AFTERNOON STORMS
LEEWARD SHADOW
& TROPICAL VORTEX
Precipitation
Rainfall, Snowfall, Sleet & Hail · Collision-Coalescence vs. Bergeron Mechanism · Convectional, Orographic & Cyclonic Rainfall (28 Micro-Realms)
From Cloud Saturation to Ground Precipitation (NCERT Chapter 11)
The process of continuous condensation in free air helps the condensed particles to grow in size. When the resistance of the air fails to hold them against the force of gravity, they fall onto the earth's surface as Precipitation. This happens via two primary microphysical mechanisms: Collision-Coalescence in warm tropical clouds and the Ice-Crystal (Bergeron) Mechanism in cold clouds. Precipitation takes various forms—Rainfall, Snowfall, Sleet, Hail, and Drizzle—and is driven globally by three principal uplift mechanisms: Convectional, Orographic (Relief), and Cyclonic (Frontal).
I. Precipitation Physics & Droplet Growth Mechanisms (NCERT)
Precipitation Definition (NCERT)
- Condensed moisture falling to the ground
- Includes liquid or solid forms of water
- After air is cooled below its dew point & saturated
Condensation vs. Precipitation
- Condensation alone does not mean precipitation
- Cloud droplets are microscopic ($0.02\text{ mm}$)
- They float freely in upward air currents (NCERT)
Droplet Growth Requirement
- Droplets must grow to at least $0.5\text{ mm}$
- So that gravity overcomes atmospheric updrafts
- Transforms suspended cloud into falling rain
Collision-Coalescence Mechanism
- Dominant in warm tropical clouds ($>0^\circ\text{C}$)
- Larger droplets fall faster, colliding and merging
- Coalescing smaller droplets until heavy enough to fall
Ice-Crystal (Bergeron) Mechanism
- Dominant in mid-to-high latitude cold clouds
- Ice crystals and supercooled water coexist
- Vapor deposits onto ice crystals rapidly, causing them to fall
II. Forms of Precipitation: Rain, Snow, Sleet, Hail & Freezing Rain
Forms of Precipitation (NCERT)
- NCERT classifies precipitation into:
- Rainfall, Snowfall, Sleet, Hail, Drizzle
- Each tied to distinct thermal vertical profiles
Rainfall Definition
- Most common form of precipitation
- Consists of liquid water drops larger than $0.5\text{ mm}$
- Originates when air temperature is above freezing
Snowfall Definition
- When temperature is below freezing ($<0^\circ\text{C}$)
- Moisture condenses directly as ice crystals
- Hexagonal crystals unite into snowflakes
Sleet Mechanics
- Frozen raindrops caused by melting snow
- Falling through a deep sub-freezing layer near the ground
- Bounces on impact as tiny ice pellets
Freezing Rain
- Raindrops remain supercooled in shallow sub-freezing air
- Instantly freeze upon hitting cold ground or trees
- Creates dangerous smooth ice sheets (Glaze)
Hail Mechanics & Hailstones
- Solid precipitation formed in Cumulonimbus clouds
- Strong updrafts carry water droplets up and down
- Builds concentric layers of ice around a frozen core
Drizzle & Virga
- Drizzle: Uniform fine water droplets ($<0.5\text{ mm}$) drifting slowly
- Virga: Rain or snow falling from a cloud that evaporates before hitting the ground
III. The Three Principal Precipitation Mechanisms (NCERT Ch 11)
1. Convectional Rainfall (NCERT)
- Air is heated by intense solar insolation
- Expands and rises vertically in convection currents
- Cools to dew point $\to$ heavy afternoon downpours with thunder
Convectional Daily Cycle
- Very common in equatorial regions daily
- Occurs regularly around 4:00 PM ("4 o'clocks")
- Also happens in continental interiors in summer
2. Orographic / Relief Rainfall
- Saturated air is forced to rise over a mountain barrier
- Cools adiabatically $\to$ heavy rain on windward slopes
- Descending air on leeward side warms $\to$ Rain Shadow!
Windward vs. Rain Shadow (NCERT)
MAX RAIN
SHADOW
- Windward slope: Maximum precipitation belt
- Leeward slope: Descending dry air (Rain Shadow)
- E.g., Mahabaleshwar gets rain, Pune (leeward) stays dry
3. Cyclonic / Frontal Rainfall
- Precipitation resulting from cyclonic activity
- Temperate fronts lift warm air gradually
- Tropical cyclones drive massive vortex convergence
Monsoonal Rainfall Mechanism
- Combination of macro land/sea breeze and orography
- Western Ghats force SW monsoon winds to dump massive rain
IV. Global Distribution of Precipitation & Extremes (NCERT)
Global Precipitation Patterns
- Precipitation decreases from equator to poles (generally)
- Controlled by pressure belts, winds, and relief
Equatorial Belt Precipitation
- Heavy daily convectional rainfall year-round
- Amazon, Congo basin, and Indonesian archipelago
- Exceeds $200\text{ cm}$ annual precipitation
Subtropical Desert Belts ($30^\circ$)
- Subtropical High-pressure sinking air zones ($30^\circ$ N/S)
- Prevent cloud formation, resulting in major deserts
- Sahara, Arabian, Australian, and Atacama deserts
Temperate West Coasts
- Westerlies bring moist air onto western coasts
- Europe, Pacific Northwest (USA), Chile
Continental Interiors
- Precipitation decreases as air moves inland
- Far from oceanic moisture sources
- Interior Asia and North America experience semi-arid steppes
World Wettest Places (Mawsynram)
- Located in the funnel-shaped Khasi Hills (Meghalaya)
- Bay of Bengal monsoon winds get trapped
- World's highest average annual precipitation ($11+\text{ meters}$!)
World Driest Places (Atacama)
- Atacama Desert (Chile) & McMurdo Dry Valleys
- Extreme rain shadows combined with cold currents
- Some weather stations have never recorded a drop of rain!
Master Precipitation Matrix
- Completes Precipitation & Mechanisms (NCERT Ch 11)
- Locks droplet growth, forms, and uplift types
- Transitions to Realm 77 (World Climate Types)
Core Distinctions
• Convectional Rainfall: Driven by intense surface heating, expanding air rising vertically, and producing afternoon thunderstorms (common at the Equator).
• Orographic Rainfall: Driven by topographic barriers forcing moist wind to rise on the windward slope, creating heavy rain and a dry rain shadow on the leeward slope.
• Cyclonic Rainfall: Driven by atmospheric convergence in storms (frontal lifting in temperate lows or vortex ascent in tropical cyclones).
• Collision-Coalescence: Operates in warm clouds ($>0^\circ\text{C}$) where larger drops overtake and merge with smaller droplets.
• Bergeron (Ice-Crystal) Mechanism: Operates in cold mixed-phase clouds where supercooled water and ice crystals coexist; vapor deposits onto ice crystals, making them heavy enough to fall.
The funnel-shaped topography of the Khasi Hills in Meghalaya traps the moisture-laden SW Monsoon winds from the Bay of Bengal, forcing extreme orographic uplift and condensation.
Precipitation Matrix
NCERT Chapter 11 Reference Guide