EYE ON THE SKY

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Highs and Lows

Large masses of air of generally consistent temperature, humidity and barometric pressure are constantly circling the earth, driven by the jet stream (very strong, high-altitude winds which blow from west to east in the Northern Hemisphere) and local winds. When a large mass of warm airy meets a colder one (or vice versa), a front is established. All along the front, the weather has a tendency to become unstable. Low fronts - ones in which low barometric pressure (30 inches or less) dominates - are the bearers of rain and snow. Air systems with high barometric pressure (30 inches or higher) bring fair weather. It is easy to tell one from another, as low fronts spin counterclockwise as they travel and high fronts spin clockwise. Knowing the wind direction, therefore, is a terrific first step in forecasting. If a cold front is approaching, the heavier cold air will push under the warm, and there may be severe weather (thunderstorms in summer) of short duration. If it is a warm front, the lighter warm air pushes over the cold, and there's likely to be protracted (but usually not very heavy) precipitation. Most often, cold fronts come and go quickly, while warm fronts give more warning and last longer. On average, a cold front will pass through every five to seven days, so you're not being paranoid if you feel as if it only rains on weekends.

* * * FORECASTING HINTS * * *

APPROACHING BADWEATHER
*Barometer falls continuously/quickly.
*Smoke from chimneys lowers.
*A ring or halo around the moon.
*Leaves show their backs.
*Strong wind in the morning.
*Red sky in the morning (the old
proverb was right).
*Temperatures distinctly above or
below normal for the time of year.
*Static on AM radio.

APPROACHING FAIR WEATHER
*Cloud bases rise.
*Smoke from stacks rise.
*Wind shifts to west, especially from east through south.
*Barometer rises quickly.
*A cold front has passed in the past four to seven hours.
*Gray early morning sky shows signs of clearing.
*Morning fog or dew.

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