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Air Mass, Temperature, and Pressure System
Name: Christopher
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
If low pressure air masses are created by warm
air rising, then why do we associate low pressure zones with
colder temperatures. In predicting weather, lower pressure
usually indicates colder temperatures, but that belies the theory
of its origin. Help! I cannot explain this hypocrisy to my students.
Replies:
Christopher -
Warm air does rise, but warm is relative. If it is warmer than the
surrounding air and if it continues to be warmer than the air as it
rises (and cools) it continues to rise... even if it is cool to
human body standards. In the northern hemisphere air being pulled
in to the low is bent in a counter clockwise fashion (Coriolus
effect) causing the air on the back side (as it moves west to east
across the US) to come down from the north. Also adding to the
coolness are the clouds that are associated with the low pressure
area. These clouds - cumulus clouds - are formed when the rising
air cools to the dew point causing the water vapor to condense.
These clouds limit the sunlight striking the surface of the earth.
In the Midwest (where I am from) a low often produces precipitation
and clouds in front of the pressure center as moist air is brought
up from the south. After the pressure center passes cooler air is
brought down from the north. Both tend to cause cooler conditions.
A high brings rather clear skies allowing the surface of the earth
to warm... warming the atmosphere.
Your are right. Lows tend to bring cooler conditions. Students are
always on the lookout for hypocrisy.
Larry Krengel
You are not being "hypocritical" about the relation of air
pressure, air temperature, and the water content of the atmosphere.
These are complicated enmeshed parameters, as the two (of many)
links that you can find if you search the term(s) "standard
atmosphere humidity" or similar term.
http://www.ae.su.oz.au/aero/contents.html
http://wahiduddin.net/calc/calc_da_em.htm
Low pressure is not always associated with colder temperature.
In fact some of the coldest temperatures in the "temperate" zones
occur when large high pressure areas come roaring out of the
northwest. Violent fronts are often the result of colder / denser
/ drier air "rides up" on top of warmer / less dense / more moist
air. The air temperature, pressure, and moisture content are all
complicated functions of one another, altitude, time of day/night,
geographic formations such as mountains, and a lot of other variables as well.
Possibly some of the "standard atmospheres" referenced above
will let your students "experiment" with weather conditions. But
you are starting off with a not-so-good premise that low pressure
is always associated with colder temperatures. It is much more
complicated than that.
By the way, it is not always a bad answer to tell young students
that some "explanations" are really complicated, which is the case here.
Vince Calder
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Update: June 2012
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