The Weather Never Sleeps

Weather At Altitude

The whole idea of flying, of course, is to get away from the ground. But no matter how high you fly, you're not going to climb out of the atmosphere, which means you aren't going to escape the weather.

A large part of the effort expended by the U.S. National Weather Service, its counterparts in other nations, and private meteorologists goes into observing and forecasting the weather aloft. These observations and forecasts are not only vital to aviators, but they also are needed to predict the weather at the Earth's surface because the winds aloft push the weather systems that bring warm or cold, wet or dry weather.

As with the observations and forecasts of surface weather, meteorologists studying weather aloft use text and maps to show what happened at a particular time in the past (when observations were taken) or what forecasters expect to happen at a certain time in the future. For most general aviation pilots, a forecast of winds and temperatures aloft, like that in Figure 1, supplies much of the information needed to plan a flight. It is a forecast for winds and temperatures at 00 Zulu on June 10, which was 12 hours after the forecast was made. AOPA members may access these forecasts online ( www.aopa.org/members/wx ).

As with all upper air charts, the temperatures are in degrees Celsius. To read the wind forecasts, think of the lines as arrows flying with the winds. The barbs give the wind speed in knots. A short line is 5 kt and a long line 10 kt, which means two long lines and a short line indicate the wind is forecast to be 25 kt. The temperatures are needed to make performance calculations for that altitude, such as the true airspeed at 70 percent power.

You could use this map if you were planning to fly at 5,000 feet, but it covers such a large area that you will probably want winds and temperatures for more places. Textual winds aloft forecasts are the source of such information. Figure 2 shows the winds and temperatures aloft for Buffalo, New York, and Sioux Ste Marie, Michigan, for 3,000, 6,000, 9,000, and 12,000 feet above mean sea level (msl).

In these forecasts each four-digit number is decoded by taking the first two digits and adding a zero at the end to find the true direction of the wind in degrees. The second two digits represent the air temperature in degrees Celsius. The Buffalo forecast for 3,000 feet shows wind from 330 degrees and temperature 10 degrees C.

While the winds and temperatures aloft text supplies the information required for performance and navigation calculations, it tells you little about the weather to be expected during a flight.

For instance, if the Buffalo forecast of winds from 330 degrees were on an early winter day and if the temperature were below zero Celsius, you might suspect that heavy "lake effect" snow is possible because the cold wind would be blowing over Lake Erie, where it would pick up moisture from the relatively warm lake. But even the most knowledgeable pilot would have to consult other forecasts and probably talk to a briefer at an FAA flight service station to make sure the weather isn't likely to spring any nasty surprises.

Area forecasts, which give a general description of the weather that's expected over large areas of the country, will include information on possible areas of poor visibility, icing, and thunderstorms. The upper air sections of low-level significant weather prog charts will show at a glance where dangerous weather is expected. But to really get a feel for the weather aloft, you need to at least glance at constant pressure charts like the one in Figure 3. The instructions for learning how to read and understand these charts are in Section 8 of FAA Advisory Circular AC 00-45E, Aviation Weather Services ( www.faa.gov/avr/afs/afs400/).