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This time of year, our attention turns to thunderstorms. From May 2003 through May 2006,
there were 11 accidents involving general aviation aircraft whose pilots inadvertently
flew into severe convective weather conditions. Ten of the encounters were fatal and the
eleventh suffered substantial damage to the aircraft. In the spring of 2006, the Federal
Aviation Administration's (FAA) air traffic organization revised the terminology and
phraseology its air traffic controllers use to describe areas of weather radar echoes in
the National Airspace System (NAS). The four terms -- "light," "moderate,"
"heavy," and "extreme" -- each represent a precipitation intensity level
paired with a dBZ range to help pilots interpret the severity of the flight conditions present.
ATC's precipitation information can also complement the information that you may already
have from your own on board weather displays or radar. While the ATC view can sometimes
provide a bigger picture of what is out there, keep in mind that the air traffic controller's
first duty priority is to separate aircraft and issue safety alerts regarding terrain,
obstructions, and other aircraft. Additional services, such as suggested headings or
radar vectors to assist pilots to avoid areas of precipitation, will be provided to the
extent possible, but the service is contingent upon higher priority duties and other
factors including limitations of radar, volume of traffic, frequency congestion, and
workload. Subject to these factors/limitations, controllers will issue pertinent
information on precipitation areas that are displayed to them on their radar scopes.
If you have done your homework, you will already have a good idea of what type of weather
system you will encounter during your flight. Are conditions ripe for convective
activity? If they are, will the storms be organized in lines of frontal activity or
disorganized and widely scattered with storms popping up randomly like popcorn here and
there? Your pre-planning is important because the air traffic radar cannot detect the
presence or absence of clouds. In fact, ATC radars don't show weather areas. They only show
precipitation, which could be in the form of rain, snow, virga, hail, etc. In other words,
a controller can tell you where precipitation is, but cannot tell you what kind it is.
Some ATC radars can determine the intensity of the precipitation area and some cannot.
Those that can will use the terms, "Light," "Moderate," "Heavy,"
and "Extreme." When the intensity cannot be determined, the controller will state,
"Intensity Unknown."
The precipitation areas that the controllers see on their radar scopes can be as old as
six minutes before the weather data is updated. This is important to remember because
convective weather is transient and can change rapidly. Thunderstorms can develop at rates
exceeding 6,000 feet per minute, which is faster than the updates. To rely solely on ATC
as a source for weather avoidance is not entirely prudent. It is the pilot's responsibility
to obtain a preflight weather briefing. Any ATC reported weather information, along with
periodic contacts with Flight Watch while airborne, will supplement what was learned
during the preflight briefing, The ATC reports of precipitation areas are of value because
they can give you a global view of what is in the area. Pilots who have onboard weather
radar or lightning detection systems can benefit from the big picture that ATC can paint
and can use the aircraft's onboard systems to pick the best tactical route to avoid
severe weather.
ATC can tell you what is in your immediate path, but won't tell you what to do. It's up to
you. ATC can tell you whether or not an area of precipitation awaits you and some can tell
you if it is Light, Moderate, Heavy, or Extreme. It is up to you to decide what to do.
Be prepared to tell ATC what you want to do. ATC can provide approval for you to deviate
from your assigned course so that you can skirt around the weather yourself. Do you want
assistance? ATC can provide you with a suggested heading to take you to one side or other
of the weather, but remember, ATC radars cannot detect the presence or absence of clouds
or turbulence, so the headings convey no guarantee that you will not encounter hazards
associated with convective activity. If you wish to circumvent the area at a specific
distance, you must make your desires clearly known to ATC at the time of the request for
the service.
Rainfall rates are difficult to associate with the intensity levels because they can
vary significantly depending upon whether they occur in convective or non-convective
conditions. Since Mother Nature can be capricious, suffice it to say that in convective
conditions, once you get near the Moderate range of precipitation, you should expect
difficult conditions. All thunderstorms and convective activity should be expected to
have turbulence associated with them. Operation in and around such conditions should be
approached with great caution because the severity of turbulence can be markedly greater
than the precipitation description might indicate. Turbulence should be expected to occur
near such areas, even in clear air. A good rule of thumb is to give thunderstorms a wide berth.
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