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July-August 2011

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Forecast Center

Spring evokes thoughts of brisk showers, warm weather, and a fickle wind. But to meteorologists, one of the key signatures is the appearance of rich moisture from the Gulf of Mexico. If a stout high pressure area is located over the Atlantic coast or the ocean itself, a “southerly gradient” sets up that draws the moisture northward. Dewpoint temperatures which once ranged from 20°F to 40°F during the wintertime dramatically rise into the 60s and 70s on these south winds. Moisture, defined not by relative humidity but by dewpoint or mixing ratio, is by far the biggest contributor to the instability of an air mass and the appearance of showers and thunderstorms. Such a dramatic rise may mean dangerous weather, and this is the scenario we'll cover for this issue.

This weather map depicts a midday situation in April. Draw isobars every four millibars (996, 1000, 1004 mb, etc.) using the plot model example at the lower right as a guide. As the plot model indicates, the actual millibar value for plotted pressure (xxx) is 10xx.x mb when the number shown is below 500, and 9xx.x when it is more than 500. For instance, 027 represents 1002.7 mb and 892 represents 989.2 mb. Therefore, when one station reports 074 and a nearby one shows 086, the 1008 mb isobar will be found halfway between the stations. Then try to find the locations of fronts, highs, and lows.s.

TIM VASQUEZ is a former Air Force forecaster and author of Digital Atmosphere, a weather forecasting software program. He lives near Norman, Oklahoma, where he keeps busy as a weather consultant and software developer.

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