FOG
• Fog is a cloud (usually stratus) that is in
contact with the ground.
– Relatively stable air ie. Shallow lapse rate needed
– Temperature to dew point spread is small, wind
may be present, requires condensation nuclei
– Usually needs a cooling process
• Types of Fog:
Radiation
Upslope
Frontal
Advection
Steam
Ice
Radiation Fog: over land, clear night, light
winds, high humidity. Often in a high pressure
area, maritime air mass, and near industrial
pollution
Pg 10-7
+
+
+
Radiation Fog
•
Radiation fog sometimes thickens at sunrise due to
slight heating from the sun which creates a weak
turbulence and more of the moist air circulates
against the colder ground surface.
“Burning Off”
• When the sun begins to significantly heat
the surface, the air will begin to warm and
the fog will begin to evaporate.
• Radiation fog tends to “Burn Off” from the
bottom up usually within two hours of sun
rise.
Upslope Fog
Pg 10-9
Upslope fog
Calgary
Winnipeg
Montreal
Pg 10-8
Advection Fog:
-Warm moist air
moving over colder
surface
land or sea
-Requires horizontal
movement
eg. Warm air moving
over an ocean
- sometimes strong
winds (+25kts)
Frontal Fog: addition of water vapour
evaporating from the falling rain raises
dew point under a warm front
Pg 10-10
Pg 10-8
Advection Fog:
-Warm moist air
moving over colder
surface
land or sea
-Requires horizontal
movement
eg. Warm air moving
over an ocean
- sometimes strong
winds (+25kts)
Pg 10-8
Advection Fog:
-Warm moist air
moving over colder
surface
land or sea
-Requires horizontal
movement
eg. Warm air moving
over an ocean
- sometimes strong
winds (+25kts)
Steam fog: cool moist air moving
over a warm surface
Steam Fog over Georgian Bay
Pg 10-9
Ice Fog: Byproduct of engine is
water…added to cold crisp air.
(sublimation: vapour to ice)
Pg 10-10
WATER VAPOUR – WATER - ICE
Pg 1-2
• Clear Ice:
– large super cooled water droplets spread out on wing
as they freeze
– bottom layers of cold clouds or tops of unstable clouds
– freeze just below 0° to -15°
– tend to hit wing
– high collection efficiency
– large spreading droplets
• Rime Ice:
ICE
part 9
– small super cooled droplets can exist down to -40°
– stable clouds, usually rime only -25° to -40°
– tend to flow around wing
– low collection efficiency
– leading edge only (no spread)
Icing types
Rime Ice
Clear Ice
(there is a bit of
mixed here too)
PIREP report Mixed Icing
Freezing of Super-Cooled Droplets
at Impact
Pg 9-4
Heaviest Icing – Rate of Catch
–
–
–
–
Aircraft skin temperature at or below freezing
High water content held up in updrafts in unstable air
Large drops in unstable air as droplets coalesce
Collection efficiency is inversely proportional to the
surface geometry - thick vs. thin wings
THIN WING
HIGHER SPEED
LARGER DROPLETS
Pg 9-10
ICING
•
Stratus
•
Cumulus
•
icing distributed
horizontally
if turbulent, top of cloud will
have the heaviest.
snow means less icing
•
•
•
•
icing distributed vertically
top of mature stage
In 5000 ft above 0° level
large droplets to -25°
•
•
**Warm clouds have more droplets with altitude.
**Colder clouds have fewer droplets with altitude.
Pg 9-5
Frost (Hoar Frost)
• Forms by sublimation (Gas  Ice crystals)
• Occurs when moist air comes into contact
with an object at temperatures sufficiently
below freezing for ice crystals to form.
• Most likely to form on aircraft on clear, cold
nights due to radiation cooling.
• DO NOT ATTEMPT TO TAKE-OFF WITH
FROST ON THE CRITICAL SURFACES.