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PRIMUS
R
660 Digital Weather Radar System
A28–1146–111
REV 2 A–13/(A–14 blank)
Federal Aviation Administration (FAA) Advisory Circulars
MODIFICATION OF CRITERIA WHEN SEVERE STORMS AND
RAPID DEVELOPMENT ARE EVIDENT
During severe storm situations, radar echo intensities may grow by a
factor of ten each minute, and cloud tops by 7,000 feet per minute.
THEREFORE, NO FLIGHTPATH THROUGH A FIELD OF STRONG
OR VERY STRONG STORMS SEPARATED BY 20–30 MILES OR
LESS MAY BE CONSIDERED TO REMAIN FREE FROM SEVERE
TURBULENCE.
EXTRAPOLATION TO DIFFERENT CLIMBS
General comment: Severe storms are associated with an atmospheric
stratification marked by large values of moisture in low levels, relative
dryness in middle levels, and strong wind shear. It is well known that this
stratification of moisture permits excessive magnitudes of convective
instability to exist for an indefinite period until rapid overturning of air is
triggered by a suitable disturbance. Regions of the atmosphere which
are either very dry or very moist throughout substantial depths cannot
harbor great convective instability. Rather, a more nearly neutral
thermal stratification is maintained, partially through a process of
regular atmospheric overturning.
D Desert Areas – In desert areas, storms should be avoided on the
same basis as described in the above paragraphs. While nonstorm
turbulence may, in general, be expected more frequently over desert
areas during daylight hours than elsewhere, THE SAME
TURBULENCE CONSIDERATIONS PREVAIL IN THE VICINITY
OF THUNDERSTORMS.
D Tropical–Humid Climates – When the atmosphere is moist and only
slightly unstable though a great depth, strong radar echoes may be
received from towering clouds which do not contain vertical velocities
as strong as those from storms over the U.S. plains. Then it is a matter
of the pilot being informed with respect to the general atmospheric
conditions accompanying storms, for it is well known that
PRACTICALLY ALL GEOGRAPHIC AREAS HAVING
THUNDERSTORMS ARE OCCASIONALLY VISITED BY SEVERE
ONES.
USE OF AIRBORNE RADAR
Airborne radar is a valuable tool; HOWEVER, ITS USE IS
PRINCIPALLY AS AN INDICATOR OF STORM LOCATIONS FOR
AVOIDANCE PURPOSES WHILE ENROUTE.