WX BRIEF

Clues to changing weather

Pilots can stack the odds in their favor by watching simple signs.


One of the most visual clues about the state of the atmosphere is the exhaust pouring out of a smoke stack. A pilot can gain instant information about wind speed and direction as well as the stability of the lower atmosphere.

As these cells rise, many become airmass thunderstorms capable of deluging a coastal airport. By early afternoon, the unequal heating of the land versus the ocean has set up an onshore flow that brings these storms on shore.

Winds are a great source of meteorological information. Both surface and upper-level winds can provide clues about weather systems, turbulence and windshear. For example, if you stand with your arms outstretched and your back to the wind, then turn clockwise by about 30° (counterclockwise in the Southern Hemisphere), your left (right) hand will be pointing more or less directly at the center of a low pressure system.

This truism is a function of winds flowing counterclockwise (clockwise) into a low-pressure area. The 30° offset to what is known as Buy Ballot's Law is to counter the surface friction that draws the air flow off its intended path.

If the low is to your west, your weather will likely deteriorate, but if the low is to the east of you, the high developing behind will generally help to improve the weather. In the tropics or near the poles, the law doesn't work as well.

These locations are dominated more by a global scale low pressure belt along the Equator, and high pressure over the subtropics and poles. In the tropics or near the poles, a guess that the low pressure will be to your south in the Northern Hemisphere and to your north in the Southern will be fairly accurate.

Once you know the positions of the highs and lows, you can also begin to get a good idea about the winds aloft. Without the interference of surface friction, the winds at altitude tend to flow around highs and lows, rather than out of and into them.

Between the two, the winds will flow parallel to the pressure gradient, which means roughly perpendicular to a line drawn between them. For example, if the low is to your east in the Northern Hemisphere and the high is to the west, the winds aloft will likely be from the north.

If the low is to your north, your upper-level winds will likely be out of the west, and you'll probably be beneath a strong part of the jet stream. This means strong winds and turbulence aloft.

Mixed signals

In addition, you can compare the wind at different levels of the atmosphere by looking at a combination of signs.

Above the surface, clouds will flow along with the prevailing wind. At the surface, friction will tend to slow the winds and also shift their direction. If you notice the windsock sticking out in one direction but the clouds above it moving in another, you'll likely encounter some mechanical turbulence (windshear) on climb-out.

A great source of information about low-level shear is smoke stacks. At most urban airports, you should be able to find at least one plume of smoke rising into the sky. Does it rise straight up? If so, you've got yourself a relatively unstable surface layer.

You may get a few turbulent jostles on your way through it, but in general it means winds are light throughout the air column, and heat will not be trapped near the surface, waiting to explode into a thunderstorm.

If the smoke is rising, but makes several unexpected twists and turns on the way up, you can bet there is a layer of wind moving above you in a direction and speed different from those you are seeing on the Metar report from the surface weather station or from the winds aloft report.

Make a note of the approximate altitude of this shift as a place for potential windshear turbulence. Smoke stack exhaust can also rise to a certain level before a strong wind catches it and whisks it away.

This should be a pretty obvious sign that the calm surface winds are not tied into the much stronger winds aloft. The level at which this wind picks up will also be a zone of moderate to strong windshear.

This sort of atmospheric condition is commonly seen at night when a weak temperature inversion separates the surface layer flow from the free atmosphere above it, and a weak nocturnal jet forms along the boundary.

During the day, however, smoke that rises to a certain level and then spreads out can portend potentially bad weather. Exhaust (smoke or water vapor) from a chimney rises because it is warmer, and thus less dense, than the air around it.

The only thing that would keep it from continuing to rise is the presence of warmer air above it. The inversion not only keeps the smoke from rising further-it also traps a lot of heat and water vapor in the lower atmosphere.

Eventually, there will be enough heat energy in places for the air to burst through this capping inversion and grow quickly into a thunderstorm. Finally, when the smoke stack is surrounded by hot and humid air, the exhaust often doesn't rise up but instead just dissipates around the chimney.

In such cases, the atmosphere is very stable. While this means an absence of clouds or turbulence, it also means that low-level visibility will be poor due to the smog and haze that is bound to build up from all the smoke stacks in the vicinity.

Looking down

Aloft, the pilot can also gain a lot of information about the weather. Just as on the ground, the clouds will provide a good indication of the stability and moisture content of the air at various altitudes.

The angle you hold to maintain course along your assigned airway will quickly tell you the approximate speed and direction of the wind.

Aloft, this means you can figure out the position of highs and lows using the same Buy-Ballot's Law but without the 30° correction. When approaching to land, look around at the landscape.

Does it appear crisp and colorful or somewhat out of focus? Note that haze and fog tend to be relatively shallow events, and can be completely overlooked from the air. Although you may not be able to perceive a shallow fog layer, if one is present it will tend to wash out colors and make all but the closest objects appear somewhat blurry.

You might look for some depressions in the landscape as well. If fog is present, it may be thick enough in low places to be visible from the air. Even before you pick up the latest ATIS, you'll have an idea of the level of reduced visibility you may find as you flare out of final approach.

In the early days of aviation, many pilots flew by the seats of their pants. Weather maps were rudimentary at best and usually many hours out of date. Forecasts were also notoriously inaccurate, and for many locations the only weather data a pilot could get would come from the windsock of whatever airport they were circling.

Despite the relative lack of aviation meteorology, more pilots than not survived their flights by relying instead on their innate understanding of the weather signs that Mother Nature posted for them.

Although today there is a seemingly infinite supply of meteorological information at our fingertips from the radio, Internet or cockpit display, the basics of reading the weather still apply, and that primitive understanding can go a long way toward a safe, weather-ready 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 and was a scientist with NASA's Global Change Master Directory. Shein holds a commercial license with instrument rating.

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