PRIMUS
r
880
Digital
W
eather
Radar
System
A28-1146-102-00 FederalAviation Administration (FAA) Advisory Circulars
A-7
HAIL
DHailcompetes with turbulence as the greatest thunderstorm hazard
to aircraft. Supercooled drops above the freezing level begin to
freeze. Onceadrop has frozen, other drops latch on and freezeto
it,so the hailstonegrows-sometimes intoahuge iceball. Large hail
occurs with severe thunderstorms with strong updrafts that have
builtto great heights. Eventually,the hailstones fall, possibly some
distancefrom the storm core. Hail may be encountered in clear air
several miles from dark thunderstorm clouds.
DAshailstones fall through air whose temperature isabove0_C, they
begin to melt and precipitation may reach the ground as either hail
orrain. 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 alarge cumulonimbus. Hailstones
larger than one-half inch in diameter can significantly damage an
aircraft in afew seconds.
LOW CEILING AND VISIBILITY
Generally,visibility is near zero within athunderstorm 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
increasedmany fold when associated with other thunderstorm hazards
ofturbulence, hail, and lightning whichmakeprecision instrument flying
virtually impossible.
EFFECT ON ALTIMETERS
Pressureusually falls rapidly with the approach of athunderstorm, then
rises sharply with the onset of the first gust and arrival of the cold
downdraftand heavy rain showers, falling back to normal as the storm
moveson. This cycle of pressure change may occur in 15 minutes. If
the pilot does not receive acorrected altimeter setting, the altimeter
may be more than 100 feet in error.