fss 108.2b - distinguishing middle and high étage clouds

Description of Altostratus (AS)

• Uniform layer

• Continuous veil or sheet

• Grey or bluish colour

• Sun or moon appears as if seen through ground glass

• Sun or moon may be completely hidden if it is thick

• No shadows

Description of Altocumulus (AC)

• White or grey colour

• Often flattened globular masses

• Regularly arranged elements fairly small

• Layers or patches

• Elements arranged in groups, lines or waves following one or two directions

Description of Altocumulus Castellanus (ACC)

• White to light grey

• Common flat base

• Slight vertical development in form of turrets or castles

Description of Cirrus (CI)

• Pure White

• Delicate fibrous filaments

• Silky appearance

• Tufts, hooks, commas

Description of Cirrostratus (CS)

• Generally white, can be light gray

• Continuous uniform layer

• Fibrous

• Often produces a halo

• Outline of sun or moon clearly visible

• Sun or moon casts shadows through it

Description of Cirrocumulus (CC)

• Pure White

• Small white flakes

• Small globular masses

• Arranged in groups or lines

• Ripples - resembling those of the sand on the sea shore

• Frequently associated with Cirrus or Cirrostratus

Heights of Cirrus, Cirrocumulus and Cirrostratus

20,000 to 40,000 feet

Heights of Altostratus, Altocumulus, and Altocumulus Castellanus

6,600 to 19,000 feet

Thickness of Altostratus

1,000 to 10,000 feet

Thickness of Altocumulus

100 to 3,000 feet

Thickness of Altocumulus Castellanus

500 to 5,000 feet

Thickness of Cirrus and Cirrostratus

300 to 1,000 feet

Thickness of Cirrocumulus

300 to 2,000 feet

What clouds are composed of only ice crystals?

Cirrus, Cirrocumulus, and Cirrostratus

Which cloud is composed of water droplets and ice crystals?

Altocumulus Castallenus

Which mid ètage cloud(s) is composed of water droplets, ice crystals, and super-cooled water droplets?

Altostratus and Altocumulus

How is Altostratus formed?

• May form from Altocumulus or change into Altocumulus

• Usually associated with warm front

How is Altocumulus formed?

• May form from Altostratus or change into Altostratus

• Spreading out of Towering Cumulus or Cumulonimbus

How is Altocumulus Castellanus formed?

Small diameter updrafts

How is Cirrus formed?

• Frontal overrunning

• Detached tops of Cumulonimbus

How is Cirrostratus formed?

• Frontal overrunning

• Can be associated with approaching warm front

How is Cirrocumulus formed?

• Forms in turbulent overrunning air

• Degeneration of Cirrostratus

Precipitation Produced by Altostratus

• Rain or freezing rain

• Snow

• Ice Pellets

• Intensity: light to heavy

Precipitation Produced by Altocumulus

• Usually nil

• Intermittent light rain or light snow

Precipitation Produced by Altocumulus Castellanus (ACC)

• Rain and/or snow showers

• Virga

• May produce dry microbursts

Precipitation Produced by Cirrus, Cirrostratus, and Cirrocumulus

Nil

What is the significance of Cirrocumulus?

Indicates presence of turbulence in the high étage

What other phenomena occurs from Cirrocumulus?

Virga

What is the significance of Altocumulus Castellanus?

• Often forerunner of thunderstorms

• Indicates unstable air aloft