WX Watch: Front formation

Turning temperature boundaries into full-blown fronts

September 1, 2013

Fronts begin when adjoining air masses of different temperatures appear at the surface (far left). Divergent air created by a trough aloft moves over the air masses and create a low pressure center on the front (center). The low deepens, and a classic frontal complex ensues. Later, the complex moves east, following the 500-mb height contours.

Pilots are always on the lookout for fronts. We’re always gawking at The Weather Channel or checking the prognosis and surface analysis charts to see if any are lurking around, or are predicted to occur. Sometimes I think many of us believe that fronts just magically appear because they so often crop up in the space of a forecast cycle. One surface prog chart shows nothing, and 12 hours later, voila! There’s a front right along the proposed route of flight.

But fronts don’t just spring up without a reason. And if we’re to be air savvy the way our passengers would like us to be, then a little education can go a long way toward developing the weather-smarts we ought to have in the first place.

Typically, frontogenesis relies on dynamics both at the surface and aloft. It often begins with temperature differences near the surface. A boundary between cooler and warmer air forms. And this boundary needn’t be a very big deal at all. Very often the boundary may be a mere stationary front only a hundred miles long, and bearing little in the way of clouds. Sometimes you see these bits of fronts on surface analysis or prog charts, floating out there all alone without a parent low. But just because it seems benign now doesn’t mean you should ignore it.

This temperature boundary will remain just that until a trough aloft moves over it. Troughs aloft look like southward-dipping “U” shapes on constant pressure charts. Check the 500-millibar chart, a pressure level that roughly corresponds to 18,000 feet, to see them. Meteorologists call these troughs short waves, and short waves bring with them vorticity and divergence. Think of vorticity as counterclockwise spin, and divergence as air that’s spreading out.

What happens when vorticity and divergence move over the temperature boundary below? Pressures drop, and a low forms along the stationary front. Soon, a counterclockwise circulation around the low creates warm and cold fronts. Temperature gradients tighten, winds pick up, and rising air creates cloudiness and precipitation. If there’s enough vorticity and enough rising air, thunderstorms are possible.

As the surface low moves east, it’s pulled under the area of maximum divergence aloft. This is the stage at which the surface low and its fronts are most intense. After all, if there’s divergent air aloft, there must be converging air at the surface to feed the system. If the winds aloft are strong enough to create a core of jet stream winds, then watch out: the system is all the more likely to become convective.

Eventually, the cold front closes its distance to the warm front and an occlusion forms. Now the system loses strength as the surface low moves east of the zone of maximum divergence aloft and encounters higher-pressure air. And after, say, three days, our frontal drama ends.

You can watch all this beforehand as you check out both surface prog charts and upper-air constant pressure charts. On the ADDS website (www.aviationweather.gov/adds) you can see winds aloft predicted for the upcoming four days, and the same website shows surface prog charts out to 48 hours. So the next time you see a lonely bit of a front, don’t blow it off. See what the prog and constant-pressure charts have to say, then look over the Storm Prediction Center’s Convective Outlooks (you can link to these from the “Convection” tab on the ADDS site) for word of any adverse weather on the Days One, Two, or Three map and text explanations. Short wave moving over that mini-front? Better pay attention!

Email tom.horne@aopa.org

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.