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Flavors of Low
Thu Dec 09, 2004
Listen in RealAudio 
You have probably heard your favorite meteorologist using the terms "extra-tropical,"
"subtropical," or just plain "tropical" when talking about low pressure systems. Well,
they’re all storms, so they are all probably pretty similar, right? Not at all! Hi, I’m Bryan
Yeaton, and this is The Weather Notebook.
These variants on the Low Pressure theme differ not only in origin, but also in
structure. Your everyday, garden variety low, the "extra-tropical," or "mid-latitude," is
generally born in an area where very large cyclonic masses of air are swirling — off the
southwest coast of Alaska, or over Iceland, to name a couple. In these storm factories,
bits of swirl become teenagers, and break away to form their own lives. One of the
main discerning components of the extra-tropical low is the frontal boundary: the
often-tempestuous zone between it and an area of higher pressure. The fronts are
where the heavy winds are found, sometimes hundreds of miles from the center of
circulation.
Tropical lows, on the other hand, are children of the warm equatorial waters — but not
right over the equator, at least not until last year. If conditions are right, with winds light
aloft but converging at the surface, and unstable air up high, a wave of thunderstorms
may organize into an intense low — sometimes eventually into a hurricane! These are
sometimes called "warm-core" systems, and the winds are strongest near the center
of circulation.
Subtropical systems exhibit characteristics of both tropical and extra-tropical, and are
often formed when those two meld together. Depending on circumstances, the
subtropical can morph into one of those other systems. Probably the most famous of
these occurred when the remnants of Hurricane Grace merged with a strong
extra-tropical system in October 1991. It became known as "The Perfect Storm."
The Weather Notebook is a program of The Mount Washington Observatory, funded by
Subaru of America.
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