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AIM

10/12/17

7−1−58

Meteorology

relative humidity is low in any layer between the

surface and 15,000 feet. Then the lower altitudes may

be characterized by strong out flowing winds and

severe turbulence.

e. The probability of lightning strikes occurring to

aircraft is greatest when operating at altitudes where

temperatures are between minus 5 degrees Celsius

and plus 5 degrees Celsius. Lightning can strike

aircraft flying in the clear in the vicinity of a

thunderstorm.

f. METAR reports do not include a descriptor for

severe thunderstorms. However, by understanding

severe thunderstorm criteria, i.e., 50 knot winds or

3

/

4

inch hail, the information is available in the report

to know that one is occurring.

g. Current weather radar systems are able to

objectively determine precipitation intensity. These

precipitation intensity areas are described as “light,”

“moderate,” “heavy,” and “extreme.”

REFERENCE−

Pilot/Controller Glossary− Precipitation Radar Weather Descriptions

EXAMPLE−
1. Alert provided by an ATC facility to an aircraft:
(aircraft identification) EXTREME precipitation between
ten o’clock and two o’clock, one five miles. Precipitation
area is two five miles in diameter.
2. Alert provided by an FSS:
(aircraft identification) EXTREME precipitation two zero
miles west of Atlanta V−O−R, two five miles wide, moving
east at two zero knots, tops flight level three niner zero.

7−1−29. Thunderstorm Flying

a. Thunderstorm Avoidance. Never regard any

thunderstorm lightly, even when radar echoes are of

light intensity. Avoiding thunderstorms is the best

policy. Following are some Do’s and Don’ts of

thunderstorm avoidance:

1. Don’t land or takeoff in the face of an

approaching thunderstorm. A sudden gust front of

low level turbulence could cause loss of control.

2. Don’t attempt to fly under a thunderstorm

even if you can see through to the other side.

Turbulence and wind shear under the storm could be

hazardous.

3. Don’t attempt to fly under the anvil of a

thunderstorm. There is a potential for severe and

extreme clear air turbulence.

4. Don’t fly without airborne radar into a cloud

mass containing scattered embedded thunderstorms.

Scattered thunderstorms not embedded usually can

be visually circumnavigated.

5. Don’t trust the visual appearance to be a

reliable indicator of the turbulence inside a

thunderstorm.

6. Don’t assume that ATC will offer radar

navigation guidance or deviations around thunder-

storms.

7. Don’t use data-linked weather next genera-

tion weather radar (NEXRAD) mosaic imagery as the

sole means for negotiating a path through a

thunderstorm area (tactical maneuvering).

8. Do remember that the data-linked NEXRAD

mosaic imagery shows where the weather was, not

where the weather is. The weather conditions may be

15 to 20 minutes older than the age indicated on the

display.

9. Do listen to chatter on the ATC frequency for

Pilot Weather Reports (PIREP) and other aircraft

requesting to deviate or divert.

10. Do ask ATC for radar navigation guidance

or to approve deviations around thunderstorms, if

needed.

11. Do use data-linked weather NEXRAD

mosaic imagery (for example, Flight Information

Service-Broadcast (FIS-B)) for route selection to

avoid thunderstorms entirely (strategic maneuver-

ing).

12. Do advise ATC, when switched to another

controller, that you are deviating for thunderstorms

before accepting to rejoin the original route.

13. Do ensure that after an authorized weather

deviation, before accepting to rejoin the original

route, that the route of flight is clear of thunderstorms.

14. Do avoid by at least 20 miles any

thunderstorm identified as severe or giving an intense

radar echo. This is especially true under the anvil of

a large cumulonimbus.

15. Do circumnavigate the entire area if the area

has 6/10 thunderstorm coverage.

16. Do remember that vivid and frequent

lightning indicates the probability of a severe

thunderstorm.