Short notes 09 : Vertical Motion & Cloud
Clouds are visible manifestations of atmospheric processes and are critical indicators of stability, instability, weather hazards, icing, turbulence, and visibility . For aviation, cloud type, height, and structure directly influence flight safety and operational decision-making .
Cloud Classification by Height Cloud Level Height Range Typical Clouds Low Surface – 6,500 ft ST, SC, NS (base), CU Medium 6,500 – 23,000 ft AC, AS, NS (upper part) High 16,500 – 45,000 ft CI, CS, CC Great Vertical Development Surface → Tropopause TCU, CB
Stratiform (Layer) Clouds – Stable Air
Stratus (ST)
Low, uniform grey layer resembling lifted fog
Little vertical development
Produces drizzle (DZ) or mist
Indicates stable air and poor visibility
Stratocumulus (SC)
Low cloud with cellular appearance
Rarely produces precipitation (occasional drizzle)
Altostratus (AS)
Medium-level sheet cloud
Grey/blue-grey appearance
Common ahead of warm fronts
Produces continuous or intermittent rain/snow
Nimbostratus (NS)
Thick, dark, extensive cloud layer
Continuous rain or snow (moderate to heavy)Vertical extent: Surface to ~15,000 ftCan span low and medium levels
Associated with warm fronts and overrunning
Moderate to severe icing possible
Cumuliform (Heap) Clouds – Unstable Air
Cumulus (CU)
Cauliflower-shaped cloud
Formed by convection (thermals)
Indicates unstable air
Fair-weather CU causes bumpy air
Cumulonimbus (CB)
Cloud of great vertical development
Extends from surface to tropopause
Contains liquid water, supercooled droplets, and ice
Produces:
Heavy showers
Thunderstorms
Hail
Severe turbulence
Moderate to severe clear icing
Most hazardous cloud to aviation
CB Types
CB Calvus: Rounded top, no anvilCB Capillatus: Fibrous cirrus anvil , capped by tropopause
High-Level Clouds (Cirri-form)
Cirrus (CI)
Thin, wispy, hair-like
Ice crystals only
Nil icing and turbulence
Cirrostratus (CS)
Thin sheet cloud
Produces halo (refraction through ice crystals)
Cirrocumulus (CC)
Small ripples/grains at high altitude
Ice crystals only
Special Cloud Types & Indicators
Altocumulus Castellanus (ACC)
Turret-shaped mid-level cloud
Indicates mid-level instability
Often precedes thunderstorm development
Altocumulus Lenticularis (ACL)
Lens-shaped, stationary
Forms in mountain (lee) waves
Indicates severe turbulence and possible icing
Precipitation Types
Showers (SH)
Sudden start and end
Rapid intensity changes
Only from convective clouds (CU, CB)
Continuous / Intermittent Precipitation
From stratiform clouds (NS, AS)
Drizzle (DZ)
Very fine drops (<0.5 mm)
From Stratus / Stratocumulus
Hail (GR)
From Cumulonimbus only
Strong updrafts essential
May fall beneath the anvil
Virga
Precipitation evaporates before reaching ground
Indicates dry air below cloud
May intensify downdraughts / windshear
Freezing Precipitation
Freezing Rain (FZRA) / Freezing Drizzle (FZDZ)
Supercooled liquid drops
Freeze on contact → clear (glaze) ice
Severe icing hazard
Common near warm fronts
9.8 Optical Phenomena Phenomenon Cause Associated Cloud Halo Refraction through ice crystals Cirrostratus Corona Diffraction through small water droplets AS / AC
9.9 Contrails
Mintra Level: Minimum altitude for contrail formationMaxtra Level: Maximum altitude for contrail formationRequire cold, humid air
No contrails below Mintra or above Maxtra
9.10 Cloud Ceiling & Instruments
Ceiling: Lowest cloud layer covering >4 oktas
BKN (5–7 oktas)
OVC (8 oktas)
Measured using a Ceilometer
Height reported AGL
9.11 Stability Summary Stability Cloud Type Weather Stable Air ST, SC, AS, NS Smooth air, poor visibility, continuous precipitation Unstable Air CU, CB Turbulence, showers, good visibility outside precipitation
Key Points
Showers → CU / CB only
Continuous rain/snow → NS
Most hazardous cloud → CB
Drizzle → Stratus
Halo → Ice crystals (CS)
Lenticular clouds → Mountain waves
NS spans low + medium levels
CB spans all levels
Severe icing → CB, FZRA
