"Icebound" wrote in message
...
This posting is in response to the "weather watch" article in the
December issue of EAA's Sport Pilot Magazine. I hope you forgive the
cross-posting to these three groups.
Obviously, the article was not meant to be real deep on detail, but
there are a few significant statements and implications that warrant
some discussion.
1. "Warm air can hold more water vapour than cooler air so cooler air
will reach its dew point ... sooner"
Whether the air is warm or cool is not the issue. The issue is the
spread between the temperature and the dew point.
Warm air with a small dewpoint spread is much more dangerous than
cooler air with the same spread.
*Because* warm air (say: above 20 or 25 deg C) can hold more moisture
(and therefore has more moisture to condense), the visibility will
start to lower with dewpoint spreads as much as 3 or 4 degrees. When
the temperature is down around minus 10 degrees C or less, visibility
rarely deteriorates until the dewpoint spread is less than 1 or 2
degrees C.
2. "Humid air rises relative to dry air because of the lightness of
water vapour as a gas."
That may be technically true, but air-masses tend to have more-or-less
the same humidity characteristics throughout the entire air mass. So
there is no "relative to dry air" for the supposed convection to take
place.
Humid air tends to exhibit instability more than dry air in
practice... because humid air, in rising, will reach the condensation
point sooner. Thereafter it will not cool as rapidly, and is more
likely to remain warmer than the rest of the environmental air, and
hence more buoyant.
Dry air if often the result of a high pressure system, which produces
a general subsidence in the area, and is not at all conducive to
rising anything.
3. "A unit of air rising will cool 5.5 deg F per 1,000 feet of
altitude in relation to the larger mass of air surrounding it, until
it reaches temperature equilibrium. If this rising air does so at a
rate less than the DALR (dry adiabatic Lapse Rate), the air is stable.
Air is unstable when rising at a rate greater than the DALR."
I thought air rose *AT* the DALR, not "less than" or "greater than".
I am sure the statement was *supposed* to say something like....
"...after rising, if the resultant temperature of the risen air is
less than the ambient temperature of the environment at that level,
then the air is stable. Air is unstable if the resultant temperature
of the risen air is greater than the ambient temperature of the
environment at that level."
4. In a later paragraph: "Air rising at or slower than the MALR
(moist adiabatic lapse rate) is more stable than air rising at or
faster than DALR".
WHAT?
I have no idea what she is trying to say. The MALR and DALR have
nothing to do with speed. And if we are talking about the speed of
temperature change, the MALR is ALWAYS "slower" than the DALR, and in
any case, the air is either cooling *AT* the MALR or *AT* the DALR.
5. There is the description of the famous formula for determining
cloud height from dewpoint spread, along with the statement: "It
indicates changes in base heights taken over time; that is, are the
clouds lowering?"
This formula is useful primarily for the determination of the bases of
*cumulus* clouds in fair weather, and possibly stratocumulus following
a cold front. Be *very, very* careful trying to apply it to any
other meteorological situation. In fact... DON'T!
6. In a sidebar on Density Altitude is the statement: "Even though you
may think that humid air is more dense than dry air, the fact is that
the molecules of water suspended in the air force other gas molecules
farther apart to make room for them. So, even though humid air may
weigh more per unit volume, there is actually less air for your wing
and propeller to use."
In humid air, the molecules of water replace the molecules of air.
Since air is mostly nitrogen and oxygen, whose atomic masses are 28
and 32 respectively, and water has an atomic mass of 18... every
replaced molecule results in a lighter and lighter air mass. Humid
air does never, Never, NEVER "weigh more per unit volume" than dry
air. Your wing and propeller uses molecules of water vapour just the
same as the molecules of any other gas. It is just that these
molecules happen to be lighter, thus are easier to displace, thus
provide less "reaction", and therefore less lift.
I don't believe you can expect much in the way of technically competent
editing in an EAA publication.
John Lowry
Flight Physics
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