Icebound
December 17th 04, 03:59 PM
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.
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.