Aero-Tips!

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Aero-Tips 09.23.06

You'll often hear a METAR (hourly weather report) identified as a "special". What are the criteria for issuing a METAR outside the normal reporting schedule? Why is it important to a pilot?

Note: Scheduled METARs are observations taken at 45 minutes before each hour to straight up on the hour, and are available for briefings between 50 past the hour and straight up on the hour.

Wind

If the wind direction changes 45 degrees or more in less than 15 minutes a "special" is required, but only if the wind speed remains above 10 knots the entire time. (Comment: lighter, variable winds, or stronger, shifting winds that don't exceed a 45-degree shift do not warrant a special, despite potential to greatly affect an aircraft).

Visibility

If visibility goes down (or up) beyond the one-, two- or three-mile point a special is required. Same goes if the visibility goes below (or comes up above) the lowest authorized instrument procedure minimum for that airport. If none is published, then the "minimums" special is triggered at 1/2 mile. Similarly, a special is required if a Runway Visual Range (RVR) goes below or comes above 2400 feet.

Sky condition

If a cloud layer or obscuration forms below 1000 feet (Above Ground Level), a special is required. It's also required if a ceiling forms or dissipates either below or above one of these levels: 3000 feet, 1500 feet, 1000 feet, 500 feet, or the lowest ceiling authorized in an instrument approach for that airport. If none is published, the special is required for ceilings going below or coming above 200 feet.

Miscellaneous

Special METAR observations are also required for beginning or ending of thunderstorms, tornadoes, funnel clouds, or waterspouts; beginning, ending or a change in intensity of freezing precipitation, ice pellets or hail; beginning of a volcanic eruption; and any other phenomenon judged by the observer to be "critical." Lastly, a special METAR is necessary on first notification of an aircraft mishap, unless a scheduled observation has been issued between the time of the mishap and this notification (note: the intent here is to capture weather data for a possible mishap investigation).

Aero-tip of the day: Know what triggers a Special METAR -- and when you hear "special observation" perk up your ears for the reason a special was issued. 

FMI: Aero-Tips