Clouds, Precipitation & Fog 4 3 . 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, when condensation nuclei are present, and when there is a cooling of air.

Lifting Agents Orographic lift Frontal lift Convection 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 Agents Convergence - Turbulence 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 When water droplets (visible as a cloud) grow sufficiently in size and weight to fall due to gravity.

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

PRECIPITATION CLOUD TYPE Drizzle, freezing drizzle, snow Snow or rain (continuous) Ice pellets (continuous) Hail, ice pellet showers Snow shower, rain showers Snow or rain (intermittent) Ice prisms PRECIPITATION CLOUD TYPE Stratus and stratocumulus Thick altostratus and nimbostratus Thick altostratus and stratocumulus Altocumulus, heavy cumulus, cumulonimbus Cumulonimbus Any rain cloud (below freezing) No clouds necessary Snow pellets Heavy cumulus

Confirmation Stage NOTE: You must use the buttons in the Confirmation Stage

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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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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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.

Types of Fog

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.

Forms when warm moist air moves over a cold surface. ADVECTION FOG Forms when warm moist air moves over a cold surface.

scan Advection Fog COLD LABRADOR CURRENT WARM MOIST AIR 9

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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.

example of upslope fog scan Stable, moist easterly flow MSL YQR YBR Regina YBR Brandon YPG Portage La Prairie YWG Winnipeg Stable, moist easterly flow

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.

FRONTAL FOG SATURATION OF COLD AIR BY EVAPORATION OF FALLING PRECIPITATION NS FOG ST 32

Precipitation-induced Fog Caused by the addition of moisture to the air through evaporation of precipitation.

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 Stage NOTE: You must use the buttons in the Confirmation Stage

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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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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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!