Ask A Scientist Weather Archive

From: Steve

I'm currently studying for my private pilot's license
and while I get some of the basics of weather, I'd
rather be a safer pilot by understanding how it
actually works.  I know that air flows from an area of
high pressure to an area of low pressure, and that
while temperature increases are directly proportional
to pressure increases in a closed container, the
atmosphere doesn't work that way.

What I get confused about, however, is the relations
between barometric pressure, temperature, troughs and
ridges.  If cold air forms high pressure areas because
it is heavier, why isn't barometric always higher when
it's colder?  Is this because of the water vapor in
the air that could cause it to be lighter than dry,
hot air?  When they mean high pressure areas, are
those that are high in general, or are they just
higher than the air masses surrounding us, and lastly,
are highs always going to be cool air descending?  I
don't see how we could have a high of hot air, since
hot air always rises.
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Steve,

Confusion about this is normal.  High and low pressure
systems are high and low in relative terms.  A low pressure
system has lower pressure than an adjoining high pressure
system.  The pressure in either of these (highs and lows)
has a tremendous range: for the center of highs the range
can be anything from 970 to 1100 millibars and for lows it
can be anything from 890 (strong hurricane) to 990 millibars
You'll note that there is some overlap in the ranges.

High pressure occurs because there is more atmosphere (and
therefore more weight of air) in the column of air above.

Low pressure occurs because there is less atmosphere (and >therefore less weight of air) in 
the column of air above. >
Air moves from higher to lower pressure, as you say.  Because
there is less air in the low pressure system column, air from
a high pressure area tends to flow towards it at the surface,
feeding into the low pressure center.  Therefore, the
air in a high pressure area is generally descending.  That's why
continuous decks of clouds are suppressed in highs.  The air
needs to rise to cool and produce clouds.  So in highs, only
small scale, locally rising thermal plumes of air can produce
clouds (cumulous).

In lows, air is being pulled into it at the surface and the
air is rising, attempting to equalize pressure (in other
words, equalize the weight of air) with adjoining
high pressure areas.  The rising air leads to continuous cloud
decks and sometimes violent thunderstorms embedded in cold
fronts.

Highs and lows as we know them would not exist if it weren't
for the rotation of the Earth.  The rotation causes turning of the
atmosphere over the Earth's surface.  The strength of that
turning depends largely on the difference in pressure between
adjoining low and high pressure areas; that difference is the main
forcing element. However, horizontal temperature gradients are also
very important.  The temperature gradients don't always line
up with pressure gradients, as you mentioned, primarily because
temperature gradients are basically the consequence of a north to
south difference in solar radiation received in the atmosphere
and at the Earth's surface.  This results in differential heating
(generally greater at the equator than at the poles).  The pressure
gradients generally result from the turning of the large air
masses by the Earth's rotation.

The temperature in each of these systems is almost irrelevant,
as you can see that the air in the center of high pressure areas
tends to be colder than lows in the winter, whereas in summer
the opposite is often true.
 
 Warm fronts lead movements of warm air (usually moving northward
or eastward, in the northern hemisphere), whereas cold fronts trail
from low pressure centers.  Warm air tends to move towards (and over)
cold air in front of it, thereby allowing cold air to fill in
from behind.  The low pressure center forms and further
develops at the point of lowest pressure (least weight of air) in a
trough (which is just an elongated area of pressure that is lower
than the adjoining areas on either side). A cold front tends to
form in the trough, since warm air is usually to the east and cold
air is to the west. A ridge is just an elongated area of higher
pressure, analogous to a trough of lower pressure.  Troughs and
low pressure centers often form below the jet stream, a result of
the jet causing air to tend to rise underneath it.

With that as background let's get to your specific questions.

From the discussion above, you can see that cold air doesn't form
high pressure areas because it is heavier (it is only denser, meaning
that there are more air molecules per volume).  The total amount of
molecules in the column above determines the pressure.

Water vapor does displace dry air molecules, making the air less dense
(this is a small effect), but again, it's the total amount of
molecules in the column above that determines the pressure.

You ask, "when they mean high pressure areas, are those that are high
in general, or are they just higher than the air masses surrounding us?"
You can see from the general discussion above that it is the latter
of your two statements.  High and low pressure is relative to the
pressures of the air masses that are adjoining.

You ask, "are highs always going to be cool air descending?  I don't
see how we could have a high of hot air, since hot air always rises."
As you can see from what I wrote above, the air in a high is descending
no matter what the temperature is in it; it's weight is actually causing
it to compress the air (over all) in the high.  This causes elevated
inversions, sometimes at several altitudes, from daily surface inversions
that are carried upwards during the following morning.  When air is
descending in general, obviously it cannot rise.  Instead, as the surface
of the Earth is warmed during the daylight hours, the air above it is
warmed, the air expands, and the high pressure area grows slightly in 
height.
All the air has done is became less dense, not less heavy, so the 
tendency is
for the air to still want to descend.  In the summertime, this process can
create huge, stable, and expanding (both vertically and horizontally) high
pressure areas which can become hot, humid and smothering.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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