Dew point relative to evaporative cooling?

Question

I live in the desert but it has become increasingly humid. I'm wondering when my evaporative cooling will cease being effective relative to the dew point. It seems humidity is no longer the barometer by which to judge this by. I guess I don't understand the dew point as it relates to evaporative coolong or the comfort level of the air.

Answer 1

The temperature you're really interested in is the wet bulb temperature.  That is the temperature measured by a thermometer with a small wet sock on it that is constantly being ventilated.  It will be cooler than the actual temperature and warmer than the dewpoint, and it will give an estimate of what sort of temperature that you will get out of your evaporative cooler.  Technically the wet bulb temperature is the temperature that air can be cooled to by evaporating water into it. Theoretically that sounds a lot like what you'll get out of your swamp cooler, but in reality you won't do quite as well as the wet bulb temperature.

Answer 2

What exactly is your question? I suppose you might want to know that the dew point is the temperature to which air must be cooled for it to become saturated (100% humidity). Evaporation stops when this happens because the air cannot hold any more moisture. Evaporative means loss of heat due to evaporation. Energy is needed to evaporate water, and that energy is used in the form of heat.

Answer 3

I think you are correct in saying that you don't want to put too much emphasis on the actual percentage value of relative humidity.  Since relative humidity is dependent on temperature, and the higher the temperature the more water vapor it can hold, then your RH values aren't necessarily that high when expressed as a percentage in the summer.

That doesn't mean it isn't humid and you should look exclusively at dewpoints.  If you are in the desert southwest of the U.S., then your dewpoints should be slowly increasing as you are approaching the moonson period.  

As for your evaporative cooler (a device correct?) you should probably cease using this device once dewpoints reach the 55-60.  Any dewpoint reaching above 55 is plenty of moisture to provide a comfortable environment that isn't painstakingly dry.