Clouds and Precipitation

Objectives

1) Identify the major cloud types and the two types of fog
2) Describe how dew and frost are formed
3) Describe the necessary conditions for precipitation
4) Describe using diagrams the three major methods by which moist air can be forced to rise and create precipitation
5) Describe the air masses of North America, giving their source regions, paths of movement, and generally associated weather conditions

In this lesson, you will look at the various categories of clouds and the three major mechanisms for producing precipitation.

Cloud Types

See Figure 9.6b on page 175 of Planet Earth: A Physical Geography

A) High Clouds: formed above 6 000 metres to 12 000 metres; composed of ice crystals.

1) cirrus: thin, feather-like wisps, stream with the wind forming what looks like mares tails

2) cirrocumulus: small puffs that look like cotton batting; arranged in bands by high altitude winds

3) cirrostratus: thin sheet or layer; the sky has a milky appearance and a halo appears around the sun or moon.

B) Middle Clouds: 3 000 metres to 6 000 metres; composed of water droplets

1) altocumulus: large puffs of cotton arranged in bands; slightly darker and thicker than cirrocumulus.

2) altostratus: a thin layer of cloud; grey or bluish in colour.

Note: If you can see mountains with clouds above, they are likely middle or high clouds.

C) Low Level Clouds: from ground level to 3 000 metres.

1) stratus: low, uniform, rather dark layers with broken patches called fracto stratus beneath the base.

2) stratocumulus: softly rounded, covers most of the sky but with spaces in between.

3) nimbostratus: thick stratus from which rain falls: nimbus means "rain cloud".

D) Clouds of Vertical Development: the base can be as low as 500 metres and rise as high as 10 000 metres with cumulonimbus clouds.

1) cumulus: thick, bubbling, flat base usually with blue sky surrounding them; smaller patches on a pleasant day are called fair weather cumulus.

2) cumulonimbus: towering thunderheads, ascend from low levels to great heights on strong updrafts; when reach maximum height, upper level winds fan out the cloud giving it an anvil top composed of clouds made of ice crystals.

Dew, Frost, and Fog

dew: occurs when the air has given up some of its water vapour. It forms on cool surfaces when the night is clear and still. The moist air comes into contact with a cool surface (i.e. grass), condenses, and forms water droplets.

frost: if the process of forming dew occurs at or below freezing, the condensed vapour forms frost.

When the cooling of the air by contact with a cold surface extends through the layers of the atmosphere, the resultant condensation is fog.

radiation or ground fog: results from the cooling of the earth on calm, clear nights when heat is radiated from the earth's surface into space. This type of fog occurs most frequently in valleys and low places where heavy cold air sinks. It is also common around cities where smoke and dust particles act as condensation nuclei.
The sun is usually able to burn off this type of fog quite quickly.

advection fog: occurs when moist air moves over a cold surface. This persistent fog is common along coastlines where warm air moving off the water comes into contact with a colder land mass (i.e. the Pacific Coast). It is more common in late Fall and Winter when land masses are colder. Advection fog also occurs when warm air comes into contact with a cold ocean current (eg Gulf Stream air meeting the Labrador current near the Grand Banks off of Newfoundland).

Note: In terms of cloud classification, wide scale fog is simply stratus clouds at ground level.

Precipitation

Necessary Conditions

1) A source of moisture - water bodies, plants, etc.

2) Heat for evaporation to get water vapour into the air.

3) Condensation
- occurs when there is 100% relative humidity
(i.e. when the dew point is reached)
- for condensation to begin, the air has to cool. This is because the colder the air is, the less water vapour it can hold.

Most of the time, water vapour needs to rise in order to cool.

adiabatic cooling: the cooling of the air by expansion. When air is forced to rise it expands resulting in lower air pressure. When a gas expands it does work and therefore uses up energy. The loss in energy results in a loss of heat.

dry adiabatic lapse rate: when air is forced to rise, it cools at a rate of 1 deg. C / 100 m as long as the air temp. is above the dew point.

wet adiabatic lapse rate: occurs when the dew point is reached and condensation of the air takes place. As a result of condensation, the air does not cool as fast when it rises
(i.e. 0.6 deg. C / 100 m).

Note: There is also something known as the environmental lapse rate - the rate at which air temperature decreases with altitude under stable conditions at a rate of 1 deg. C per 156 metres. Stable conditions indicate that the air is not being forced to rise.

4) Nuclei: water vapour needs particles to condense onto.
Nuclei sources: dust, pollution, sea salt.

5) Rain droplets need to coalesce into large enough droplets to fall to earth.
- ice particles aid the process as well as wind and electrical charge from lightning.

Types of Precipitation
eg rain, snow, hail, sleet, drizzle

Processes of Precipitation
- convectional
- frontal
- orographic

Processes of Precipitation

With each process of precipitation, there is a mechanism to get moist air to rise. Once it begins to rise, the water vapour in it cools, condenses, forms clouds and precipitation.

*See page 179 diagrams in Planet Earth: A Physical Geography*

a) With orographic precipitation, the mechanism to get the air to rise is the mountains.

b) With convectional precipitation, the mechanism is the heat of the ground warming the air above it.

c) With cyclonic or frontal precipitation, the mechanism is two different air masses meeting. Air masses are large bodies of air that form over a period of days or weeks above extensive, fairly uniform surface areas of the world. They take on the temperature and moisture characteristics of their source region (i.e. the land or water they are over).

Air masses formed over land areas are designated continental (c) and are dry.

Air masses formed over the ocean are designated maritime (m) and are humid.

Air masses formed in the tropics are designated Tropical (T) and are warm.

Air masses formed in the upper latitudes are designated Arctic (A) or Polar (P) and are cold with the former being even colder.

*See page 171 in Planet Earth: A Physical Geography for a map of Air Masses Affecting North America*

Air masses do not like to mix when they encounter one another. Hence, there is a boundary left between them called a front. When a cold air mass and a warm air mass meet, the warm one being lighter will rise above the colder one. However, once the air begins to rise it cools, condenses, forms clouds, and if there is enough moisture, precipitation results.

In a later lesson, we will look in greater detail at frontal precipitation and the various types of fronts.

Assignment Work

1) Carefully read the information presented above.

2) Download the following assignment which is in a document file.

Insert your answers after each question.