Decoding the Fronts

Guidelines for deciphering frontal characteristics

When it comes to teaching about frontal characteristics, aviation weather courses often paint misleading pictures. For example, how many times have we had it drummed into our heads that cold fronts have steep slopes, move relatively fast, and produce cumulus cloud formations? Or that warm fronts have shallow frontal slopes, move slowly, and produce stratus cloud formations? While these statements are generally true, they are also gross oversimplifications, and ones that are often disproved in the real world.

The fact is no two fronts behave exactly alike, and the differences between them can be very great, even though they may fall into very neat general categories-be they cold, warm, stationary, or occluded. Let's look at two case studies to prove the point. August 25, 1988, was hot and humid across the eastern half of the United States, just another day in the scorching hot spell that took hold over the nation this past summer. But a cold front from a low pressure center in southern Ontario spread from New England to Georgia. Someone fresh out of ground school might think that a nice band of thunderstorms would be in the offing, followed by a drop in temperature as the colder air mass moved east. But this was not to be. The difference between the air masses ahead of and behind the front was not that great. It was just as warm in Winston-Salem, North Carolina, as it was in St. Louis. The stability of the air masses was roughly the same, too. Neither was there much in terms of wind, either at the surface or aloft. So what the textbooks might lead us to believe would be a very convective day turned out to be just another arid scorcher.

On October 11, 1988, another cold front passed over the eastern United States, but this time the front was much stronger and brought the first hint of winter. The front moved quickly-at some 30 knots- and the cold, Arctic air behind it dashed with the warmer conditions that had been giving Easterners a very nice Indian summer. Surface winds hit 50 mph in many areas; sigmets were out for turbulence and icing in clouds and precipitation. In New England it snowed. Along the rest of the frontal boundary, rainshowers predominated, along with overcast skies, ceilings in the 2,000- to 3,000-foot range, and tops in the neighborhood of 25,000 feet. This was a situation more like those "typical" cold fronts that ground school instructors talk about.

Two cold fronts, but two very different ones.

Meteorologists use codes to define frontal characteristics, and we can see these codes on certain aviation weather products-namely the surface analysis and significant weather prognostic ("prog") charts. By knowing the codes, you can better understand the nature of a front and be better prepared to make your weather decisions.

The code uses a sequence of three numbers, with the first or last number framed by a bracket. The first number represents the type of the front, the second tells its intensity; and the third gives its character. The translations for the three categories of numbers are below (they can also be found in Aviation Weather Services, AC 00-45B, pages 57 and 59).

So the numbers mean a great deal. The cold front of August 25 was classified as a 420; the one on October 11 was a 462. Which one would you rather fly in? Without being asked for these descriptions, a flight service station specialist would probably never give you this information, but it's there-on every surface analysis and prog chart. Be sure to ask for it. Otherwise, you may find that the front you believe to be a 411 is really a 793. And that's a big difference.