403 - Clouds, Precipitation and Fog

What are clouds composed of?

Water droplets, supercooled water droplets, ice crystals

How big are the particles that form a cloud?

0.02mm or 20 micrometers

What are the three factors that determine cloud characteristics?

Temperature of the air
Stability of the air
Moisture available

Define stratiform clouds

Clouds composed of rolls or elements, sometimes uniform in structure and normally stretched out in layers

What characters of precipitation are associated with stratiform clouds?

Continuous and intermittent

Define cumuliform clouds

Clouds which are composed of individual elements or bases and tend to have large vertical extent.

What character of precipitation is associated with cumuliform clouds?

Showery

Define cloud

Cloud is a hydrometeor consisting of minute particles of liquid water or ice, or both suspended in free air and not usually touching the ground

Where are cloud bases found?

At the condensation level

Where is the condensation level found

At the point in the atmosphere where temperature and dew point are equal.

Will cloud bases be lower or higher in dry air?

Higher

What impacts the height of cloud tops?

The vertical motion that exists at altitude

In unstable air where will cloud growth stop?

Where the temperature reaches equilibrium between the rising parcel of air and the surrounding air

In stable air where will cloud growth stop?

When the lifting agent ceases to operate

Which cloud types are classified as stratiform?

CI CC CS AC AS NS ST SF SC

Which cloud types are classified as vertical development clouds?

CF CU TCU CB ACC

Other than humidity and stability, what factors affect the formation of clouds when it comes to orographic lift?

Slope steepness
Height of terrain
Wind direction
Wind speed

Other than humidity and stability, what factors affect the formation of clouds when it comes to mechanical turbulence?

Rough ground
Strength of winds

Other than humidity and stability, what factors affect the formation of clouds when it comes to convergence?

Horizontal and vertical extent of the convergence

Other than humidity and stability, what factors affect the formation of clouds when it comes to convection?

Uneven heating
Advection of cold air

Other than humidity and stability, what factors affect the formation of clouds when it comes to frontal lift?

Speed of the front

What leads to the dissipation of clouds formed by orographic lift?

Change of air mass
Change of wind speed
Change of wind direction

What leads to the dissipation of clouds formed by mechanical turbulence?

Change of air mass
Decrease in wind speed

What leads to the dissipation of clouds formed by convergence?

Change of air mass
Filling of the low

What leads to the dissipation of clouds formed by convection?

Heating stopping
Loss of advection

Define fog

A suspension of water droplets or ice crystals in the air at the surface, reducing the visibility to 1/2SM or less

What can fog be composed of?

Water droplets, supercooled water droplets, ice crystals

What are the requirements for fog to form?

High relative humidity
Condensation nuclei
Cooling of the air or the addition of water vapour

What are the two ways fog is formed?

Cooling of the air
Addition of water vapour

What types of fog are formed by the cooling of air?

Radiation
Advection
Upslope

What types of fog are created by the addition of water vapour?

Frontal
Steam
Ice

How does radiation fog form?

Land cools by thermal radiation after sunset, causing condensation in the air just above

Other than high relative humidity and condensation nuclei, what conditions are needed to for radiation fog?

Clear night
Light winds

What are the characteristics of radiation fog?

Often seen in the early morning
Drains into low lying areas
Dissipates during the morning
More common under highs
May be initiated by aircraft

What are the conditions required for the dissipation of radiation fog?

Daytime heating
Increasing winds

How does advection fog form?

Warm moist and stable air is blown across a cooler surface

Other than high relative humidity and condensation nuclei, what conditions are needed to for advection fog?

Advection of warm air
Moderate winds

What are the characteristics of advection fog?

Can cover large areas
Persists for a long time
Thins with daytime heating over land
If over water daytime heating as little effect

What are the conditions required for the dissipation of advection fog?

Increasing winds
Change in wind direction
Daytime heating of ground

How does upslope fog form?

Moist air is forced up the slope of a mountain and as it rises it cools to the condensation temperature

Other than high relative humidity and condensation nuclei, what conditions are needed to for upslope fog?

Sloping ground
Moderate winds

What are the characteristics of upslope fog?

Persists until a change in air mass or wind direction
Dissipates on leeward side of the upslope

What are the conditions required for the dissipation of upslope fog?

Daytime heating
Increasing winds
Change in wind direction

How does frontal fog form?

Precipitation falls into dryer air below and the liquid droplets evaporate, increasing the dew point to the temperature

Other than high relative humidity and condensation nuclei, what conditions are needed to for frontal fog?

Warm air over cold air
Frontal precipitation

What are the characteristics of frontal fog?

Can form rapidly

What are the conditions required for the dissipation of frontal fog?

Movement of the front and pressure system
Precipitation moves away

How does steam fog form?

Cold air is blown over a warmer body of water

Other than high relative humidity and condensation nuclei, what conditions are needed to for steam fog?

Cold air over warmer water

What are the characteristics of steam fog?

Thin
Only form over water
Can drift over land
Forms on cold autumn mornings
Common in the arctic

What are the conditions required for the dissipation of steam fog?

Daytime heating
Change of the air mass
Freezing of the open water

Other than high relative humidity and condensation nuclei, what conditions are needed to for ice fog?

Temperatures -10C or below

What composition of the clouds leads to the Bergeron process?

A mixture of ice crystals and supercooled water droplets

During the Bergeron process what process causes the ice crystals to grow?

The sublimation of supercooled water vapour onto the ice crystal

What is the collision process?

When precipitation falls it collides with other ice crystals or water droplets and grow in size

What air currents aid in the growth of water droplets and ice crystals during the collision process

Strong vertical currents