Weather Article In EAA Sport Pilot Mag

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.

"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

John T Lowry wrote:

I don't believe you can expect much in the way of technically
competent
editing in an EAA publication.

----------------------------------------------------------

Roger that.

Infomercials and ego-articles have become the order of the day.
-R.S.Hoover
-EAA 58400 (Life Member)

Thanks a lot guys. I try to take what is see as a problem partly from
reading this news group and expand your expertise into a publishable
article. I have also published articles that are submitted.

If you find a problem with an EAA article, why not write or talk to the
author. Or write an improved article. Feed back is always helpful.

No, I didn't write the weather article and I'd be the last to say that
anything I write is the last word on my subject area of safety.

Again if one of you can write a better more accurate article, I can put you
in touch with an appropriate magazine editor. But unless you talk to the
people that publish with your ideas and improvements, your complaint does
little good.

Cy Galley
EAA Safety Programs Editor
Always looking for ideas and articles for EAA Sport Pilot

wrote in message
oups.com...

John T Lowry wrote:

I don't believe you can expect much in the way of technically
competent
editing in an EAA publication.

----------------------------------------------------------

Roger that.

Infomercials and ego-articles have become the order of the day.
-R.S.Hoover
-EAA 58400 (Life Member)

On Sun, 19 Dec 2004 04:14:19 GMT, "Cy Galley"
wrote:

Thanks a lot guys. I try to take what is see as a problem partly from
reading this news group and expand your expertise into a publishable
article. I have also published articles that are submitted.

If you find a problem with an EAA article, why not write or talk to the
author. Or write an improved article. Feed back is always helpful.

No, I didn't write the weather article and I'd be the last to say that
anything I write is the last word on my subject area of safety.

Again if one of you can write a better more accurate article, I can put you
in touch with an appropriate magazine editor. But unless you talk to the
people that publish with your ideas and improvements, your complaint does
little good.

Cy Galley
EAA Safety Programs Editor
Always looking for ideas and articles for EAA Sport Pilot


I dont know why you are so defensive Cy. I havent received my magazine
yet but what was quoted in the guy's rejoinder sure seemed the correct
take on the met situation to me. I think the author of the article has
a better understanding of met conditions than our friend doing the
criticising.
I'll comment further when I've read the magazine but her take on it
seems in accord with australian commercial pilot standard metrological
training.
Stealth Pilot