Chapter 8
Vertical Motion and Clouds

1. Causes of Vertical Motion

Five Main Causes

flowchart TD
    A[Causes of Vertical Motion] --> B[Terrain/Orographic]
    A --> C[Convection/Thermals]
    A --> D[Pressure Systems]
    A --> E[Frontal Zones]
    A --> F[Wind Shear]
    B --> G[Mountain Waves → Nacreous Clouds]
    C --> H[CU & CB formation]
    D --> I[Low → Ascent; High → Subsidence]
    E --> J[WF: Gradual; CF: Abrupt uplift]
    F --> K[CAT amplified near Jet Stream]

2. Convergence and Divergence

3. Clouds — Classification

There are 10 major genera classified by Form (Stratiform, Cumuliform, Cirriform) and Height (High, Medium, Low).

Cloud Height by Region

4. High Clouds

Cirrus (CI)

White and fibrous, delicate filaments or patches, narrow bands. Composed of ice crystals. Do not cause precipitation or ice accretion.

Cirrostratus (CS)

Thin whitish veil of fibrous or smooth appearance. Covers almost whole sky. Produces Halo — a coloured circular ring around sun or moon with violet colour inside. When thick may cause any ice fall, remaining confined to high/medium levels. Hardly any ice accretion.

Cirrocumulus (CC)

Thin, white, wave-like very small puffs. Composed of ice crystals. Sky visible in between puffs. Do not cause precipitation; hardly any ice accretion.

5. Medium Clouds

Altocumulus (AC)

White-grey clouds, patches, puffs, rounded masses, rolls, etc. Round masses generally well arranged in long waves, rows or lanes. Sky visible between puffs. Altocumulus Lenticularis = lens-shaped clouds at crests of mountain waves on leeward side of high mountains. No precipitation; may cause ice accretion.

Altostratus (AS)

Uniformly greyish or whitish cloud sheets/layers of fibrous appearance. Totally or partly cover the sky. If thin → see sun through it like ground glass. Optical phenomenon Corona occurs in AS. Thick AS: continuous rain or snow. Ice accretion possible below freezing temperatures.

6. Low Clouds

Stratus (ST)

Grey cloud layer with uniform base. May stretch horizon to horizon. May give drizzle, ice prisms or snow grains. ST touching ground = fog. Ragged/patchy ST = Fracto Stratus.

Stratocumulus (SC)

Low clouds of large, irregular puffs or rolls, gray or whitish. Regularly arranged. No serious precipitation.

Nimbostratus (NS)

Dark and grey thick cloud layer. Causes continuous rain or snow. Thick enough to blot out the sun. Extends both medium and lower levels. Ice accretion possible below freezing. Ice Pallets fall from thick NS clouds especially in Warm Fronts. Ice pallets are transparent ice particles, spherical/irregular, about less than 5 mm dia.

7. Clouds with Vertical Development

Cumulus (CU)

Detached clouds, dense with sharp outlines. Base dark, nearly horizontal. Develop vertically as rising mounds, domes or towers. Bulging upper part resembles a cauliflower. Sunlit parts brilliant white. Base sometimes ragged = Fracto Cumulus. A well-developed CU with great vertical extent = Towering Cumulus (TCU).

Fair Weather Cumulus

Limited vertical extent. Caused by thermal currents due to surface heating. Occur during daytime only, develop in forenoon, reach max in afternoon, dissipate in evening. Do not cause precipitation.

Cumulonimbus (CB)

Heavy and dense cloud of large vertical extent, like mountains or huge towers. Upper portion mostly smooth, fibrous and nearly flattened — spreads out like an anvil. Base often very dark. Low ragged cumulus fracto or stratus fractus sometimes present below. Virga = shaft of rain or snow falling from distant cloud which vapourises before reaching the ground.

flowchart LR
    CU[Cumulus CU] -->|grows| TCU[Towering Cumulus TCU]
    TCU -->|further development| CB[Cumulonimbus CB]
    CB --> P1[Rain RA]
    CB --> P2[Showers SH]
    CB --> P3[Hail GR]
    CB --> P4[Snow SN]
    CB --> P5[Virga]
    CB --> P6[Anvil Top]

8. Very High Level Clouds

Nacreous Clouds (Mother of Pearl Clouds)

Infrequent. Mainly from Scotland and Scandinavia. Resemble Cirrus or Altocumulus Lenticularis. Show very strong irisation (rainbow coloured patches). May be composed of ice crystals or supercooled water droplets. Form in the upper Mesosphere.

Noctilucent Clouds

Resemble Cirrus but have bluish, silvery or orange/red colour. Occur in upper Mesosphere between 80–85 km. Visible soon after sunset or before sunrise between 72°N and 45°N, mostly around 55°N, in summers (late May–mid August). Thought to consist of ice particles (not meteoric dust, as originally believed).

9. Cloud Base, Amount & Ceiling

Reporting Cloud Base

Cloud Amount (Oktas)

Cloud Ceiling

Cloud Amount in Imaginary Oktas

For expressing cloud amount, the sky is divided into eight imaginary parts. Each part is called one Okta.

10. Cloud Species

11. Condensation Trails (Contrails)

12. Mintra, Drytra and Maxtra Levels

13. Flying in Clouds — Operational Significance

Stratiform Clouds (ST, AS)

Flying is generally smooth. In NS clouds during monsoon with thick AS and NS layers, visibility is reduced due to continuous precipitation. ST clouds reduce visibility and cause difficulty identifying the runway during landings. Ice accretion also occurs in AS and NS clouds above freezing levels.

Cumuliform Clouds (CU, CB)

CC and AC may cause bumpiness. Well developed CU, TCU and CB can cause severe turbulence, downdrafts, gusts, ice accretion, lightning, hail and strong up/down draughts. Flying through CB should be avoided during TS. Hail may be experienced in clear air below the anvil of a CB.

14. Practice Q&A

15. Master Reference Tables

All Numerical Values

Cloud Type Quick Reference

Okta/Cloud Amount Table

Answer Key — Q&A Section

(Answers from textbook Q1–20: b b b d a b c a b c a b b a b c c b a a)