Clouds formation & Types of clouds UPSC (Geography)

• A cloud is a visible mass of tiny water droplets and/or ice crystals suspended in the air, formed when water vapour condenses on aerosols as rising air cools to its dew point.
• Clouds and fog are alike in structure; clouds form aloft by the adiabatic cooling of rising air, fog at the ground. All clouds lie in the troposphere, except rare polar clouds higher up.
• All precipitation comes from clouds, though most clouds never rain. Clouds also reflect sunlight and trap outgoing heat; on balance they cool the planet, keeping days cooler and nights warmer.

Cloud Formation

• Two requirements:
  • Saturation, by cooling the air to its dew point or by adding vapour
  • Condensation nuclei: salt, dust, smoke and sulphate particles (aerosols) about 0.1–1 micrometre across, on which droplets of about 10–20 micrometres grow
• Rising air expands and cools at about 10 °C per km until it reaches its dew point at the condensation level, which marks the flat cloud base. Above it, released latent heat slows the cooling to about 4–7 °C per km.
• Air is made to rise in five ways:
  • Surface heating: thermals rise from heated ground, building cumulus
  • Orographic lifting: air forced over mountains forms layered and cap clouds on the windward side
  • Fronts: warm air glides up over a wedge of cold air along fronts
  • Convergence: air streams meeting, as at the ITCZ, are forced upward
  • Turbulence: eddies from wind shear lift and mix air, forming stratocumulus decks

Significance of Clouds

• Precipitation: clouds are the only source of rain, snow and hail, and their type tells what kind of precipitation to expect.
• Heat budget: clouds reflect incoming sunlight (cooling) and absorb and re-radiate outgoing long-wave radiation (warming). Low, thick clouds mainly cool; high, thin cirrus mainly warms.
  • Cloudy summer days are cooler and cloudy winter nights warmer than clear ones.
  • The cloud feedback is the largest single source of uncertainty in how much the Earth will warm; the IPCC Sixth Assessment Report judges it, on balance, to amplify warming.
• Weather signs and aviation: cloud type, height and amount (measured in oktas, eighths of the sky) are routine weather observations; low cloud and cumulonimbus are major hazards to flying.

Classification of Clouds

• Luke Howard (1803) gave clouds their Latin names. The World Meteorological Organization’s International Cloud Atlas (latest edition launched online on 23 March 2017) recognises 10 genera, subdivided into 15 species and 9 varieties.
• Names combine five roots: cirro- (high, wispy, fibrous), cumulo- (heaped), strato- (layered), alto- (middle level) and nimbo- (rain-bearing).
• Genera fall into height families (temperate-latitude heights; clouds sit higher in the tropics and lower near the poles):

High Clouds

• Cirrus (Ci): detached white filaments, patches or bands with a fibrous, silky look, wholly of ice. They glow yellow or red at sunrise and sunset and give no precipitation; spreading cirrus often heralds an approaching warm front.

• Cirrostratus (Cs): a thin, milky, transparent veil that may cover the whole sky. Sunlight refracted by its ice crystals produces a halo round the sun or moon, which distinguishes it from thin stratus.

• Cirrocumulus (Cc): thin patches of tiny grains or ripples without shading; a regular pattern forms a “mackerel sky“.

Middle Clouds

• Altostratus (As): a grey or bluish sheet through which the sun appears dimly “as through ground glass”, with no halo; it may give light rain or snow.

• Altocumulus (Ac): white or grey patches of rounded masses or rolls with shading (“sheep” or “woolpack” clouds); thin edges produce a coloured corona.

• Nimbostratus (Ns): a thick, dark grey layer that blots out the sun and gives steady, prolonged rain or snow, without thunder or hail. Its base sinks into the low level as precipitation continues, and ragged streaks of rain evaporating before reaching the ground form virga.

Low Clouds

• Stratocumulus (Sc): grey or whitish lumpy rolls or a honeycomb with darker parts; the world’s most widespread cloud, covering the subtropical oceans off cold-current coasts.

• Stratus (St): a uniform grey layer with a flat base, giving only drizzle; when it touches the ground it is fog. Its flat top marks an inversion.

Clouds of Vertical Development

• Cumulus (Cu): detached, dense, cauliflower-like mounds with brilliant sunlit tops and flat, dark bases at the condensation level. Fair-weather cumulus forms on sunny afternoons over land and dissolves at night; growing cumulus is the first stage of a thunderstorm.

• Cumulonimbus (Cb): a towering thunderstorm cloud whose glaciated top spreads into a flat anvil at the tropopause. It brings heavy showers, thunder, lightning, hail, squalls and sometimes tornadoes.
  • In India it produces the pre-monsoon nor’westers (kalbaisakhi) of Bengal and Odisha, the monsoon’s heavy convective bursts and Himalayan cloudbursts.

Special Clouds and Optical Effects

• The 2017 Atlas added a new species, volutus (roll cloud), the wave-like feature asperitas, and “special clouds” named by origin:
  • flammagenitus (over fires and volcanoes), homogenitus (from human activity, such as aircraft contrails), homomutatus (contrails spreading into cirrus), cataractagenitus (over waterfalls) and silvagenitus (over forests)
• Lenticular (lens-shaped) clouds stand over mountain waves; mammatus pouches hang beneath anvils.
• Nacreous (mother-of-pearl) clouds form in the polar stratosphere at about 15–25 km, and noctilucent clouds in the mesosphere at about 80–85 km, the highest clouds of all.
• Halos (refraction by ice crystals in cirrostratus) and coronas (diffraction by water droplets in altocumulus) help identify clouds.

Cloud Seeding

• Cloud seeding (artificial precipitation) adds particles to existing clouds to speed up the growth of drops or ice crystals. It cannot create clouds and works only on clouds already near producing rain.
• History: Vincent Schaefer made snow fall from a supercooled cloud with dry ice on 13 November 1946; Bernard Vonnegut (1947) found silver iodide to be an efficient ice nucleus; both worked with Irving Langmuir.

• Indian experience:
  • CAIPEEX Phase IV (Indian Institute of Tropical Meteorology) seeded convective clouds with salt over Solapur, a rain-shadow district of Maharashtra. Its report (July 2023) found an average 18% ± 2.6% more rainfall over about 100 km² downwind under suitable conditions.
  • Karnataka’s Project Varshadhari (2017) seeded clouds over drought-hit districts.
  • Delhi (28 October 2025): IIT Kanpur and the Delhi government fired silver-iodide and salt flares over Burari, Karol Bagh, Mayur Vihar and Badli to clean smoggy air. The clouds held only about 15–20% moisture, and no measurable rain fell in Delhi.
  • Mission Mausam (approved September 2024, ₹2,000 crore) includes research on weather modification, with a cloud chamber at Pune.
• Uses: water supply and drought relief, hydropower (Tasmania), snowpack enhancement, fog clearing at airports and hail suppression.
• Concerns: results are hard to verify; seeding may “rob” rain from downwind areas; costs are high; silver residues raise environmental questions.