CLOUDS & RAIN
CLEAR & SUNNY
EYEWALL WINDS
Cyclones & Anticyclones
Surface Convergence & Ascent · Divergence & Subsidence · Tropical vs. Temperate Cyclone Mechanics (28 Micro-Realms)
Atmospheric Vortices: Convergence, Ascent, and Storm Systems (NCERT Ch 10)
Large-scale atmospheric circulation systems organize around pressure extremes. A Cyclone is a low-pressure system characterized by surface inward convergence, counter-clockwise spiraling in the Northern Hemisphere, and strong vertical air ascent that produces clouds and heavy precipitation. Conversely, an Anticyclone is a high-pressure system featuring surface outward divergence, clockwise spiraling in the Northern Hemisphere, and descending air (subsidence) that brings clear, stable weather. Globally, these divide into Temperate (Extra-Tropical) Cyclones driven by frontal polar-front interactions and Tropical Cyclones powered by warm ocean heat engines ($>26.5^\circ\text{C}$).
I. Cyclonic Circulation & Surface Inward Convergence (NCERT Core)
What is a Cyclone? (NCERT)
- System with low pressure at the center
- High pressure surrounding it on all sides
- Winds blow inward towards the center (NCERT)
Surface Inward Convergence
- Surface friction makes winds cross isobars
- Spiraling inwards into the low-pressure eye
- Piles up mass at the storm center
Vertical Ascent & Storms
- Converging air cannot accumulate indefinitely
- Forced to rise vertically (Ascent)
- Cools adiabatically to form towering rain clouds
Northern Hemisphere Cyclone
IN N. HEMISPHERE
- Due to rightward Coriolis deflection in NH
- Inward spiraling winds turn Anti-Clockwise
- Standard cyclonic rotation north of equator
Southern Hemisphere Cyclone
IN S. HEMISPHERE
- Due to leftward Coriolis deflection in SH
- Inward spiraling winds turn Clockwise
- Standard cyclonic rotation south of equator
II. Anticyclonic Circulation & Surface Outward Divergence (NCERT)
What is an Anticyclone? (NCERT)
- System with high pressure at the center
- Low pressure surrounding it on all sides
- Winds blow outward from the center (NCERT)
Surface Outward Divergence
- Air subsiding from aloft hits the surface
- Spreads and diverges outwards
- Crossing isobars away from high center
Subsidence & Fair Weather
- Sinking air (Subsidence) warms adiabatically
- Dissipates clouds and relative humidity
- Brings clear skies, calm winds, and sunshine
Anticyclone Spin Directions
CLOCKWISESH:
ANTI-CLOCK
- Opposite to cyclonic spin!
- NH Highs spin Clockwise (outward)
- SH Highs spin Anti-Clockwise (outward)
III. Tropical vs. Temperate (Extra-Tropical) Cyclones (NCERT Ch 10)
Two Great Cyclone Families
CYAN
FRONTAL
- 1) Extra-Tropical (Temperate) Cyclones
- 2) Tropical Cyclones
- Differ in origin, structure & geographic zones
Temperate Cyclones ($35^\circ\text{–}65^\circ$)
- Occur in temperate regions ($35^\circ\text{–}65^\circ$ N/S)
- Form along the Polar Front (NCERT)
Frontal Genesis (Polar Front)
- Warmed tropical air meets cold polar air
- Forms a sharp boundary called a Front
- Wave disturbance initiates frontal cyclone
Warm vs. Cold Fronts
GENTLE SLOPE
STEEP WEDGE
- Warm Front: Gentle slope ($1:200$), steady drizzle over days
- Cold Front: Steep wedge ($1:50$), violent thunderstorms & squalls
Occluded Fronts
- Fast-moving cold front overtakes warm front
- Lifts warm sector entirely off the ground
- Signals the mature decaying stage of storm
Temperate Weather Sequence
- Predictable sequence: Cirrus clouds $\to$ halo $\to$ altostratus rain $\to$ cold clear clearing
