On the lookout: Cold fronts

Like sailors, we need to stay aware of our surroundings to become weather savvy. Of all the weather events, fronts—boundaries between two different air masses—probably attract the most attention. Warm fronts occur when warmer air replaces colder air. Stationary fronts are, well, stationary, and don’t move much until a new system comes along.

Cold fronts get a lot of attention because they can generate plenty of lifting forces and cause a lot of adverse weather in a short time. How can you, standing on a ramp at your airport, sense that a cold front is approaching?

The first clue will be the wind direction. It will be out of the southerly points of the compass, and increasing in strength. Air is being drawn toward the cold front’s parent low pressure center somewhere to the north, and now its direction parallels the approaching cold front’s northeast-southwest orientation. As the front gets closer, surface winds will pick up, and reach 20 knots—maybe more, if the pressure gradient is strong enough. Is the altimeter setting high at your airport? Say, 30.20 inches of mercury, and falling rapidly? That’s when you’ll see the “PRESFR” (pressure falling rapidly) note in any nearby METARs, that’s a sign of a big pressure difference, so the gradient’s steep, making strong surface winds a pretty sure thing. If there’s a barometer in your flight school office, you’ll see that its needle confirms a drop in pressure.

You’ll notice that high temperatures accompany those prefrontal winds, which is what you might expect from winds out of the south. It will be humid, too. You’re in the warm sector of the frontal complex, and it can get muggier as the cold front makes its entrance.

A shift in wind direction is a first sign that the front has arrived. Where once winds blew out of the south, a check of the windsock will show it swinging around to the west or northwest. Now the front is barging ahead, pushing all that warm, muggy air ahead of it. The arrival of cooler temperatures is another sign that the cold front has arrived. The colder air wedges beneath the warm air, creates strong lifting motions. The result: The rising air cools as it rises, causing condensation aloft—and plenty of clouds. If there’s enough moisture, count on rain—and perhaps thunderstorms.

After an hour or two the cold front will have passed to the east. Surface temperatures are noticeably cooler, and as atmospheric pressure rises, gusty westerly winds could easily reach 20 knots, with gusts to 30 or more knots. Now’s not the time to practice takeoffs and landings, unless you have an instructor aboard.

And that barometer in the office? It’s moving to the right with determination—confirming the front’s passage. Want to see it in writing? You might just see a new note in the METARs: FROPA, short for frontal passage. Soon, skies will clear, and the stage will be set for a few days worth of good VFR weather. Until the next front comes along, that is.

No two fronts are alike, but the scene we just visited is fairly typical of summertime cold fronts. It’s good to know that the faster a cold front moves, the bigger the temperature and pressure differences across the front, and the higher the prefrontal moisture ahead of it, the more violent the weather.

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