Re: relative transparency
- From: tony_flanders@xxxxxxxxx
- Date: 23 Aug 2006 03:05:40 -0700
Tom Rauschenbach wrote:
We're all familiar with the notion of relative humidity, so there's no
need to go into that.
Actually, relative humidity is widely misunderstood and misreported.
I wish that weather stations *never* reported it, and *always* reported
dew point instead. The problem with relative humidity is that it swings
wildly over the course of a day as the actual temp varies, whereas
the dew point is fairly stable. So people often report things like
"humiidity of 90% and temp of 90 degrees," combining the morning's
humidity with the afternoon's temp, and not realizing that this is
a nonsensical result.
Tonight I was out at 90% relative humidity. Ambient air temperature was
65F and the dewpoint (I am told, I have not checked these numbers against
each other) was 50 degrees F.
According to the relative humidity calculator at
www.esa.act.gov.au/firebreak/humidity.html
at temp of 65F and a dew point of 50F yields a relative humidity of 50%
--
a far cry from the 90% you reported.
I was expecting dew like mad, yet my
optics stayed dry.
Yes, that's very common. The other critical variable you didn't report,
though, is the wind. Even a light breeze inhibits dew dramatically.
Also, most dew is re-condensation from the ground, so when the
ground is dry, the dew tends to be light even when the humidity is
fairly high.
- Tony Flanders
.
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