FOG Fog is a cloud usually stratus that

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 (+25 kts)

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 (+25 kts)

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 (+25 kts)

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 • • icing distributed horizontally • if turbulent, top of cloud will • have the heaviest. • snow means less icing • • Cumulus 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.