2.3. Weather Processes and Phenomena Flashcards
Formation of Mist and Fog
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.
Mist
Occurs when visibility is between 1000m and 5000m and relative humidity is over 93%
Fog
Occurs when visibility is below 1000m. Dense fog occurs when the visibility is below 200m.
Advection Fog
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.
Radiation Fog
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.
Dew Point
The temperature at which relative humidity is 100% and condensation occurs
Condensation Level
the height at which the relative humidity (RH) of an air parcel will reach 100% and condensation occurs
Dew
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
Evaporation
Liquid to gas
Condensation Process
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.
Condensation Nuclei
Condensation requires particles or nuclei onto which the vapour can condense. These are condensation nuclei.
Humidity
A measure of the water vapour content of the air
Absolute Humidity
The amount of water in the atmosphere, usually measured in units of grams of water vapour per cubic meter of air
Relative Humidity
The water vapour present expressed as a percentage of the maximum amount of air that temperature can hold.
Precipitation
All forms of deposition of moisture from the atmosphere in either solid or liquid states.
Clouds
Clouds are formed of millions of tiny water droplets held in suspension
Convectional Rainfall
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.
Frontal Rainfall
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.
Orographic Rainfall
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.
Hoar Frost
Radiation fog but with a temperature below freezing
Hail
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
Snow
Snow is frozen precipitation. Snow crystals form when the temperature is below freezing point and water vapour is converted into a solid.
Frost
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.
Where can condensation occur
In the air (clouds) or on ground (fog and mist)
Evaporation Definition (Water)
Process of water liquid changing into water vapour (gas) due to the addition of heat
Condensation Definition (Water)
Process of water vapour (gas)changing into water liquid due to the addition of heat
Freezing Definition (Water)
Water moving from a liquid state (water) to a solid state (ice)
Melting Definition (Water)
Water moving from a solid state (ice) to a liquid state (water)
Deposition Definition (Water)
Water moving from a gas (vapour) state to a solid state (ice)
Sublimation Definition (Water)
Water moving from a solid state (ice) to a gas (vapour) state. Occurs in very cold environments where the humidity is low.
Low Altitude Clouds
Nimbostratus (tall stratus), Stratus, Stratocumulus, Cumulus (clouds with vertical development)
Middle Altitude Clouds
Typically start with “Alto”: Altocumulus, Altostratus
High Altitude Clouds
Typically start with “Cirro”: Cirrocumulus, Cirrostratus (halo around sun), Cirrus
All level clouds
Cumulonimbus (anvil top)
