403.2 - composition, formation, and dissipation of fog

Fog can occur with the passage of ______, but some areas are more susceptible to ______.

fronts, radiation and advection fog

Fog can also be seasonal, in that some areas are more susceptible to fog in _____.

the spring and fall

Fog is a suspension of very small water droplets or ice crystals in the air, reducing the visibility to ______ at the earth’s surface.

1/2 SM or less

_____ in the fog may produce phenomena such as small haloes and luminous pillars around lights, the sun or the moon

Ice crystals

When fog is composed of water droplets, it is reported as:

• FG (fog) if the visibility is _______

• BR if visibility is ______

less than 5/8 miles, 5/8 to 6 miles

When fog is composed of supercooled water droplets, it is reported as:

• _____ if the visibility is less than 5/8 miles

• BR if the visibility is _____

FZFG, 5/8 to 6 miles

When fog is composed of ice crystals, it is reported as FG or BR, usually when temperature is _____.

less than -30°C

The ideal conditions for the formation of fog are:

• ____ relative humidity

• The presence of _____

• Cooling of the air (decrease in ______) or addition of water vapour (increase in _____)

• High

• condensation nuclei

• temperature, the dew point temperature

Name the fog types formed by cooling of the air (decreasing temperature):

Radiation fog, advection fog, upslope fog

Name the fog types formed by adding water vapour (increasing dew point):

Frontal fog, steam fog, ice fog

Radiation fog is formed by the cooling of land ____ by thermal radiation in ___ conditions with ____ sky. The cool ground produces condensation in the nearby air by heat conduction.

after sunset, calm, clear

In perfect calm, the radiation fog layer can be less than ____, but ____ can promote a thicker layer.

a metre deep, turbulence

Radiation fog is formed as a result of ____ or _____.

nighttime cooling, cold air drainage in valleys

When might radiation fog be taller than a metre or so deep?

With wind speeds as low as 3 to 5 knots

Conditions for formation of radiation fog:

• Clear night

• Light winds (3 to 5 kts)

• High relative humidity

• Condensation nuclei

Conditions for formation of dissipation of fog:

• Daytime heating

• Increasing winds (fog lifts and becomes ST)

Characteristics of radiation fog:

• Can form or thicken at sunrise. Often seen in the early morning hours.

• Drains into low lying areas such as valleys or ditches

• Dissipates during the morning, usually due to daytime heating

• More common under highs or ridges

• May be initiated by aircraft

Advection fog forms when quite warm, moist, and stable air is blown across _____. The air temperature falls until the dew point is reached and condensation occurs. Fog then forms over _____.

a cooler surface (land or water), the colder surface

Conditions for formation of advection fog:

• Advection of warm, humid air over a cold surface

• Moderate winds (13 to 17 kts)

• High relative humidity

• Condensation nuclei

Conditions for dissipation of advection fog:

• Increasing winds

• Change in wind direction

• Daytime heating of the ground

Characteristics of advection fog

• Can cover a large area

• Can persist for long periods

• Forms in the warm sector of frontal waves

• Can be blocked by higher terrain

• Thins with daytime heating of the ground

• If over water, daytime heating has little effect

Upslope fog forms when moist air is forced up the slope of a mountain and as the air rises with the terrain, it cools to the condensation temperature. Fog forms on the slope on account of _____ and, to a lesser extent, the ____.

adiabatic cooling, drop in pressure with altitude

Conditions for formation of advection fog:

• Sloping ground (orographic lift)

• Moderate winds (13 to 17 kts)

• High relative humidity

• Condensation nuclei

Conditions for dissipation of advection fog:

• Daytime heating

• Increasing winds (fog lifts becoming ST)

• Change in wind direction

Characteristics of advection fog

• Persists until a change in air mass or wind direction

• Dissipates on the leeward side of the upslope (adiabatic heating)

Frontal fog forms as precipitation falling from ______ saturates the colder air below. The water vapour cools, and at the dew point, it condenses and fog forms. This is caused by the _____.

the warmer air aloft, evaporation of precipitation

The moisture content of the air increases to the point that it reaches the air temperature and fog forms. Frontal fog is formed when the dew point is increased to ____.

the air temperature

Conditions for formation of frontal fog:

• Warm air over cold air

• Frontal precipitation

• High relative humidity

• Condensation nuclei

Conditions for dissipation of frontal fog:

• Movement of the front and pressure pattern

• Precipitation moves away

Characteristics of frontal fog:

• Can form rapidly (fast movement of front)

Steam fog is also known as _____. During the early winter season, before the ice freezes, any colder air blowing above it can easily generate Arctic sea smoke or steam fog.

Arctic sea smoke

Steam fog forms over bodies of water overlain by _____; it can also lead to _____ forming.

much colder air, steam devils

“Streamers” are long rows of TCU clouds that build up moisture as they move over water before dumping snow in a swath over portions of southern Ontario.

This phenomenon occurs when the Great Lakes _____, causing ______ coming off Lake Huron and Georgian Bay.

do not freeze over, lake-effect snow

Lake effective fog is a _____ phenomenon, resulting in fog which can be quite a bit denser, deeper, and fluffier in appearance. Most other fog is _____.

stratiform

Steam fog is a type of advection fog, but it is formed by _____.

air saturation

Conditions for formation of steam fog:

• Cold air above warmer water

• High relative humidity

• Condensation nuclei

Conditions for dissipation of steam fog:

• Daytime heating

• Change of the air mass

• Freezing of the open water

Characteristics of steam fog:

• Usually thin

• Only forms over water

• Can drift over land after formation

• Often forms over lakes/rivers on cold autumn mornings

• Common in the Arctic; forms in winter above “leads” (cracks) in the ice

Ice fog is a type of fog consisting of fine ice crystals _____. In an ice fog situation, the temperature becomes too cold for only supercooled water to occur. Generally, the temperature must be _____ in order for ice fog to occur.

suspended in the air, -10°C or colder