Crozet Weather Almanac: July 2013

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Old Ladies & Dew Point Dilemmas

By Roscoe Shaw and Heidi Sonen

Chart courtesy Heidi and Roscoe
Chart courtesy Heidi and Roscoe

Back in my TV weatherman days, I answered a phone call from an elderly female viewer who not so politely said “Look mister, I don’t give a [expletive] about dew point.”  When I stopped laughing, I called Heidi to tell her about my nasty viewer call.  Heidi worked at the competing TV station and she had heard the same thing before, probably from the same lady.

But the rude old lady has a point.  Meteorologists have done a terrible job at communicating humidity information.  The dew point temperature is actually quite simple and useful but very few people understand it.  Relative humidity, on the other hand, is very confusing and often misused, but people think it’s simple.  I’ll try to explain it, but unfortunately you’ll probably just get more confused and not give a [expletive] about dew point either.

The problem with using relative humidity is that it is “relative” to the temperature.  On a typical summer day in the morning, the temperature might be 70 with a dew point of 70.  The air is fully saturated with a relative humidity of 100%.  Dew forms on the grass and perhaps fog.  Later in the afternoon, the temperature rises to 95 while the dew point remains at 70.  The fog and dew disappear (or “burn off”) and the relative humidity drops to 45%.

In this scenario, there is just as much humidity in the afternoon as there was in the morning.  But since the temperature rose, the ‘relative’ humidity dropped from 100% to 45%.  Dew point is a measure of absolute humidity and it stayed the same at 70.

Are you confused yet?  Good.  You should be.

If you want to know about summer humidity, I suspect you just become friends with dew point and ignore “relative humidity.” Once you get used to using dew point, it’s very simple.  The higher it is, the more muggy it feels and the harder it is to cool off.  If the temperatures drops down close to the dew point, then you get dew (or frost) or fog.  For summer humidity, just use this handy dandy chart.

 Dewpoint        

  • > 75 extremely humid 
  • > 70  very humid summer muggies
  • 65-70 typical sticky summer weather 
  • 60-65 pleasant summer humidity
  • 50-60 dry, refreshing, Canadian air
  • < 50 unusually dry summer air

In summertime, the low temperature usually drops pretty close to the dew point.  So a forecast low of 73 generally means very humid while a low of 60 is much more comfortable.

Oh, by the way, hot and humid air isn’t “heavy.” It’s actually much lighter than cold, dry air. Almost everybody gets that wrong, but it’s true. Your car will get better gas mileage in summer and it’s easier to hit a home run on a hot muggy day.

July Recap

July was our sixth straight month of cooler than normal weather.  The hottest here so far this year is just 91 degrees, which is remarkable.  The rain keeps falling and the grass keeps growing which is also remarkable for this late in summer.

Rain Totals

  • Crozet   5.08”
  • Greenwood   5.53”
  • Waynesboro   4.59”
  • Flattop Mountain  5.02”
  • Nelleysford    7.53”
  • Univ. of VA    7.03”
  • CHO Airport  3.85”