WX BRIEF

Keeping weather flying rules in mind

Basic atmospheric knowledge that's essential for pro pilot flying.




Fronts

This 500-hPa chart corresponds to conditions at about 18,000 ft. Pilots should quickly note the trough and strong winds over the eastern US. In addition to likely turbulence around the base of the trough, adverse weather is most likely found beneath the downwind leg of the trough—indeed, at the time, a major blizzard was paralyzing east coast air traffic.

Above the surface layer, fronts are common culprits for bad weather. Fronts are the leading edges of the large-scale movement of cold and warm air around a low-pressure center. The cold air represents an obstacle around which warm air must be displaced.

If surface dew points in the warm air are relatively close to the air temperature, clouds will result. The higher the dew points, the thicker the clouds are likely to be, and the more likely they are to produce precipitation.

In a warm front, the cold air is retreating, which leaves a gradual upslope for the warm air to ride up. Except for the most vigorous warm fronts, the result is normally nonconvective stratus clouds that may be only a few hundred feet thick, but with bases that can near the ground as the front is approached.

Occasionally, especially near the surface frontal boundary, nimbostratus may develop, resulting in persistent precipitation. Stratus is also common with stationary fronts. Conversely, cold air advancing behind a cold front pushes forward as more of a wall, over which the displaced warm air is forced at a rapid pace.

The rapid uplift of the warm air often destabilizes it into significant cumulus clouds and frequently a line of cumulonimbus. Rapidly moving cold fronts—moving as fast as 30 kts—can also produce an atmospheric wave that, if there is sufficient heat and moisture in the air, can kick up a squall line 50–100 miles ahead of it.

Both cold and warm fronts tend to be preceded by high-altitude cirrus, and conditions tend to be more benign behind the fronts. However, the closer you fly to the center of the surface low, the more likely you are to encounter adverse weather regardless of where you are relative to any fronts.

Behind a cold front, even though the skies may be clear, winds are usually a factor. All pilots should be aware that closely spaced isobars on a weather map mean strong wind. The isobars are simply a visualization of pressure changes over a distance—and, as previously discussed, the greater the change, the greater the windspeed.

The winds around the low will always be moving counterclockwise in the Northern Hemisphere and clockwise in the Southern, and they always spiral inward toward the low at the surface. Winds are opposite, and usually much weaker, around a surface high. Above the surface, away from its frictional effect, the winds will not spiral inward, but rather will parallel the isobars.

Coasts also present a front of sorts. The transition between land and water also represents a substantial change in the local diurnal heating and cooling pattern. Pilots who fly around the coasts are all too familiar with sea breezes that blow inland during the day and offshore at night, carrying with them lines of thunderstorms that will hit a coastal airport at almost the exact same time each day.

During the day, many pilots flying a coastal route will stay offshore to minimize turbulence and clouds, while at night they will fly a land-based route.

Finally, while there are a few research pilots who fly into thunderstorms on purpose, every pilot should recognize the omnipresent and often mortal danger presented by thunderstorms.

A thunderstorm should never be penetrated intentionally, regardless of its strength, as even the weakest thunderstorms contain enough concentrated energy to exceed the structural limits of most aircraft. All thunderstorms that you know about—whether visually or otherwise—should be given a berth of at least 20 miles, and never flown beneath.

So what amount of meteorological knowledge should every pilot have? Certainly, the preceding is a good start—and, arguably, no pilot should know less. For some pilots, simply knowing how to read a weather map or the clouds is sufficient, while for others only a comprehensive understanding of the inner workings of weather models will do.

Ultimately, the answer to the question of how much a pilot should know about the weather is however much they need to be confident in their ability to be weatherwise and conduct a safe flight.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University. Shein holds a commercial license with instrument rating.



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