Short notes 11 : Precipitation
Rain Shadow (General Concept)
- Always on leeward/downwind side of mountains.
- Mechanism: Moist air rises on windward side → cools & rains → dry air descends on leeward side (warms adiabatically, Foehn effect) → dry region.
- Windward: Wet, rising air.
- Leeward: Dry, descending air.
Western Ghats (SW Monsoon)
- Moist SW winds (June–Sept) hit western slopes → heavy rain.
- Leeward (East) becomes dry rain shadow region.
Eastern Ghats (NE Monsoon)
- Moist NE winds (Oct–Dec) strike east coast → coastal rain (TN, AP).
- Leeward (West) is rain shadow zone.
Precipitation Formation Theories
1. Coalescence / Giant Nucleus Theory (Warm Clouds)
- Warm clouds (>0°C); only liquid droplets.
- Mechanism: Large droplets collide/coalesce → rain.
- Favored over sea (hygroscopic salt nuclei).
- Result: Drizzle or rain, no icing hazard.
2. Ice-Crystal / Bergeron Process (Cold Clouds)
- Cold clouds (<0°C) with ice crystals + supercooled water.
- SVP (water) > SVP (ice) → vapor moves to ice → ice grows, droplets evaporate.
- Dominant in mid/high latitudes.
Cold Clouds
- Extend into subfreezing levels; contain ice + supercooled water droplets (SWD).
- CB/TCU: Strong updrafts → severe icing.
- AS/NS: Layered cold clouds → prolonged icing.
Warm Clouds
- Entirely >0°C; only liquid droplets.
- No icing, precipitation via coalescence.
- Common in tropics.
Artificial Rainmaking / Fog Dispersal
- Cloud Seeding: Introduce AgI or dry ice → enhance rain or clear fog.
- Cold Fog Clearing: Dry ice converts SWD to ice → fall out → temporary clearing.
- Warm fog seeding is inefficient/short-lived.
Cloud & Weather Types
- Convective Clouds (CU, CB): Form by surface heating; Tmax ~1500 LMT → max growth.
- Anticyclone: Descending, warming air → clear skies; only ST/SC under inversion.
- Turbulence Clouds (ST, SC): Form by mechanical mixing below inversion; flat tops.
- Fair Weather CU (morning): May evolve into afternoon TS/CB; morning turbulence likely.
Precipitation Types & Visibility
- Drizzle: <0.5 mm, from ST; poor visibility (~500 m).
- Continuous Rain: From Nimbostratus (NS), stable air.
- Showers: From CU/CB, short, intense, variable.
- Cloudburst: Intense shower rain from CB.
- Flash Flood: Sudden rise due to cloudburst or slow TS.
- Snow Visibility: Heavy snow → visibility 50–200 m.
Special Weather Concepts
- Sleet: Mix of rain & snow (0–6°C); ICAO: tiny ice pellets (refrozen).
- Snow Survival: Up to +4°C at surface if air is dry.
- Rainy Day: ≥2.5 mm rain in 24 hr.
- Polar Air over Warm Sea: Heating from below → instability → CU/CB showers.
- Tropical Rainfall: Afternoon max due to convection; nocturnal coastal max from land breeze.
- Temperate Rainfall: Winter max from Polar Front Depressions.
- Western Disturbances: Bring winter rain/snow to J&K.