Clouds

Clouds and Fog Formation

● Cloud type is dependant on stability and moisture content.

● In order to get clouds or fog the water vapour must change into a liquid (water droplets) or solid (ice crystals) to form.

➢ In other words for clouds to form the air must become saturated.

● There are a number of ways that this can happen:

➢ By lowering the air temperature to the dew point temperature, most common is by rising air (lifting process).

➢ By adding water vapour into the air.

➢ By mixing warm moist air with cold air

Cloud coverage

● Cloud coverage is reported in oktas or eighths of the celestial dome.

● For weather reports these are broken up into five basic categories:

➢ SKC: Clear Sky.

➢ FEW: 1/8 to 2/8 of the sky covered.

➢ SCT: Scattered, 3/8 to 4/8 of the sky covered.

➢ BKN: Broken, 5/8 to less than 8/8 covered.

➢ OVC: Overcast, 8/8 covered.

Ceilings 

● A ceiling is said to exist whenever the sky is either BKN or OVC.

○ Remember, in this case the cloud coverage is equal to or greater than 5/8ths of the sky covered.

○ VFR pilots are NOT permitted to fly above a BKN or OVC layer.

○ (Unless they have a VFR OTT rating )

● A ceiling will also be defined by a Vertical Visibility (VV) on a METAR or in a TAF.

○ Example: VV001=Ceiling obscured at 100 feet

● Ceilings are depicted on the GFA with a scalloped border

Cloud Classification 

● Clouds are classified into four families based on their height and vertical development, i.e. strati-form or cumuli-form.

● Stratiform develop horizontally and cumuliform family develop vertically.

● Stratus family is associated with fog and low clouds with poor visibility.

● This system of classification describes the approximate altitude and appearance

● High Clouds

● Middle Clouds

● Low Clouds

● Clouds with Vertical Development

High Clouds

● These are clouds found at altitudes between 20000 ft and 40000 ft.

● They include:

➢ Cirrus (CI)

➢ Cirrostratus (CS)

➢ Cirrocumlulus (CC)

Cirrus (CI) 

● High and Wispy.

● Typically found at heights greater than 20 000 feet.

● Generally occur in high pressure systems, and fair weather.

● They will point in the direction of air movement at their elevation.

● Mainly composed of ice crystals.

● A ragged windswept appearance of the Cirrus Clouds is always an indication of very strong winds or jet streams

Cirrostratus (CS) 

● Sheet-like high-level clouds composed of ice crystals

● Cirrostratus clouds tend to thicken as a warm front approaches signifying an increased production of ice crystals

Cirrocumulus (CC) 

● Somewhat rare.

● Appears as a white sheet with a pebbly pattern.

● Characterized by high ice crystals

Middle Clouds

● Found at altitudes between 6500 ft to 20000 ft.
● Types:
➢ Altostratus (AS)
➢ Altocumulus (AC)
➢ Altocumulus Castellanus (ACC)

Altocumulus (AC)

● These are puffy “cotton ball” type clouds.

● On a warm and humid summer morning they may be followed by thunderstorms as the day progresses.

● Icing is usually present in the cloud above the freezing level and will be especially heavy near the top of the cloud

Nimbostratus (ST)

● Low layer cloud. Smooth air.

● Resembles fog but does not rest on the ground.

● No waves or patterns, “grey.

Altostratus (AS) 

● Layer cloud with no definite pattern.

● Steely or bluish in color.

● Sometimes the sun or moon can be seen dimly through.

● Altostratus clouds make the sun seem to appear to be behind heavily frosted glass.

● Incapable of producing heavy precipitation but often cause light drizzle

Alto Cumulus Castellanus (ACC)

● Created from instability associated with air flows having marked vertical shear and weak thermal stratification

Low Clouds

● These are clouds found at altitudes between the surface to 6500 ft.

● Types:

➢ Stratus (ST)

➢ Nimbostratus (NS)

➢ Stratocumulus (SC)

➢ Stratus Fractus (SF)

➢ Cumulus Fractus (CF)

Stratus (ST)

● Stratus clouds are known for drizzle or freezing drizzle.

● Updrafts associated with Stratus clouds are very weak so that the larger droplets in the cloud tend to settle earthward.

● As they sink they strike and coalesce with other droplets and grow.

● Finally they sink out of the cloud base as drizzle

Nimbostratus (NS) 

● Dark, low-level clouds often accompanied by steady, light to moderate continuous precip.

● Low clouds are primarily composed of water droplets since their bases generally lie below 6 500 feet.

● The prefix ‘nimbus’ indicates precipitation

Stratocumulus (SC) 

● Low, lumpy layer of clouds.

● Sometimes accompanied by weak intensity precipitation.

● Precipitation with varying intensity, from light to heavy showers.

● Main producer of drizzle.

Stratus Fractus (SF)

● Stratus cloud that has been torn by wind into fragments.
● Drizzle may fall from these clouds

Cumulus Fractus (CF)

● Stratocumulus torn by wind .

● These can be differentiated from Stratus Fractus by their more rounded tops

Cumulus (CU) Fair Weather 

● Appearance of floating cotton or "popcorn popping" and have a lifetime of 5-40 minutes.

● Indicates the presence of significant vertical currents at low levels without precip.

● With the proper conditions, harmless fair weather cumulus can later develop into TCU and then CB

Clouds of Vertical Development

● Cumulus (CU).
● Towering Cumulus (TCU).
● Cumulonimbus (CB).
● Alto Cumulus Castellanus (ACC).
● Have high proportion of supercooled water while developing.
● As the cell ceases to grow ice crystals will be found.
● In new growth CBs for example, icing will be severe.
● Horizontal extent of icing is minimal

Towering Cumulus (TCU)

● Growing cumulus cloud.
● On the way to becoming a Cumulonimbus

Cumulonimbus (CB) 

● Violent vertical currents that at times are in excess of 50 KT and contain the greatest turbulence.

● Depending on the height of the troposphere and the buoyancy of the updraft, the tops of cumulonimbus clouds can reach up to 60 000 feet.

Mammatus

● Powerful cumulonimbus clouds may have appendages protruding from the base of them called Mammatus clouds.

● Mammatus clouds indicate that the atmosphere is extremely unstable.

● Severe weather is imminent.

● Funnel clouds may also be associated, STEER WELL CLEAR!!

Orographic Clouds

● Developed when air is forced to rise by the earth's topography

Mountain Wave Clouds

Lenticular Clouds 

● Forms in the wave crest very high and hundreds of miles long

Rotor Clouds

● Associated with mountain wave activity.
● Form downwind and below each mountain wave crest.
● These are dissipating and forming at the same time due to the
rotation of the air

Cap Cloud
● Lies over the top of the mountain and extends partially down the leeward slopes, indicating an extremely strong downdraft

Roll Cloud

● Associated with thunderstorms.

● May form near the main cloud base in the shear area where the downdraft comes out