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880
Digital
W
eather
Radar
System
A28-1146-102-00
Federal Aviation Administration (FAA) Advisory Circulars
A-6
ICING
D Updrafts in a thunderstorm support abundant liquid water with
relatively large droplet sizes; and when carried above the freezing
level, the water becomes supercooled. When temperature in the
upward current cools to about -15 _C, much of the remaining water
vapor sublimates as ice crystals; and above this level, at lower
temperatures, the amount of supercooled water decreases.
D Supercooled water freezes on impact with an aircraft. Clear icing
can occur at any altitude above the freezing level; but at high levels,
icing fromsmaller droplets may berime or mixed withrime and clear.
The abundance of large, supercooled droplets makes clear icing
very rapid between O _C and -15 _C and encounters can be
frequent in a cluster of cells. Thunderstorm icing can be extremely
hazardous.
NAUTICAL MILES
0 5 10 15
WAKE
WAKE
GUST FRONT
MOTION OF STORM
WARMAIR INFLOW
COLD AIR
OUTFLOW
AD-37561@
DRY AIR
INFLOW
COLD
AIR
OUTFLOW
WARM AIR
INFLOW
Schematic Cross Section of a Thunderstorm
Figure A-1