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Date: 04 Aug 2006 10:40:03
From: William C. Keel
Subject: Seeing in extremely summery weather
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Like most of the area of the 48 contiguous US states, Alabama has
shared the baking weather of the last few weeks (albeit it's
not all that much of a departure from the norm as it is in
some other places). Since much of my intuition about weather
and the quality of seeing was honed in the western part of the
country, I have been especially struck by noticing that there
have been a string of nights with seeing almost as good as it ever
gets hereabouts. I've seen this from the campus observatory and
my front yard, so it's not something extremely local. (I could
do without the sting of concentrated perspiration pouring into the
eyes, though, and the eyepieces could as well).
What's the general experience regarding seeing when it's been
very hot? Humidity may enter; in the mountain West, hot
weather often brings large diurnal temperature swings,
but in the southeastern states, the ridiculously high
specific heat of water spares us that issue.
Bill Keel
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Date: 04 Aug 2006 16:29:46
From: Chris L Peterson
Subject: Re: Seeing in extremely summery weather
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On 4 Aug 2006 10:40:03 -0500, "William C. Keel"
<keel@bildad.astr.ua.edu > wrote:
>What's the general experience regarding seeing when it's been
>very hot? Humidity may enter; in the mountain West, hot
>weather often brings large diurnal temperature swings,
>but in the southeastern states, the ridiculously high
>specific heat of water spares us that issue.
Here in the high Rockies, the best seeing is generally during hot
weather. For us that means 75-80°F in the day, dropping to the low 50s
at night. Summer humidity can approach 40%, which does moderate the
temperature swings somewhat. These more humid nights deliver better
seeing. Between monsoons the humidity drops to 20%, the nighttime
temperatures are in the 40s, and seeing is usually worse.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 04 Aug 2006 16:26:35
From: RMOLLISE
Subject: Re: Seeing in extremely summery weather
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William C. Keel wrote:
>
> What's the general experience regarding seeing when it's been
> very hot? Humidity may enter; in the mountain West, hot
> weather often brings large diurnal temperature swings,
> but in the southeastern states, the ridiculously high
> specific heat of water spares us that issue.
>
> Bill Keel
Down here in Lower Alabama, high pressure systems often bring very
steady seeing and hazy skies. Good transparency is often rare...except
when thunderstorms have moved through and cleaned things out. Right
now, though, it's clouds every evening.
Peace,
Rod Mollise
Author of: _Choosing and Using an SCT_, and _The Urban Astronomers's
Guide_.
<http://skywatch.brainiac.com/astroland >
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Date: 04 Aug 2006 15:14:41
From: Davoud
Subject: Re: Seeing in extremely summery weather
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William C. Keel wrote:
> Like most of the area of the 48 contiguous US states, Alabama has
> shared the baking weather of the last few weeks (albeit it's
> not all that much of a departure from the norm as it is in
> some other places). Since much of my intuition about weather
> and the quality of seeing was honed in the western part of the
> country, I have been especially struck by noticing that there
> have been a string of nights with seeing almost as good as it ever
> gets hereabouts. I've seen this from the campus observatory and
> my front yard, so it's not something extremely local. (I could
> do without the sting of concentrated perspiration pouring into the
> eyes, though, and the eyepieces could as well).
>
> What's the general experience regarding seeing when it's been
> very hot? Humidity may enter; in the mountain West, hot
> weather often brings large diurnal temperature swings,
> but in the southeastern states, the ridiculously high
> specific heat of water spares us that issue.
In my view (pun not unnoticed) it's not the heat, it's the humidity. I
haven't been out of late -- too hot, and I'm rehabbing the pier in my
observatory <http://www.davidillig.com/observatory15.shtml >.
I managed to steal a quick look at the moon the other night, however.
It was the ugliest, sickest looking shade of yellow that I could
imagine, but the air was very still, and it would have been a great
time to make photographs. It seems to me that very humid air works well
for Solar System objects and moderately humid air works well for deep
sky.
I base this partly on my early observing experiences in Indonesia and
Thailand with humidity in the 80 to 90% range, and from the high desert
in Yemen, with humidity in the 5 to 10% range. The high desert had no
light pollution, some dust, and seeing that was fair to good -- not
great.
Davoud
--
usenet *at* davidillig dawt com
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Date: 04 Aug 2006 10:23:07
From: Dan Mckenna
Subject: Re: Seeing in extremely summery weather
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William C. Keel wrote:
> Like most of the area of the 48 contiguous US states, Alabama has
> shared the baking weather of the last few weeks (albeit it's
> not all that much of a departure from the norm as it is in
> some other places). Since much of my intuition about weather
> and the quality of seeing was honed in the western part of the
> country, I have been especially struck by noticing that there
> have been a string of nights with seeing almost as good as it ever
> gets hereabouts. I've seen this from the campus observatory and
> my front yard, so it's not something extremely local. (I could
> do without the sting of concentrated perspiration pouring into the
> eyes, though, and the eyepieces could as well).
