dewpoint and temperature will meet at when complete saturation occurs. Another reason that evaporative cooling is a significant weather changer is that it can cause a cold rain to turn into wintery precipitation. Evaporative cooling can be enough to cause the ground surface temperature to drop below freezing which leads to ice on the ground and travel problems. For example, the temperature could be 34 F with a dewpoint of 10 F. When rain falls into this air, evaporative cooling will cause the dewpoint to increase and the temperature to decrease. After saturation, the new temperature will be below freezing. This can change precipitation type from rain to snow, sleet or freezing rain. Weather forecasters keep a close eye on the evaporative cooling potential when the temperature is initially just above freezing since the weather could change dramatically to icing conditions after evaporative cooling. |