August 31, 2004
Weather fronts separate different air masses?
What are weather fronts? One of the first concepts we learned in meteorology was that of the large air masses that dominate our atmosphere. Weather fronts are just the thin boundaries between them. See more belowQ. What is an air mass?
A. An extended body of atmosphere with little change in temperature or humidity as you move across it.
Q. Across it?
A. Yes, if you move up or down, temperature and dewpoint can change dramatically. Usually both decrease at higher elevations.
Q. Is this a perfect definition?
A. No, things like cities and lakes can cause small scale but sudden changes in both these properties, and others, within an air mass. Without defining a new air mass.
Q. Are temperature and moisture content the only defining properties?
A. No, meteorologists talk about temperature humidity indices such as web bulb potential temperature (temperature corrected for elevation, pressure and humidity) as well as stability (the air's ability to resist flipping over and mixing). Graphs help us with the complicated mathematics involved with these concepts.
Q. How big are air masses?
A. A thousand miles (kilometres) or more across. But they can be squeezed together and made smaller.
Q. What is a weather front?
A. The division between air masses. Cold fronts where the colder air mass is replacing a warmer one, and warm fronts where the colder air mass is getting replaced by a warmer one.
Q. Don't they call these "zones"?
A. Yes. A baroclinic zone is the area between two masses where the properties change quickly over a relatively short distance. As opposed to a barotropic zone, inside a single air mass, where there is almost no change over even a long distance.
Q. How do air masses develop?
A. Air masses form when air sits or slowly moves across a large uniform region. Eventually the air acquires properties from the surface of this region.
Q. Is there a pattern of air pressure within air masses?
A. Often, but not always, the pressure is highest near the center of the air mass.
Q. Do they have names?
A. Yes, the two letter designations tell you something about the air mass.
Types of air masses:
Temperature - Capital letters
T - tropical, warm or hot like a tropical rainforest climate. They form in near the equator but can wander as far poleward as Canada in the summertime.
P - polar, average. Considered cool in summer and mild in winter, depending on where you live.
A - arctic, coldest. Welcome to Alberta Canada Weather.
Moisture: - lower case letters.
c - continental, dry.
m - maritime, moist.
A capital letter above paired with one of the lower case letters tells you a lot about the air mass, such as mT for maritime tropical: warm moist tropical air.
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