Expert: Dave Dombek - 6/13/2010

Question
I am considering adding a mist cooling system to my patio outside.  I have spent a lot of time researching them to understand the science of temperature reduction through water evaporation.  It seems to me that it will reduce the dry bulb temperatures, but the human body will not like the lower temperature with much greater humidity.  It appears to me that there may be some benefit to the slightly lower temperature(after adding back the negative impact of higher humidity).  There are a lot of other negative impacts like rusting of anything metal and electronic devices outdoors.  I live in Dallas Texas and it is often very humid.  Right now it is 9:30 p.m. and it is 87 degrees and 67% relative humidity and pressure of 29.91.  I calculate the wet bulb temperature to be 78 degrees. If my misters lower the temperature by 9 degrees the relative humidity will then be 100% and my body will not feel 9 degrees cooler.  What is the true real feel change by using misters and foggers to lower the air temperature while at the same time greatly increase the humidity?  I think there may be a slight benefit that is greatly offset by the very high humidity.

Thanks...

Answer
Kendall,

You are correct about the relative humidity going up when you put mist into the air. However, evaporation if water is always a cooling process, no matter how high the dew point is. So, you will get the desired cooling effect with this system, despite some of its drawbacks. The only really effective way to cool the air on your patio without significantly raising the relative humidity is to close it in and install an air conditioner.

Sincerely,

Dave Dombek
AccuWeather.com