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cloud base forecasting question



 
 
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  #1  
Old June 23rd 05, 12:33 AM
William_F
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Default cloud base forecasting question

I'm not a pilot, but I've got a question pertaining to aviation
forecasts. I'm a mountain climber who has recently discovered
aviation forecasts that could be very useful in determining cloud base
heights. The idea being that alot of times I'll see a forecast that
calls for rain yet I find out on my climb that I'm easily above the
clouds for great climbing weather. My question has to do with
interpreting cloud base forecasts. For instance, at
http://usairnet.com/cgi-bin/launch/c...nge +Location,
I note that on Saturday at 11:00 am they are calling for a cloud base
of 3000-6500'. I'm assuming (maybe I'm way off, but that's why I'm
asking) that this means if I was climbing a mountain at 10,000', I'd
likely be above the clouds. Yet at 11:00 am on Friday at
http://usairnet.com/cgi-bin/launch/c...nge +Location,
I note a forecast for clear skies and a cloud base of 12,000' to
"unlimited." I'm not sure what this means, does this mean that even
though it is supposed to be sunny, I may encouter clouds above
12,000'?

Can someone set me straight on what this data is supposed to mean?

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  #2  
Old June 23rd 05, 01:08 AM
Peter Duniho
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"William_F" wrote in message
...
[...]
Can someone set me straight on what this data is supposed to mean?


To understand, you should probably look at two things: the raw data, and the
NWS explanation of the raw data.

All that the web site you're looking at does is map the NWS raw data to
plain English, essentially by just using the tables the NWS provides in the
first place.

You can find the raw data by clicking on the links near the bottom of the
pages you provided. The key for that data can be found he
http://www.nws.noaa.gov/mdl/synop/mavcard.htm

As far as your specific questions go:

First, it's important to understand that the forecast beyond about 6 hours
or so is extremely unreliable, especially with respect to cloud coverage and
heights.

Second, clouds can and often do come in layers. A cloud base of "3000-6500
feet" does not preclude the possibility of clouds above that height. Note
also that the range is quite wide; the reason being that there's a very wide
range of uncertainty in the forecast. They aren't saying that the bases
will actually range from 3000 to 6500 feet on that day; they are saying that
the bases, which will generally be relatively uniform, will be at a height
that is somewhere in or near that range.

Likewise, the "12000' to Unlimited" describes a potential range of
conditions. They are basically saying there won't be conditions favorable
for low clouds, but that there may be middle-altitude or higher clouds, or
there may be no clouds at all.

So, during a time period for which clear skies are predicted, the
"Unlimited" (which is really just a misleading transcription of the NWS term
"unlimited ceiling") is the more likely condition to prevail. On the other
hand, on a day when cloudy skies are predicted, the "12000'" (with the
implied "or higher") is the more likely condition to prevail.

So, if you want to know whether you'll see the sun, look at the "Sky"
category. If the "Sky" category indicates clouds, you can learn more detail
about the predicted lowest cloud heights in the "Cloud Base" category (but
it won't tell you how high the clouds are likely to go).

A forecast for low clouds could mean you'd be above the clouds on your
climb, or it could mean clouds extend all the way from below your climb to
well above it. There are other aspects to interpreting weather forecast
information that would give you more detail with respect to the extent of
the clouds, but the data you're referencing right now won't provide that.

As pilots, especially those flying recreationally, are painfully aware, any
forecast a day or more in the future is not to be relied upon. It gives you
a vague sense of what the most likely weather conditions may be, but that
forecast is almost never exactly correct. Even large scale questions like
"will there be rain?" or "will it be sunny?" can be wrong either in terms of
degree or time of occurrence, and fine details like temperature, wind speed,
cloud bases, and the like are almost impossible to predict with any accuracy
that far in advance.

For your purposes, I think the most useful information is "sunny or cloudy"
and "low clouds or high clouds". Looking for more detail than that from
these forecasts is likely to simply lead to disappointment.

Pete


  #3  
Old June 23rd 05, 01:09 AM
George Patterson
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Default

William_F wrote:

I note a forecast for clear skies and a cloud base of 12,000' to
"unlimited." I'm not sure what this means, does this mean that even
though it is supposed to be sunny, I may encouter clouds above
12,000'?


The aviation weather people typically don't provide info for clouds above
12,000'. Basically what they're saying is that they aren't forecasting for
clouds to occur, but maybe there will be some above 12,000'.

