"Ali Ghorashi" wrote in message
...
Can the temperature/dew point forecast be used to get a "rough" idea of
the
ceiling? They have those forecasted for several days in advance on most
popular weather sites.

-Ali



Well, yes and no.

If you have a reasonable suspicion that the cloud forecast is for convective
cumulus in a well-mixed boundary layer...then you could use the standard
"400 times (the spread in degrees celsius)" or "222 times (spread in degrees
F)", to get the base AGL in feet. Remember: this is ONLY accurate for
convective cumulus in a well mixed boundary layer.

One thing that you CAN use the temperature-dewpoint forecast for (if they
are accurate).... is to try to determine what airmass the forecasters are
putting you in, relative to the airmasses you see around the area. That can
help tell you whether you are "behind the cold front", or if it is still to
come... or are you "in front of the warm front" and can expect lowering
ceilings in the warm-front precipitation, etc.

Remember that, in many ways, the dewpoint forecast may be a better
indicator of the airmass than the temperature. If they are forecasting
"cloudy", or especially "rain", they will depress the forecast
"High-temperature" quite a bit due to no sun. But the dewpoint is
conserved fairly well within an airmass, regardless of other properties.

If you get a good estimate of your forecast airmass, and look at the
relationship of the site to the forecast pressure pattern, you certainly
might have a good guess whether to expect IFR, MVFR or VFR, especially if
you also have access to a "text" forecast specifying the cloud and
precipitation.

For example if your site is forecasting a temperature/dewpoint of 14/10 C
and "cloudy, chance of rain" as the outlook for that day...

.... and to the south of you, you see forecasts of 25/12 C and "partly
cloudy", then you and him could actually be pretty close to the same
airmass...

.... but if you also check the pressure-pattern forecast and find that you
pretty close to a low pressure system and slightly to the east or northeast
of it, and he is quite a bit south of there....

.... then it is a pretty good bet that you are being forecast as being right
*at* the warm front, and are probably going to be IFR or at best MVFR.

Will the ceiling be 200, 700, or 1200....??? I wouldn't hazard a guess,
even with the dewpoint forecast. (He, being well south, in the warm sector,
will probably be VFR in hazy Cumulus and/or an isolated airmass
thunderstorm, and I could certainly guess the bases at 5000-5500 there.)

....but say that the outlook for that day, rather than "cloudy, chance of
rain", was simply "cloudy"? Well, the estimate for the cloud base with that
4-degree spread could be anything. With the warm front, you can probably
expect the 8-thsnd-plus sort of middle cloud that has cut off the sun and
will hold the temperature down. But its not forecast to rain so will there
be any low level moisture to form low cloud? And if there is, will it form
the cloud at 3 thsnd or 3 hundred? I don't believe the dewpoint spread will
really help you there. For me, I would simply be prepared for IFR, only
because it is at the front, with a low nearby, and the potential for the
very low cloud exists.

Beware that you are not using the pressure forecasts from one agency, "text"
forecasts from another, and "dewpoint" forecasts from still another. In
these days of free-enterprise meteorology, the media produce some pretty
bizarre stuff with their own in-house staff. Choose who you trust, I guess,
but in North America, I would stick to NWS in the US, and Environment
Canada.... These are, I believe, the only providers of "official"
information to the FSSs.

As I said before.... YMMV ... greatly.