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Name: Christopher
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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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