- Moves West-to-East driven by Westerlies
IV. Tropical Cyclones, Prerequisites & Eye Structure (NCERT Ch 10)
Tropical Cyclones ($5^\circ\text{–}30^\circ$)
- Violent storms originating over tropical oceans
- Between $5^\circ$ and $30^\circ$ North and South latitude
- Compact diameter (150–500 km) with extreme winds
Prerequisite #1: Warm Water
- Requires continuous sea surface temp $>26.5^\circ\text{C}$
- Extending to a depth of at least 60 m
- Supplies massive latent heat engine
Prerequisite #2: Coriolis Force
- Must be at least $5^\circ$ away from the equator
- So that Coriolis force is strong enough
- To impart rotational spin to converging winds
Prerequisite #3: Low Shear
- Little change in wind speed/direction with height
- Prevents storm core from tilting and shearing apart
- Allows thunderstorm towers to cluster
The Eye of the Cyclone (NCERT)
- Circular center of 10–50 km diameter
- Characterized by calm winds and clear skies
- Air gently descends (subsidence) here (NCERT)
The Eyewall (NCERT)
- Surrounds the calm eye like a towering wall
- Contains strongest hurricane-force winds ($200+\text{ kmph}$)
- Most destructive part of the storm (NCERT)
V. Global Basin Names, Monsoonal Depressions & Storm Surges
Global Basin Nomenclature
- North Atlantic & NE Pacific = Hurricanes
- NW Pacific (China Sea) = Typhoons
- Indian Ocean = Cyclones; Australia = Willy-Willies
Tropical vs. Temperate Summary
NO FRONTS · OVER SEA
FRONTAL · OVER LAND/SEA
- Tropical: Thermal engine, no fronts, dies over land
- Temperate: Frontal contrast, large area, lasts longer over land
Indian Monsoonal Depressions
- Frequent low-pressure depressions form in Bay of Bengal
- Move northwest across Indian mainland
- Provide vital rainfall pulses to Central/North India
Storm Surges (NCERT)
- Abnormal rise of sea level during landfall
- Driven by low barometric pressure & onshore winds
- Causes 90% of tropical cyclone fatalities (NCERT)
Anticyclone Weather Impacts
- Persistent blocking high-pressure systems
- Cause severe summer heatwaves and droughts
- Trap urban air pollution near ground
Satellite Meteorology
- Geostationary satellites track cloud spirals
- Dvorak technique estimates storm intensity
- Provides critical early landfall warnings
Master Circulation Matrix
- Completes Cyclonic Circulation (NCERT Ch 10)
- Locks Convergence/Ascent vs Subsidence logic
- Transitions to Realm 70 (General Circulation of Atmosphere)
Core Distinctions
• Cyclone (Low Pressure): Surface winds converge inward and spiral up (ascent). This adiabatic cooling creates clouds, storms, and heavy rain.
• Anticyclone (High Pressure): Surface winds diverge outward while air sinks from above (subsidence). This adiabatic warming dries out the air, bringing clear, sunny, and stable weather.
• Tropical Cyclones: Formed over warm tropical oceans ($>26.5^\circ\text{C}$) between $5^\circ\text{–}30^\circ$ latitude; powered by latent heat; have no fronts; feature a calm central Eye and destructive Eyewall; die rapidly over land.
• Temperate Cyclones: Formed along the Polar Front in mid-latitudes ($35^\circ\text{–}65^\circ$); powered by horizontal temperature contrasts (fronts); feature warm and cold fronts; cover vast areas and last for days over land and ocean.
• Cyclones (Low): NH = Anti-clockwise / Counter-clockwise; SH = Clockwise.
• Anticyclones (High): NH = Clockwise; SH = Anti-clockwise.
Cyclonic Circulation Matrix
NCERT Chapter 10 Reference Guide