Definition and Formation of Clouds

• Clouds are visible masses of minute water droplets or ice crystals suspended in the atmosphere.
• They form when moist air rises, cools adiabatically, and reaches its dew point, causing condensation around hygroscopic nuclei like dust, salt, or smoke particles.

Clouds are a key link between condensation and precipitation in the hydrological cycle.

Role of Clouds in Climate Regulation

Clouds play a dual role in Earth’s energy balance and climate system:

Cooling Effect:

• Reflect a portion of incoming solar radiation (albedo effect).
• Thick, low clouds (e.g., stratus) have high albedo, keeping surfaces cooler.

Warming Effect:

• Trap outgoing longwave radiation emitted from Earth’s surface.
• Thin, high clouds (e.g., cirrus) enhance greenhouse warming.

The net impact depends on cloud type, altitude, and coverage.

Reasons for Cloud Colour and Appearance

• White Clouds: Appear white due to scattering of all visible light wavelengths by the water droplets/ice crystals.
• Grey or Dark Clouds: Appear dark because they are thicker and denser, allowing less sunlight to pass through.
• Brilliant Colours at Sunrise/Sunset: Due to Rayleigh scattering of shorter wavelengths and diffraction through moisture layers.

Types of Clouds

Clouds are classified based on:

(A) Shape (form or appearance)

(B) Altitude (height above ground level)

A. Classification Based on Shape

1. Cirrus Clouds

• Appearance: High, thin, wispy, feather-like.
• Altitude: 8,000 – 12,000 m
• Composition: Ice crystals
• Weather: Indicate fair weather, but may signal approaching disturbance.

2. Cumulus Clouds

• Appearance: White, fluffy, cotton-like with flat bases.
• Altitude: 1,000 – 6,000 m
• Weather: Fair weather; if vertical development occurs → may turn into cumulonimbus (thunderclouds).

3. Stratus Clouds

• Appearance: Low, uniform grey layer covering the sky like a blanket.
• Altitude: Up to 2,000 m
• Weather: Dull, overcast days; may bring light drizzle.

B. Classification Based on Altitude

1. Low Clouds (Surface to ~2,000 m)

a) Stratocumulus

• Low, lumpy, grey clouds.
• May bring light rain or drizzle.

b) Stratus

• Flat, uniform grey clouds.
• Associated with gloomy, overcast conditions.

c) Nimbostratus

• Thick, dark grey rain clouds.
• Produce continuous, steady rainfall or snowfall.

2. Middle Clouds (2,000 – 6,000 m)

a) Altostratus

• Grey or blue-grey sheets, covering the entire sky.
• May allow diffused sunlight; bring light rain or snow.

b) Altocumulus

• White or grey patches, often arranged in groups.
• May indicate instability and possible thunderstorms.

3. High Clouds (Above 6,000 m)

a) Cirrus

• Wispy, ice-crystal clouds (already discussed above).
• Appear before weather changes.

b) Cirrostratus

• Thin, veil-like clouds covering the sky.
• May cause halo around the sun or moon.

c) Cirrocumulus

• Small, white flakes or ripples in patches.
• “Mackerel sky” appearance; no precipitation.