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Name: DTuc
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Date: Tuesday, April 23, 2002

An episode of one weather type usually gives way to another weather type, very-abruptly, in a single day. Why is this?

Dear DTuc-

When referring to "weather episodes," I assume you mean dry vs precipitation. Precipitation tends to occur along boundaries of differing air masses, and areas of precipitation, usually are narrow in width. As these boundaries (fronts) sweep across a given area, the weather changes can be dramatic.

Here is a link that has some good explanations of what drives weather systems. Check especially the topic on "Storms and Fronts."




What you describe occurs because so many dramatic weather events or situations occur along "boundaries" between different air masses or underneath usually narrow jet streams. Good examples are the thunderstorm lines (squall lines) that form a hundred miles or so in front of a cold front and move very quickly through a local area or a cold frontal passage with its accompanying rain or snow, followed within a day by colder and clear air within the trailing high pressure area. Even frontal related thunderstorms can be caused by an overlying jet stream that is aligned with the front; once this passes or loses strength you get an abrupt change to another very different weather regime.

Opposite to this are the periods of several consecutive days of nice weather associated with a high pressure system, separated by many miles from any meteorological boundary.

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

Well, DTuc, most of what we call weather - clouds, wind, rain, snow - is associated with fronts. A front is where two air masses meet. The characteristics of one air mass is different from the one taking its place. The frontal passage can be very abrupt (but not always). Cold fronts can move at over 50 miles per hour. When one of these comes through, it could seem very abrupt.

Larry Krengel

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