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Date: 22 Aug 2006 22:25:44
From: Tom Rauschenbach
Subject: relative transparency
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We're all familiar with the notion of relative humidity, so there's no
need to go into that. But is there a meaningful metric for
humidity/transparency such that a less transparent sky allows less
radiative cooling and thus causes less dew ?
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. I was expecting dew like mad, yet my
optics stayed dry. The night was clear, maybe 10% cloud cover or less,
but not very transparent. The naked eye just didn't see many stars.
Was the lack of dew caused by the same thing that caused the bad
transparency ? Was the lack of dew caused by the bad transparency (not
the same question) ? Has anybody quantified this ? Would it be useful to
do so ?
I think my next eyepiece just morphed into a weather station!
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Date: 23 Aug 2006 03:05:40
From:
Subject: Re: relative transparency
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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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Date: 23 Aug 2006 10:36:59
From: Tom Rauschenbach
Subject: Re: relative transparency
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On Wed, 23 Aug 2006 03:05:40 -0700, tony_flanders wrote:
> 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.
That's why I took pains to point out that I didn't perform the check that
you did ! Also, I had no means to determine either the dewpoint or the
relative humidity (or ambient temp for that matter) I just took numbers
off a weather web page.
>
>> 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.
No *that's* a useful observation ! While considering how I might quantify
this phenomenon (assuming it exists) I'll be sure to keep that in mind.
I guess the next question might be : is there a quantified metric of
transparency ?
>
> - Tony Flanders
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Date: 23 Aug 2006 15:41:51
From: Chris L Peterson
Subject: Re: relative transparency
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On Wed, 23 Aug 2006 10:36:59 -0400, Tom Rauschenbach
<tomsusenet@tomsdomain.org > wrote:
>I guess the next question might be : is there a quantified metric of
>transparency ?
Transparency is closely related to limiting magnitude. Instrumentally I
think you could just look at the absolute intensity of a reference
object.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 23 Aug 2006 05:51:19
From: Chris L Peterson
Subject: Re: relative transparency
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On Tue, 22 Aug 2006 22:25:44 -0400, Tom Rauschenbach
<tomsusenet@tomsdomain.org > wrote:
>We're all familiar with the notion of relative humidity, so there's no
>need to go into that. But is there a meaningful metric for
>humidity/transparency such that a less transparent sky allows less
>radiative cooling and thus causes less dew ?
>
>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. I was expecting dew like mad, yet my
>optics stayed dry. The night was clear, maybe 10% cloud cover or less,
>but not very transparent. The naked eye just didn't see many stars.
>
>Was the lack of dew caused by the same thing that caused the bad
>transparency ? Was the lack of dew caused by the bad transparency (not
>the same question) ? Has anybody quantified this ? Would it be useful to
>do so ?
I've made some limited comparisons between atmospheric water vapor (I
have an IR camera) and the temperature of objects. There is a simple
cloud detector that compares the temperature on the top and bottom of a
plate which depends on this relationship.
On very transparent nights, I've had water bowls freeze over when the
air temperature never dropped below 50°F. If it weren't for the fact
that the dewpoint here is usually tens of degrees below ambient, that
would be a big problem.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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