Every automotive air conditioning system performs four basic functions to help improve passenger comfort and maintain temperature control. Besides cooling the air in the vehicle, the A/C system also dehumidifies the air. That is the process of removing the moisture from the air passing over the evaporator.

The dehumidification process occurs because all of the hot outside air entering the vehicle must pass over the A/C evaporator first. When the A/C system is working properly, that evaporator is very cold. When the hot air encounters the cold evaporator, any humidity in the air is instantly cooled and condensed back to a liquid.

Humidity is just water vapor. We know that when that vapor is cooled, it will condense back to a liquid form. That vapor also releases any heat it may have absorbed when it originally changed state to vapor. The release of that excess heat is why auto A/C systems are under such sever heat load on days when there is high humidity.

You can see the result of the dehumidification processes on any air conditioned vehicle. Typically on air conditioned vehicles, there will be constant drip of water or condensation from the evaporator and that usually drains just behind the passenger side front wheel.

By removing the humidity from the air, the auto A/C system provides increased passenger comfort, specially on those hot sticky days where the relative humidity is very high.