>
> What's the general experience regarding seeing when it's been
> very hot? Humidity may enter; in the mountain West, hot
> weather often brings large diurnal temperature swings,
> but in the southeastern states, the ridiculously high
> specific heat of water spares us that issue.
>
> Bill Keel
>
Bill,
Most of the seeing is in the ground layer due to night time
radiation cooling.
Heavy water vapor loading reduces the cooling and improves the seeing
same with cirrus.
d
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Date: 07 Aug 2006 12:32:29
From: dan chaffee
Subject: Re: Seeing in extremely summery weather
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On Fri, 04 Aug 2006 10:23:07 -0700, Dan Mckenna
<dmckenna@as.arizona.edu > wrote:
>
>Most of the seeing is in the ground layer due to night time
>radiation cooling.
Ground layer as in PBL (planetary boundary layer-- up to ~3000' AGL),
not 5 or 10 feet as some may interpret. Although it is true that
getting a scope 15-20' above the ground can help a bit; it's nothing
like getting above the lowest 3000'. Even mid-level inversions don't
inhibit turbulence at the gound level the way the low level inversions
often do.
Dan C.
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Date: 07 Aug 2006 08:04:13
From: Dan Mckenna
Subject: Re: Seeing in extremely summery weather
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dan chaffee wrote:
> On Fri, 04 Aug 2006 10:23:07 -0700, Dan Mckenna
> <dmckenna@as.arizona.edu> wrote:
>
>
>
>>Most of the seeing is in the ground layer due to night time
>>radiation cooling.
>
>
> Ground layer as in PBL (planetary boundary layer-- up to ~3000' AGL),
> not 5 or 10 feet as some may interpret. Although it is true that
> getting a scope 15-20' above the ground can help a bit; it's nothing
> like getting above the lowest 3000'. Even mid-level inversions don't
> inhibit turbulence at the gound level the way the low level inversions
> often do.
>
> Dan C.
Depends on where you are.
On a peak above the PBL in good weather, like Mauna Kea you can get 1
arc second of seeing in the first 20 Meters in some spots.
Depending on the wind direction Mauna Kea will also have regions where
the ground seeing layer is thinner and you get really good seeing.
On flat land the PBL will drop after sunset to less than 200 meters and
as the night progresses grows deeper. At the inversion interface waves are
often present that can produce "seeing layers".
In a drainage flow the first 100 M above the site can produce
very bad seeing. This is a density current driven by nurnal cooling.
The only time I saw Venus twinkle was at the mouth of a canyon
just after sunset.
On a peak with an inversion above it you can have waves that are
generated several Km in altitude as the waves propagate upward.
Most of the time on a Mt peak we have observed a factor of ten or more
seeing in the first 100 meters or less than the upper layers >2Km above
site altitude.
Dome seeing is also in part generated by radiation cooling and can be
intense and "thin".
Getting above the PBL is a good start but most of the time you can do
better by selecting a good location. For example, the Florida coast can
have sub arc second seeing. However that does not mean ocean side is
always better. I have watched layers with 10"s of arc second layer (
near horizontal) develop over inland bays.
Good seeing to you
d
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Date: 04 Aug 2006 10:12:06
From:
Subject: Re: Seeing in extremely summery weather
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William C. Keel wrote:
> What's the general experience regarding seeing when it's been
> very hot?
>the ridiculously high specific heat of water
Here in MO we get a little bit of every type weather. Best seeing
always occurs when a cold front "stalls" within a couple hundred miles
after passing. Wet humid nights with ground fog after a cloudy rainy
day. April and November are the months most likely to have decent
seeing. Hot doesn't help.
The naked eye "good seeing" clue to look for here is very large and
slow "twinkles". When the stars shine steadily this always indicates
the seeing distortions are occurring faster than the response rate of
your eye.
Doug Kniffen
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Date: 05 Aug 2006 01:16:19
From: canopus56
Subject: Re: Seeing in extremely summery weather
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"William C. Keel" <keel@bildad.astr.ua.edu > wrote in message
news:44d36a53@news.ua.edu...
> What's the general experience regarding seeing when it's been
> very hot? Humidity may enter; in the mountain West, . . .
In the Intermountain West, we get about one-and-one-half to two weeks per
month where the monsoonal weather clears out. As Chris notes, the humidty
drops and the seeing gets good, but only at high altitudes IMHO. The major
limiting factor during clear dry spells is photochemical smog. In general,
even if I drive horizontally for 60 or 70 miles from an urban zone and into
rural areas and national forests, the layer is still there. It is only when
you drive vertically above 8,500-9,000 feet that the layer tops out and you
look down on the layer. With the lack of smog extinction the seeing
improves, but you have to contend with mountain winds. In general, after
12am above 8,500ft, the clear air becomes still and seeing is excellent.
You lose about 1 to 1/2 weeks per year to wildfires. Any wildfire within
100 miles gunks the air up for two or three days with a visible extinction
layer. - Canopus56
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