Heat Index 101

I rambled in a post here last July about the difference in using dew point versus using humidity in describing how humid it feels outside, with me trying to make a case that dew point is quantifiable as opposed to a relative measurement of atmospheric moisture.   Most folks wouldn't know that 78% humidity on a 60 degree morning isn't humid whereas they would know that a dew point of 53 on a 60 degree morning in summer is rather comfortable.

We're asking in a poll here if you feel heat or humidity is worse -- and so far the results aren't surprising as humidity wins handily.   Taking it to another level, I figured that I would help explain the tie between dew point (and humidity) and temperature when it is hot out.   Most know what a heat index is -- the "feels like" temperature that combines the impact of heat and humidity together.   Some call it the summer equivalent of wind chill although that's a bit of a malaprop and misnomer since heat index does not account for wind nor does it feel chilly on a hot and humid day.   Regardless, the heat index is really the tell tale sign of how it really feels outside.   The higher the dew point (or humidity level) at a constant temperature, the higher the heat index.

Heat index is a critical measurement used to determine excessive heat warnings and heat advisories locally.   Typically, 100 degree heat index values are the standard to issue heat warnings in the Delaware Valley but in the early parts of the summer the criteria is lower (96 before June 15th, 98 in the latter half of June).

When we have a 90 degree day, we generally feel that it is very warm if not hot outside.   Just how hot it feels depends greatly on the dew point outside.   If we have a comfortable atmosphere with dew points in the 50's, the apparent temperature (heat index) is lower than the air temperature as evaporation off of our skin from sweat and regular ol' moisture transport around the body will cool us off somewhat, not accounting for any additional wind that may help keep us comfortable.  As dew points reach around 60, that evaporation benefit is negated and temperatures feel as warm as advertised.   It's hot but most everyone can deal with it feeling like 90 degrees out.   The higher the dew point gets, the higher the apparent temperature becomes on a hot day.   If the actual temperature is 90 and the dew point is 75, the heat index is 100 on that given day and we're all sweltering.

The heat index is a sliding scale, meaning the higher the dewpoint and/or higher the temperature, the progressively worse it will feel.   If we keep a constant 75 degree dew point but increase the temperature from 90 to 95, the heat index is now 107 (seven degrees worse).  Increasing the temperature to 100 with a dew point of 75 brings us an ungodly heat index of 114, whereas a dew point of 50 at 100 degrees yields a heat index of "just" 97.   At some point, even the "dry heat" folks will begin to complain when the air temperature gets too hot, despite a lack of atmospheric moisture.