Wx Watch: Fronts on the Move

Making surface analysis charts come alive

October 1, 2008

Pilots are always looking for easy answers to weather questions, most of which relate to determining forecast conditions. Unfortunately, the terms “easy” and “forecasting” seldom go together.

But if there’s anything you can hang your hat on, knowing the basics of the cyclone model can help simplify your visualization of the weather to come. The cyclone model is a fancy term for the way low pressure centers and fronts are organized. It’s a conceptualization that explains a great deal about how weather works—in a very general sense, of course—and it’s something you can see at work on a surface analysis chart.

A low pressure center is at the hub of a classic cyclone model, and fronts radiate from it. Typically, a cold front extends to the south or southwest of the center of low pressure, and a warm front extends to the east. And because air moves counterclockwise around the low pressure center (in the northern hemisphere), the fronts move counterclockwise, too. Meanwhile, the whole complex moves in an easterly direction thanks to steering winds aloft.

Sectors

Think of low pressure complexes in terms of cold and warm sectors, with fronts marking the dividing lines between the differing air masses. You can also think of fronts as war zones where clashing air masses create some of the worst possible flying weather.

As a matter of fact, weather fronts were named after the frontal warfare in World War I. Up until that time, there was no concept of weather as occurring along lines of air-mass disparity. Instead, precipitation and thunderstorms were thought to happen in more or less random patterns. It took a group of Norwegian meteorologists in 1916 to come up with the concept of fronts.

The cold sector contains air from northern, or cooler, areas experiencing comparatively high barometric pressure. Because cool air is dense, it tends to sink, and this is why conditions behind a cold front are relatively stable.

Cold front

When a cold front arrives you can expect the following:

  • Its colder, denser air wedges beneath the preceding warm air mass, causing the warm air to rise along the frontal slope, which in turn can cause:
  • Thunderstorms and/or showery precipitation,
  • Surface winds out of the southerly points of the compass, followed by winds out of the northwest—possibly strong and gusty,
  • Falling altimeter settings, and
  • Turbulence and icing associated with cumulus clouds and convection.

Warm sector

Once the cold front has passed, expect:

  • Clearing skies,
  • Lower dew points,
  • Lower freezing levels,
  • Rising altimeter settings, and
  • Possible strong and gusty surface winds out of the northwesterly points of the compass.

The warm sector—behind the warm front, but ahead of the cold front—consists of less dense air. That’s why the leading edge of the warm front rides up and over the cooler, ground-hugging air ahead of it; it’s too light to wedge itself beneath the preceding air mass. This process, called overrunning, is what causes the widespread cloudiness and precipitation associated with warm fronts.

Warm front

As a warm front passes, expect:

  • Winds out of the south,
  • Rising altimeter settings,
  • Widespread stratus clouds in layers,
  • Continuous precipitation,
  • Warmer surface temperatures,
  • Higher dew points,
  • Widespread convective weather, and
  • Higher freezing levels.

In the winter

Winter warm fronts deserve special mention. That’s because their shallow frontal slopes can produce large areas of varying types of precipitation. Think about it: Warm air aloft, packed with moisture, shedding rain into cold air below. Depending on the temperatures on the way down, and the distance the precipitation has to fall, conditions could range from snow to freezing rain to ice pellets to just plain rainfall. The biggest problems occur when snow falls through a warm layer of air, and melts to raindrops, which then become supercooled as they pass down through a subfreezing layer of air. The moral: Watch temperature aloft very closely when flying in or near winter warm fronts. And keep an eye on your outside air temperature gauge.

You can expect icing conditions—usually rime, but clear ice happens most often in temperatures between minus 10 and 0 degrees Celsius—in clouds and precipitation any time a winter warm front is around. That sort of goes without saying. But here’s something else to remember: If rain falls into the cold air retreating ahead of a warm front, then freezing rain can happen in what can look like clear, cloud-free conditions!

Conveyor belts

Frontal complexes obeying the rules of classic cyclone-model behavior are fed by upper-air flows that give the system its different moisture and temperature properties. Meteorologists call these flows “conveyor belts,” and you can often see them on satellite imagery. A warm conveyor belt feeds the warm sector with winds out of the south, carrying large amounts of moisture—often from the Gulf of Mexico. This belt rises in altitude as it makes its way north, then gets blown eastward at about 30,000 feet or so by the prevailing westerlies at those altitudes.

A cold conveyor belt works north of the warm front. It brings cold air aloft from the east, parallels the warm frontal alignment, and then wraps around the surface low pressure center. It’s this conveyor belt that spirals around the low, bringing “wraparound” precipitation and icing conditions with it.

Finally, there’s a dry conveyor belt. This flows northeastward, right behind the cold front. As the name implies, the dry conveyor belt imports low-humidity air and makes for the clear skies so frequently found after a cold front’s passage.

If conditions are right, you can see all three conveyor belts at work on satellite imagery. The warm conveyor belt makes an S-shape—the top part of the “S” being the portion turned to the east. The cold conveyor belt makes a spiral band of clouds, and the dry conveyor belt leaves a nice wedge of clear air on the image, as represented by a dark, cloud-free area behind the cold front. This is called the “dry slot.”

Next time you scan a surface analysis chart, test your knowledge of the cyclone model. Note the station models and the wind directions, temperatures, and dew points at each. You’ll quickly identify the warm and cold sectors. Ditto satellite imagery, especially if you can impose satellite imagery on surface analysis symbology. I had a meteorology professor who said that understanding the cyclone model is perhaps the most useful concept in learning how weather works. That’s something worth thinking about.

E-mail the author at 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.