2.3. Weather Processes and Phenomena

Question 1

Formation of Mist and Fog

Answer 1

For mist and fog to form, condensation nuclei, such as dust and salt, are needed.
- Fog - condensation must take place near ground level. Condensation can take place in 2 major ways: air is cooled, more water is added to atmosphere.

Question 2

Mist

Answer 2

Occurs when visibility is between 1000m and 5000m and relative humidity is over 93%

Question 3

Fog

Answer 3

Occurs when visibility is below 1000m. Dense fog occurs when the visibility is below 200m.

Question 4

Advection Fog

Answer 4

Warm moist air moves over cold land or cold moist air moves over warm land. Contact cooling at a cold ground surface may produce saturation. As warm moist air passes over a cold surface it is chilled. Condensation takes place as the temperature of the air is reduced and the air reaches dew point.

Question 5

Radiation Fog

Answer 5

Occurs in valleys with calm, cloudless conditions when the ground loses heat rapidly at night by long-wave radiation due to a lack of clouds. The cold air sinks down to the bottom of the valley, with warmer air on top. This is a temperature inversion. The ground becomes cold. When there is enough moisture, there is radiation fog.

Question 6

Dew Point

Answer 6

The temperature at which relative humidity is 100% and condensation occurs

Question 7

Condensation Level

Answer 7

the height at which the relative humidity (RH) of an air parcel will reach 100% and condensation occurs

Question 8

Dew

Answer 8

Dew is the direct deposition of water droplets onto the surface and vegetation. It occurs in clear, calm anticyclonic conditions (stability) where there is rapid radiation cooling by night. The temperature reaches dew point, and further cooling causes condensation and direct precipitation onto the ground and vegetation

Question 9

Evaporation

Answer 9

Liquid to gas

Question 10

Condensation Process

Answer 10

When Relative Humidity reaches 100% saturation. During condensation process, water molecules lose the latent heat that was added during the evaporation process. When latent heat is released it is converted into sensible heat which warms the surrounding air, increasing the temperature.

Question 11

Condensation Nuclei

Answer 11

Condensation requires particles or nuclei onto which the vapour can condense. These are condensation nuclei.

Question 12

Humidity

Answer 12

A measure of the water vapour content of the air

Question 13

Absolute Humidity

Answer 13

The amount of water in the atmosphere, usually measured in units of grams of water vapour per cubic meter of air

Question 14

Relative Humidity

Answer 14

The water vapour present expressed as a percentage of the maximum amount of air that temperature can hold.

Question 15

Precipitation

Answer 15

All forms of deposition of moisture from the atmosphere in either solid or liquid states.

Question 16

Clouds

Answer 16

Clouds are formed of millions of tiny water droplets held in suspension

Question 17

Convectional Rainfall

Answer 17

When the land becomes very hot and heats the air above it. This air expands and rises. As it rises, cooling and condensation take place. If it continues to rise rain will fall. This is very common in tropical areas like the ITCZ.

Question 18

Frontal Rainfall

Answer 18

Frontal Rainfall occurs when warm air meets cold air. The warm air, being light and less dense, is forced to rise over the colder, denser air. As it rises it cools, condenses and forms rain. This is a warm front and stratus clouds form. If cold air collides with the warm air along a cold front, the more dense cold air can force the warmer air ahead to rise rapidly creating vertically developed cumulus clouds.

Question 19

Orographic Rainfall

Answer 19

The forced rise of air when it collides with a mountain. As air strikes the windward side (side exposed to wind) it is uplifted and cooled. Windward slopes of mountains tend to be the rainy sides, while the leeward side (side sheltered from the wind is dry). Dry climates like deserts are normally found here because of the rain shadow.effect.

Question 20

Hoar Frost

Answer 20

Radiation fog but with a temperature below freezing

Question 21

Hail

Answer 21

Made up of alternate shells of clear and opaque ice, formed by raindrops being carried up and down in vertical air currents in large cumulonimbus clouds. Freezing and partial melting may occur several times before the pellet is large enough to escape from the cloud. As a raindrop is carried high up in the cloud it freezes. As the hailstone falls, the outer layer may be melted but can freeze again with further uplift. The process can occur many times before the hail finally falls to the ground, that is, when its weight is great enough to overcome the strong updraughts of air

Question 22

Snow

Answer 22

Snow is frozen precipitation. Snow crystals form when the temperature is below freezing point and water vapour is converted into a solid.

Question 23

Frost

Answer 23

Frost is a deposit of fine ice crystals onto the ground or vegetation. It occurs on cloud-free nights wen there has been radiation cooling to below freezing point. Water vapour condenses directly onto these surfaces by sublimation.

Question 24

Where can condensation occur

Answer 24

In the air (clouds) or on ground (fog and mist)

Question 25

Evaporation Definition (Water)

Answer 25

Process of water liquid changing into water vapour (gas) due to the addition of heat

Question 26

Condensation Definition (Water)

Answer 26

Process of water vapour (gas)changing into water liquid due to the addition of heat

Question 27

Freezing Definition (Water)

Answer 27

Water moving from a liquid state (water) to a solid state (ice)

Question 28

Melting Definition (Water)

Answer 28

Water moving from a solid state (ice) to a liquid state (water)

Question 29

Deposition Definition (Water)

Answer 29

Water moving from a gas (vapour) state to a solid state (ice)

Question 30

Sublimation Definition (Water)

Answer 30

Water moving from a solid state (ice) to a gas (vapour) state. Occurs in very cold environments where the humidity is low.

Question 31

Low Altitude Clouds

Answer 31

Nimbostratus (tall stratus), Stratus, Stratocumulus, Cumulus (clouds with vertical development)

Question 32

Middle Altitude Clouds

Answer 32

Typically start with “Alto”: Altocumulus, Altostratus

Question 33

High Altitude Clouds

Answer 33

Typically start with “Cirro”: Cirrocumulus, Cirrostratus (halo around sun), Cirrus

Question 34

All level clouds

Answer 34

Cumulonimbus (anvil top)