|WX CHANGERS- EVAPORATIVE COOLING
METEOROLOGIST JEFF HABY
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.