Wx Watch: FROPA Basics

Signs of a change in weather

March 1, 2006

You're standing on the ramp, about to climb into your airplane and take off. You notice that the wind has picked up, and your airport's latest AWOS (automated weather observation system) broadcast says that the altimeter setting has just dropped a bit. What do you make of this? Does it have any significance to your flight? Those with a good handle on the basics of frontal passages might become curious enough to walk back to the FBO's weather terminal for a last-minute check of any new developments.

There are several markers that indicate the passage of a "typical" surface front. Knowing them can give you an edge when forecasts are few — or very old — or when forecasters are vague about the timing of a frontal passage. And you don't need a lot of fancy equipment, although a barometer — or the altimeter — can make for a nice way to quantify pressure changes. All you need to know are wind speed and direction, temperature, and humidity. Here's why each element plays a part in revealing frontal passage — FROPA, in meteorological slang.

Atmospheric pressure. Barometric pressure always falls before frontal passage, reaches its lowest as the front moves by, then rises after passage. Fronts cause air masses to lift, and it's this upward motion that lessens the effective weight (and therefore, pressure) of the air molecules acting on the surface. If you had a barograph (an instrument that plots atmospheric pressure on a rotating drum) you'd see that these pressure fluctuations are many times quite easy to see. Of course, the faster the pressure fall ahead of a front, the more likely it is to bring thunderstorms and/or heavy precipitation. Your altimeter — being a calibrated barometer — also can indicate pressure falls and rises. If, during your preflight inspection, you noticed your altimeter showing a reading, say, 500 or 1,000 feet higher than it did when you parked the airplane yesterday, then you know that a front might be nearby. Conversely, a reading lower than before may indicate that the front has moved off, and that higher pressure is building.

Wind. Most fronts bring wind shifts. Ahead of a cold front, winds are typically out of the southerly quadrants of the compass — say, from the south or southeast. As the front passes, winds switch around to the westerly quadrants, and blow from the west or northwest. When a warm front goes by, winds typically switch from an easterly or southeasterly direction to southerly directions. With really strong cold fronts, the prefrontal surface winds can be violent, with gusts to 30 or 40 knots — and even higher in any prefrontal thunderstorms. On a surface analysis chart, closely packed isobars are other surefire indicators of an aggressive cold front's strong, gusty winds.

Temperature. This one's easy. You'll know a cold front has passed when temperatures drop. With warm fronts, temperatures rise. Strong, warm winds out of the south often presage the passage of a strong, fast-moving cold front. The greater the temperature differences between the pre- and post-frontal air masses, the more likely the frontal passage will be a violent one.

Humidity. Humidity, as evidenced by the temperature-dew point spread, can be another sign of frontal passage. Ahead of a typical cold front, temperature-dew point spreads will be close, and humidity will be relatively high. As the front moves by, the temperature-dew point spread widens, and humidity decreases. With warm fronts these changes may not be as evident because of the shallowness of the warm front's slope and the more gradual change in the weather.

Clouds. Ever wonder why they made you learn cloud types in ground school? One big reason is that cold fronts and warm fronts typically bring cumulus and stratiform clouds, respectively, with them. Cold fronts wedge cold air under warmer air to make the strong vertical motions — and thunderstorms and turbulence! — so often associated with their steeper cold frontal surfaces. Warm fronts ride up over the colder air ahead of them, so there's often less abrupt lifting, and therefore, stratus-type clouds predominate.

Put all these elements together and you can develop quick instincts about the weather. Just ask yourself where the wind is coming from, and how strong it is. Has the barometric pressure changed? If so, has it risen or fallen? Are clouds building or dissipating? Answer these questions and you'll have your own "nowcast."

I remember one trip when flight service advertised a cold frontal passage to occur three hours after my planned departure time. No problem, I thought, as I drove to the airport. I'd be en route, flying away from the front, before it arrived. But as I walked out to the airplane a noticeably warm wind out of the south picked up. Flags snapped in the breeze, which rapidly became a gale (wind speeds of 28 to 55 knots on the Beaufort scale). Overhead, lowering, deepening stratocumulus clouds raced out of the south. The first drops fell from what would soon become a line of thunderstorms overtaking the field. The front had picked up speed, and would arrive sooner rather than later. No flying that day!


E-mail the author at tom.horne@aopa.org.


Wx Briefs

Tip of the month

Fog is likely to form on cold, clear nights. Everybody should know that. That's when heat radiates away from the Earth's surface and lowers temperatures to the dew point. When temperatures and dew points get within a few degrees of each other, presto! Fog forms. But when can you expect the densest fog, the kind that can lower visibilities to a quarter-mile or less?

This can happen the night after an afternoon's worth of soaking rains, in low-lying areas. The rain saturates the earth, which in turn imparts lots of moisture to the air above it. As nighttime temperatures drop, temperatures meet dew points and cold air sinks to low points such as valleys and riverbeds — which are where many airports are located. Fog might have formed anyway, but the extra moisture from the rainfall (and its evaporative cooling) is a virtual guarantee of the kind of dense fog that makes any pilot think twice about taking off — or landing. These sorts of situations often occur after a slow-moving, rainfall-laden frontal passage, when skies clear late in the day.

Hemingway's Hurricane

In the days before satellites sent us cloud imagery and there were no hurricane-penetration flights, hurricanes snuck up unannounced. Fast-moving hurricanes simply appeared without warning. So it was with the Labor Day hurricane of 1935, which killed hundreds in the Florida Keys. One group of 250 doomed workers — World War I veterans — building a highway to Key West was supposed to be picked up by a rescue train, but it arrived too late. Famed author Ernest Hemingway, a Key West resident at the time, drew attention to the tragedy, and congressional hearings eventually were held. But the damage was done, and all that remained were the lessons. Among them: Don't give weather observers vacations on Labor Day.

Phil Scott, a regular contributor to AOPA Pilot, has written Hemingway's Hurricane, a dramatic narrative that chronicles the botched evacuation plan and profiles key characters of the disaster. It's a fast-moving, highly educational read for those with a historical bent and for anyone interested in meteorological history. It's especially poignant given Hurricane Katrina's impact — and that was a storm that was tracked extensively by satellite and direct observation. If you liked The Perfect Storm, you'll love Hemingway's Hurricane. This 288-page $24.95 hardcover book is available at bookstores everywhere, and online ( www.amazon.com), where it earned five-star reviews. — TAH

Thomas A. Horne

Thomas A. Horne | AOPA Pilot Editor at Large, AOPA

AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.