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4 0 3 . 0 9. Clouds, Precipitation & Fog. Cloud Formation. Clouds form when invisible water vapour that is present in the air changes into its visible form as water droplets or ice crystals, via condensation or sublimation. This occurs when: the relative humidity is high,

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Clouds, Precipitation & Fog

cloud formation
Cloud Formation
  • Clouds form when invisible water vapour that is present in the air changes into its visible form as water droplets or ice crystals, via condensation or sublimation. This occurs when:
    • the relative humidity is high,
    • when condensation nuclei are present, and
    • when there is a cooling of air.
lifting agents
Lifting Agents
  • Orographic lift
    • Air blowing against a range of hills or mountains is forced upward into a region of lower pressure, expands and cools. Condensation will occur when the dew point is reached.
  • Frontal lift
    • A mass of warm air is forced aloft and rises over the cold air. The rising warm air cools by expansion and clouds are formed.
  • Convection
    • As the sun heats the earth unevenly, rising currents will occur. This is convection. The rising air expands and cools. This produces condensation and cumuliform cloud forms at the top of the column of air. Further ascent and cooling causes rain.
lifting agents1
Lifting Agents
  • Convergence -
    • Air is forced into a region, as at the center of a low, convergence is occurring. The excess air is forced to rise, as it rises it expands and cools. When the condensation level is reached, cloud form.
  • Turbulence
    • As wind blows over an uneven or rough surface or when uneven heating occurs, vertical currents are created. Air in the upward motion cools by expansion and if enough moisture is there condensation occurs. This results in cumulus cloud.
precipitation
Precipitation

When water droplets (visible as a cloud) grow sufficiently in size and weight to fall due to gravity.

forms of precipitation
Forms of Precipitation
  • Drizzle, rain, freezing drizzle, freezing rain
  • Hail, snow pellets (soft hail)
  • Snow
  • Ice prisms
  • Ice pellets
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PRECIPITATION

CLOUD TYPE

Drizzle, freezing drizzle, snow

Stratus and stratocumulus

Snow or rain (continuous)

Thick altostratus and nimbostratus

Snow or rain (intermittent)

Thick altostratus and stratocumulus

Snow shower, rain showers

Altocumulus, heavy cumulus, cumulonimbus

Hail, ice pellet showers

Cumulonimbus

Ice pellets (continuous)

Any rain cloud (below freezing)

Ice prisms

No clouds necessary

Snow pellets

Heavy cumulus

confirmation stage

Confirmation Stage

NOTE: You must use the buttons in the Confirmation Stage

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Meteorology

Clouds, Precipitation & Fog

Let's try a few review questions on Meteorology:

Question #1 - A mass of warm air is forced aloft and rises over the cold air. The rising warm air cools by expansion and clouds are formed. This phenomena is known as .

A

Orographic Lift

B

Frontal Lift

C

Convection

Convergence

D

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slide14

Meteorology

Clouds, Precipitation & Fog

Let's try a few review questions on Meteorology:

Question #2 - What cloud will produce continuous rain?

A

Stratocumulus

B

Stratus

C

Nimbostratus

Cirrostratus

D

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slide19

Fog

Fog is a cloud, usually stratus, in contact with the ground. It forms when the air is cooled below its dew point, or when the dew point is raised to the air temperature through the addition of water vapour.

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RADIATION FOG

Usually forms on clear nights. Conditions that favor the formation of radiation fog are:

  • clear skies
  • moist air
  • light winds (2-5 knots)

The ground cools losing heat through radiation. The air in direct contact with the earth’s surface is cooled. If this air is moist and the temperature is lowered to the dew point , fog will form.

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ADVECTION FOG

Forms when warm moist air moves over a cold surface.

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scan

Advection

Fog

COLD

LABRADOR

CURRENT

WARM

MOIST AIR

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UPSLOPE FOG

Caused by the cooling of air due to expansion as it moves up a slope. A light upslope wind is necessary.

slide27
scan

example of

upslope fog

MSL

YWG

Winnipeg

YQR

Regina

YBR

Brandon

YPG

Portage La Prairie

Stable, moist easterly flow

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STEAM FOG

Forms when cold air passes over a warm water surface. Evaporation of the water into the cold air occurs until the air becomes saturated. The excess water vapour condenses as fog.

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Frontal Fog

Often found near fronts. Warm front fogs are the most extensive and are caused by the cold air becoming saturated by evaporation from rain falling from the warm air. It is also known as precipitation-induced fog.

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FRONTAL FOG

SATURATION OF COLD AIR BY

EVAPORATION OF FALLING PRECIPITATION

NS

FOG

ST

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Precipitation-induced Fog

Caused by the addition of moisture to the air through evaporation of precipitation.

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Ice Fog

At very low temperatures, the air may become full of ice crystals. The ice crystals, which formed by sublimation, may suddenly appear when an aircraft engine has been started (the exhaust contains water vapour, condensation nuclei and the engine create mixing).

confirmation stage1

Confirmation Stage

NOTE: You must use the buttons in the Confirmation Stage

slide36

Meteorology

Clouds, Precipitation & Fog

Let's try a few review questions on Meteorology:

Question #3 - What fog forms when warm moist air moves over a cold surface?

A

Radiation fog

B

Advection fog

C

Upslope fog

Frontal fog

D

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Flying Scholarship Program

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slide41

Meteorology

Clouds, Precipitation & Fog

Let's try a few review questions on Meteorology:

Question #4 - What fog forms when cold air passes over a warm water surface. Evaporation of the water into the cold air occurs until the air becomes saturated. The excess water vapour condenses as fog.

A

Radiation fog

B

Precipitation-induced fog

C

Upslope fog

Steam fog

D

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slide46

Meteorology

Clouds, Precipitation & Fog

Congratulations!!

You have now completed the Clouds, Precipitation & Fog lesson of the Meteorology Module. Of course, this lesson is always available to you for future reference if required.

You are now ready to move along to the next Meteorology lesson you have not completed or to any other module you wish. You can advance to the Self Test Module if you feel ready to challenge the final exam.

Good Luck!