June 2002Departments

The Weather Never Sleeps

Airmets And Sigmets


Deciphering The Hazards Of Adverse Conditions

No one likes to hear bad news, but adults learn that ignoring it or trying to pretend they didn't hear it is fruitless. For pilots, ignoring bad news about the weather is worse than fruitless; it's one of the most dangerous things they can do.

Maybe this is why the first part of a standard preflight weather briefing is the "adverse conditions" section.

Once you've heard the bad news about thunderstorms, turbulence, low ceilings and visibilities, icing, and other potentially hazardous weather, you're ready - if you still want to go flying - to begin learning about all of the current and forecast weather that could affect your flight.

Anyone who flies should be keeping up with the weather, especially in the 24 or so hours before a planned flight. A pilot who is following the weather on television or the Internet is going to know about any really threatening weather that's likely in the next day or two.

For instance, if you are a dyed-in-the-wool weather geek, you might visit the Web site of the National Weather Service's Storm Prediction Center ( www.spc.noaa.gov/products/index.html ) and pull up the next day's convective outlook, which will give you a map like the one shown in Figure 1. The area enclosed by the orange line is where ordinary thunderstorms are expected. The area inside the green line, marked "SLGT," faces a slight chance of severe thunderstorms with surface winds of 50 knots or more, or hail three-quarters of an inch or greater in diameter. By the way, the orange line across Florida and southern Georgia shows that thunderstorms are expected between the line and the coast. When a line doesn't enclose an area, you face in the direction of the arrow on the line and the thunderstorms will be to your right.

The National Weather Service, which produces the forecasts disseminated by FAA flight service stations, alerts pilots to potentially dangerous weather with sigmets and airmets. These can refer both to weather that is occurring or is forecast.

A sigmet, which stands for significant meteorological information, advises of weather that is potentially dangerous for all aircraft.

For the contiguous 48 states, the NWS issues convective sigmets for the various dangers associated with thunderstorms. (Convection refers to the up and down air movements needed for thunderstorms.) During the spring and summer these are the most common kind of sigmets.

Convective sigmets are issued for thunderstorms "greater than or equal to VIP Level 4 affecting 40 percent or more of an area of at least 3,000 square miles," the NWS says. This would be an area about 55 miles long and 55 miles wide. VIP refers to video integrated processor, which is a way for weather radar to indicate storm strength based on how much radio energy it reflects back to the radar. There are six VIP levels, and on many color radar displays levels 1 and 2 are green, 3 and 4 are yellow, and 5 and 6 are red.

Lines of thunderstorms, isolated thunderstorms with surface winds of 50 kt or more or hail at least three-quarters of an inch in diameter, and thunderstorms that produce tornadoes are also listed in convective sigmets. Thunderstorms embedded in non-thunderstorm clouds that cover a wide area also rate convective sigmets.

An ordinary sigmet can be issued for:

  • Severe icing;
  • Severe or extreme turbulence;
  • Dust storms and sandstorms lowering visibilities to less than three statute miles; and
  • Volcanic ash.

By the way, volcanic ash is extremely dangerous. It is capable of causing all engines to fail on jet airliners.

The NWS issues airmets, which are airman's meteorological information, for weather other than thunderstorms that could be dangerous to single-engine and other light aircraft and to pilots flying under visual flight rules (VFR). Pilots of larger aircraft and those flying under instrument flight rules shouldn't ignore airmets, however.

Airmets are put into three categories, each identified by a name from the international phonetic alphabet. These are:

  • Airmet Sierra (IFR) for ceilings of less than 1,000 feet or visibility less than three miles affecting more than 50 percent of the area at one time, or extensive mountain obscuration.
  • Airmet Tango (turbulence) for moderate turbulence or sustained surface winds of 30 kt or more at the surface.
  • Airmet Zulu (icing) for moderate icing and to list the freezing levels.

Only a few years ago an airmet was available only as text, as shown in Figure 2. The big hurdle was picturing the area indicated by a line between the locations given, starting in the example 80 miles northwest of the Santa Barbara, California, VOR (RZS).

You no longer have to mentally - or physically - connect the dots on a map to locate the area covered. If you go to the NWS Aviation Weather Center's Web site (http://aviationweather.noaa.gov/index. html), you can call up maps similar to the one in Figure 3 that show areas covered by airmets; in this case, for mountain obscuration and areas of low ceilings or visibility. On the map, the large red box covering almost all of the California coast is for low ceilings or visibilities, while the smaller, darker red box in Southern California is the airmet described in Figure 2.

Once you have the location, it's easy to translate the rest of the airmet (try reading it aloud) as "Coastal mountain slopes becoming obscured in clouds and mist (BR) below 2,500 feet between 0300 to 0600 Zulu. Conditions continuing beyond 0900 Zulu through 1500 Zulu."

Clicking on the link to "Standard Brief" on the Aviation Weather Center page takes you to links to all of the elements of a standard weather briefing, both text and maps, beginning with the adverse conditions.

Once you're satisfied that none of a day's adverse conditions are expected along the route of the flight you're planning, you can go ahead and gather all of the other weather information needed for the flight without worrying too much about being surprised by bad news.

But, don't become too complacent. As anyone with much weather experience can tell you, the atmosphere springs surprises from time to time. Once you are in the air you should assess the weather regularly to guard against being caught by bad news that wasn't expected when you checked the adverse conditions.

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 co-author with Dr. Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.


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