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Its not so much the heat, but the…Dew Point?

We’re in the middle of a very nasty heat wave here in Philadelphia (where Spectrum Scientifics is based). Temperatures are expected to reach 100 degrees (F) before the end of the week.

One thing we have noticed in various accounts of the weather is that most forcasters and weather reports don’t really have an accurate measure of just how uncomfortable the hot weather is when you factor in all the moisture in the air. You’ve doubtless heard the phrase “its not the heat, its the humidity” but measuring humidity doesn’t always give us a sense of how it feels to us. You get a percentage figure, but that doesn’t say much.

Forcasters have tried to factor in humidity by using what they call the ‘heat index’, but this is never a good index. A person standing in dry desert heat at 98 degrees is not having the same experience as a person in 90 degree weather with high humidity. “Heat index” seems to be one of those measurements so people can quote high numbers to say how bad things are. “Wind Chill” is another such figure and based on some poor information (it assumes you are not wearing clothes, for example), but more on that  figure another time.

So what is the best figure for determining how comfortable or uncomfortable you are going to be? The best indicator is actually a little measurement call the Dew Point.

The Dew Point is the actual temperature and object has to be for moisture in the air to condense. Think of how you take a cold soda out of the fridge and moisture forms on it. That soda is chilled and well below the dew point, so the moisture in the air turns to liquid in it.

Dew Points actually can be a better indicator of how comfortable a person will actually be, since it can cover lower temperatures where the humidity is high (clammy is the unofficial term).  When the Dew Point is below 60, most people are quite comfortable. From 61-65 it can be a little uncomfortable to be in, and a Dew Point above 65 is just going to make people unhappy.

Even Dew Point is hardly a perfect measurement. Things like clouds, wind, and other weather factors can make a somewhat higher Dew Point seem more tolerable than normal.  But at least the Dew Point is not a high number set up by TV weathermen to impress people with big numbers.

So remember to watch the dew point, and keep yourself hydrated!

Dew Point as of this writing: 72 Degrees