George Patterson
Why do men's hearts beat faster, knees get weak, throats become dry,
and they think irrationally when a woman wears leather clothing?
Because she smells like a new truck.
  #4  
Old June 23rd 05, 01:28 AM
Peter Duniho
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Default

"George Patterson" wrote in message
news:[email protected]
The aviation weather people typically don't provide info for clouds above
12,000'. Basically what they're saying is that they aren't forecasting for
clouds to occur, but maybe there will be some above 12,000'.


That's not what they are saying. If they forecast no clouds, then they are
forecasting no clouds. There's no "maybe there will be some", except
inasmuch as the forecast itself is subject to being inaccurate.

Pete


  #5  
Old June 23rd 05, 03:34 AM
William_F
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Posts: n/a
Default

Thanks for the advice, Peter. I thought I had stumbled on some secret
weapon that would allow me to pick my climbing days while friends sat
at home because rain had been forecast. I guess it's not that simple.
Oh well.

  #6  
Old June 23rd 05, 03:49 AM
BTIZ
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Start with temperature and dew point at ground level, apply standard lapse
rate to both until the expected temperature and dew point are the same or
within about 2degrees Celsius of each other..

That would be the "expected cloud base" if clouds were to form.
Depth of clouds or vertical development and resulting rain depend on other
criteria, such as instability in the atmosphere, upslope winds, amount of
moisture in the air, there may be enough for cloud development but not for
rain.

BT

"William_F" wrote in message
...
I'm not a pilot, but I've got a question pertaining to aviation
forecasts. I'm a mountain climber who has recently discovered
aviation forecasts that could be very useful in determining cloud base
heights. The idea being that alot of times I'll see a forecast that
calls for rain yet I find out on my climb that I'm easily above the
clouds for great climbing weather. My question has to do with
interpreting cloud base forecasts. For instance, at
http://usairnet.com/cgi-bin/launch/c...nge +Location,
I note that on Saturday at 11:00 am they are calling for a cloud base
of 3000-6500'. I'm assuming (maybe I'm way off, but that's why I'm
asking) that this means if I was climbing a mountain at 10,000', I'd
likely be above the clouds. Yet at 11:00 am on Friday at
http://usairnet.com/cgi-bin/launch/c...nge +Location,
I note a forecast for clear skies and a cloud base of 12,000' to
"unlimited." I'm not sure what this means, does this mean that even
though it is supposed to be sunny, I may encouter clouds above
12,000'?

Can someone set me straight on what this data is supposed to mean?



  #7  
Old June 23rd 05, 12:36 PM
Larry Dighera
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Posts: n/a
Default

On Wed, 22 Jun 2005 16:33:19 -0700, William_F
wrote in ::

I'm a mountain climber who has recently discovered
aviation forecasts that could be very useful in determining cloud base
heights. The idea being that alot of times I'll see a forecast that
calls for rain yet I find out on my climb that I'm easily above the
clouds for great climbing weather.


It seems to me, that you are interested in cloud tops as well as cloud
bases. Cloud top forecast information is difficult to find, but
pilots often report observed cloud top altitudes in what are known as
Pilot Reports (PIREPS). NOAA makes that information available he
http://adds.aviationweather.noaa.gov/pireps/java/
  #8  
Old June 24th 05, 02:55 PM
Dan D
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Posts: n/a
Default

try this:
http://www-frd.fsl.noaa.gov/mab/soundings/java/
once you learn to interpret skew T plots you can accurately forecast
cloudbase and get a good guess on top heights. I use it every day as a
soaring forecast tool.

"William_F" wrote in message
...
I'm not a pilot, but I've got a question pertaining to aviation
forecasts. I'm a mountain climber who has recently discovered
aviation forecasts that could be very useful in determining cloud base
heights. The idea being that alot of times I'll see a forecast that
calls for rain yet I find out on my climb that I'm easily above the
clouds for great climbing weather. My question has to do with
interpreting cloud base forecasts. For instance, at

http://usairnet.com/cgi-bin/launch/c...nge +Location,
I note that on Saturday at 11:00 am they are calling for a cloud base
of 3000-6500'. I'm assuming (maybe I'm way off, but that's why I'm
asking) that this means if I was climbing a mountain at 10,000', I'd
likely be above the clouds. Yet at 11:00 am on Friday at

http://usairnet.com/cgi-bin/launch/c...nge +Location,
I note a forecast for clear skies and a cloud base of 12,000' to
"unlimited." I'm not sure what this means, does this mean that even
though it is supposed to be sunny, I may encouter clouds above
12,000'?

Can someone set me straight on what this data is supposed to mean?



 




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