PRIMUS
r
880 Digital Weather Radar System
A28- 1146- 102- 00
Federal Aviation Administration (FAA) Advisory Circulars
A- 7
HAIL
D
Hail competes with turbulence as the greatest thunderstorm hazard
to aircraft. Supercooled drops above the freezing level begin to
freeze. Once a drop has frozen, other drops latch on and freeze to
it, so the hailstone grows - sometimes into a huge iceball. Large hail
occurs with severe thunderstorms with strong updrafts that have
built to great heights. Eventually, the hailstones fall, possibly some
distance from the storm core. Hail may be encountered in clear air
several miles from dark thunderstorm clouds.
D
As hailstones fall through air whose temperature is above 0
_
C, they
begin to melt and precipitation may reach the ground as either hail
or rain. Rain at the surface does not mean the absence of hail aloft.
You should anticipate possible hail with any thunderstorm,
especially beneath the anvil of a large cumulonimbus. Hailstones
larger than one- half inch in diameter can significantly damage an
aircraft in a few seconds.
LOW CEILING AND VISIBILITY
Generally, visibility is near zero within a thunderstorm cloud. Ceiling
and visibility may also be restricted in precipitation and dust between
the cloud base and the ground. The restrictions create the same
problem as all ceiling and visibility restrictions; but the hazards are
increased many fold when associated with other thunderstorm hazards
of turbulence, hail, and lightning which make precision instrument flying
virtually impossible.
EFFECT ON ALTIMETERS
Pressure usually falls rapidly with the approach of a thunderstorm, then
rises sharply with the onset of the first gust and arrival of the cold
downdraft and heavy rain showers, falling back to normal as the storm
moves on. This cycle of pressure change may occur in 15 minutes. If
the pilot does not receive a corrected altimeter setting, the altimeter
may be more than 100 feet in error.
Summary of Contents for Primus 880
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