I agree it sounds unusual, but Bill said similar readings were taken in
the same city (was that the AWS station or another, Bill?).
That said, it is also true that the humidity sensor is the most
unreliable electronic instrument, whether on a personal weather station
or an ASOS.
Here's a map of the stations in the area:
http://www.weathermatrix.net/mesonet/pa/phl/
The AWS station in the area did report 13%
http://aws.com/full.asp?id=PERKA
Unfortunately the official station KDYL didn't report for most of the
day...
http://www.accuweather.com/adcbin/current_index_large?metsta=DYL&nav=meteo&type=metar&time=12&btnMet=Decoded&Partner=6037
So I took the liberty of downloading humidities for today from 18Z-22Z.
Clearly there is a very low area of humidity around Philadelphia and
it's possible Bill was located in the driest pocket. The "official"
observations seen on this animation run as low as 10%
http://www.weathermatrix.net/2001/1025/humidity-anim.gif
(Courtesy AccuWeather.com SuperPremium)
http://www1.accuweather.com/iwxpage/adc/popup/superpremium/index.html
So although I think Bill is a little low it was certainly in the teens %
in the area, which is very dry.
Looking closer Bill's DEWPOINT reading is suspect, with the lowest
official ob in the area going down to 18 on the NJ coast, nothing else
below 20. Bill, I'd take Peasl2's advice below and try to verify the
accuracy of your station with a sling psychrometer if you have one, the
next time you see it that low. If you find it to be off more than
Davis's published specifications for the VantagePro you should drop them
an email. The sensor isn't overexposed or receiving radiation from a
roof or cement or anything is it?
Although we're not AS dry here at WeatherMatrix HQ in Central PA the
dewpoint has fallen from 60 this morning to nearly 20 this evening.
http://www.weathermatrix.net/wxstation/TWI-WRL/obs/24hour.shtml
Other responses to Bill's post below:
> FROM: peasl2{at}aol.com
> DATE: Thu, 25 Oct 2001 18:27:56 EDT
> SUBJECT: Re: [STORMREPORTS] Invasion of Incredibly DRY Air
>
> those readings right there are unreal,did you check to make
> sure that those readings are for real.because i never seen
> dewpoints at 71 in the mid morning and then early afternoon
> at zero in pa.i really dont think any dewpoints are at zero
> till mid winter
AND Frank Chernowetz wrote:
>
> I hate to be the bearer of bad news, but I think your readings are way
> off base. If you check "official" stations in your region, you'll see
> the lowest dew points were in the 18-23 degree range and humidities
> 15-20% at a minimum. Your readings of 0 and 6% are extremely unlikely. I
> would get a sling pyschrometer and check your calibration (if you're a
> stickler like me and want accurate readings!)....
>
> Meaning well....Frank
>
>
--
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Jesse Ferrell - Meteorologist/Web Dev. - jesse{at}weathermatrix.net
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