The Weather Never Sleeps

By: Jack Williams

Humble beginning

Knowledge-test prep isn't weather wisdom

Any student who knows only as much about the weather as is needed to answer the weather questions on the FAA's knowledge test is not prepared to make safe flying decisions about weather.

To see why, imagine that before heading to the airport for a flight, you tune in the local television weather and hear that Doppler radar shows that a thunderstorm west of the city has a mesocyclone, which means that it's a supercell. The thunderstorm has already produced a gust front and could generate microbursts. A low-level jet is feeding humid air into the region, and forecasters say a mesoscale convective complex is likely to form tonight.

If your weather study has been based only on the FAA's knowledge test questions, none of the above terms in italics will mean anything to you. Once you know what the terms mean, you'll see why pilots should know at least a little about them.

Doppler weather radar shows wind movements, especially those in showers and thunderstorms. The National Weather Service's national Doppler radar network enables forecasters to do a better job and has taught researchers much about how storms work.

A mesocyclone is a rotating area in a large thunderstorm-it may be 10 miles across.

A supercell is a thunderstorm with a mesocyclone. Such storms are especially long-lasting, and they produce the strongest tornadoes plus other dangerous weather.

A gust front is cool air that descends from a thunderstorm and moves across the ground, often for several miles. When a gust front arrives at an airport, it can cause a dangerous shift in wind speed or direction.

A microburst is a concentrated blast of wind coming down from a shower or thunderstorm. Airline and military pilots and those who study meteorology in college pilot training programs learn about these dangerous winds. The word, however, does not appear in the private pilot knowledge test.

Low-level jets not only feed humid air into thunderstorms, but also are an aviation hazard.

A mesoscale convective complex is a huge cluster of thunderstorms that forms during the night, usually in spring or summer and usually between the Rockies and Appalachians. It can create widespread hazardous flying weather.

All of these terms have one thing in common: Meteorologists began using them over the past couple of decades as researchers learned more and more about the weather, especially thunderstorms.

The FAA's bank of questions used for private pilot knowledge tests has approximately 120 weather questions; about half pertain to weather theory, and the other half focus on weather charts and word reports and forecasts. About a dozen of these weather questions will be on a private pilot's 60-question knowledge test. While the FAA no longer releases the actual knowledge test questions, sample questions available online give you an idea of the types of questions asked and how they are presented.

Almost all of the weather theory questions are based on FAA Advisory Circular 00-6A, Aviation Weather, which was last revised in 1975. Weather knowledge has made great strides in the interim, and pilots who don't have a clue about what meteorologists have learned since then are compromising their safety when they go flying.

Regular viewers of television weather aren't likely to hear terms like mesocyclone very often, but every day they see maps with the letter "L," showing areas of low pressure in some places, and the letter "H," for high pressure in others. These are obviously important. Otherwise why use them on a map that's simplified for the public? The areas of low pressure are the centers of extratropical storms (not all of these are especially stormy) that move across North America much of the year bringing weather changes. The highs are areas of generally clear weather that usually follow the lows.

The formation, growth, movement, and weakening of low-pressure and high-pressure areas are a key to understanding the weather.

To understand reports of the current weather and forecasts, pilots should have at least a general understanding of how the different parts of storm systems affect the weather as they move across your flight route. Yet, the FAA's private pilot knowledge test does not include any questions about low and high pressure centers-the weather systems that are a key part of our day-to-day weather. The basics of these systems have been known since early in the twentieth century, and they are described to some extent in Aviation Weather.

While the knowledge exam's weather questions have nothing to say about some important aspects of weather, some of the questions require pilots to learn about things that they will never use. For instance, two questions give you the temperature and dew point at a particular level and ask how high the bottoms of cumulus clouds that form under these conditions would be. To arrive at a correct answer, you must know that the temperature and dew point converge in rising air at the rate of about 4.4 degrees Fahrenheit per 1,000 feet of altitude gained. Thus, if the temperature and dew point are 44 degrees apart, they'll reach the same value after rising 10,000 feet. When this happens the water vapor in the air begins condensing into cloud droplets-the bottom of the cloud.

It's hard to imagine when a pilot would ever need to figure this out. The height of the bottoms of clouds is one of the important pieces of information that you get from reports of the actual weather or forecasts.

Another test question would throw a pilot who knew a fair amount about the weather. The question says: "Low-level turbulence can occur and icing can become hazardous in which type of fog? A-Rain-induced fog; B-Upslope fog; C-Steam fog."

Someone who knows a fair amount about weather would probably first eliminate answer C. Most of us think of steam fog as the wispy stuff that comes off streams and ponds in the fall when cold air blows across the water. Warm water evaporates into the cold air and immediately condenses into fog drops. That kind of steam fog is pretty harmless.

But, it turns out that C is almost surely the correct answer. Why? Here's what Aviation Weather's arctic weather chapter says about steam fog: "This fog is composed entirely of water droplets that freeze quickly and fall back into the water as ice particles. Low-level turbulence can occur, and icing can become hazardous."

The message here is that the exact words in a chapter of an out-of-date book that most student pilots don't think they'll need to read (how many plan to fly in the Arctic?) are more important than some useful weather knowledge.

This isn't to say that all of the weather theory questions on the knowledge exam are useless. For instance, the exam questions include a dozen on air stability, which is an important concept. If you know that stable air is associated with flat clouds, generally smooth air, and poor visibility-and unstable air is associated with clouds that have vertical development, turbulence, and good visibility-you'll answer several questions correctly.

This is a very simplified view of air stability, but at least it's a starting point for learning more.

The 60 or so knowledge-test questions about obtaining weather information are based mostly on another FAA advisory circular, AC 00-45 E, Aviation Weather Services, a supplement to AC 00-6A. It was updated in 1995 after the National Weather Service made major changes in its hourly aviation weather reports and its forecasts for airports.

To get the correct answers to most of these questions you'll have to know the symbols used on maps, and how to read reports and forecasts that use a meteorological code. Coded reports go back to the days of slow-speed Teletype machines. Today you can easily find translations when you obtain a weather briefing or look up weather information on the Internet. If you learn the symbols and codes, you can do a good job of answering questions, but you will still remain almost clueless about how to use weather information to decide whether a planned flight will be safe.

Obviously, students and their instructors need to go beyond the knowledge test questions to learn weather. How?

To begin, get an aviation weather text, such as Aviation Weather by Peter Lester, and use it as a guide to the weather knowledge you need to be a safe pilot. Other recommendations include The Complete Idiot's Guide to Weather by Mel Goldstein, Meteorology Today, seventh edition, by C. Donald Ahrens, and Flying America's Weather by AOPA Pilot Editor-at-large Thomas A. Horne.

Along with this, students and their instructors can plan imaginary flights, obtain weather "briefings" using information available on the Web, make the go/no-go decision, and see what happens. If the imaginary flight was a "go," would it have been safe, or would a weather surprise have added some unwanted excitement to the imaginary trip?

After a few such drills you'll begin to see your weak points and become more skilled at making good weather decisions. Such skill would also help you do a better job on the weather portion of an FAA knowledge exam.

Jack Williams is the weather editor of usatoday.com. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.