ATMOSPHERIC PRESSURE
**Definition:** Atmospheric pressure is the weight of a column of air contained in a unit area from mean sea level to the top of the atmosphere.
Measured in **millibars (mb)** — standard unit for meteorological measurements
At sea level, average atmospheric pressure = **1,013.2 mb** or **1,013.25 mb**
Pressure exists due to **gravity** — air at the surface is denser, hence higher pressure
Air pressure decreases with height — primary cause of **wind** (horizontal air movement from high to low pressure)
Measured using **mercury barometer** or **aneroid barometer**
Vertical Variation of Pressure
Pressure decreases rapidly in the lower atmosphere
Decrease amounts to approximately **1 mb per 10 m increase in elevation**
Rate of decrease is **not constant** — decreases more rapidly near the surface
**Standard atmosphere values** (from NCERT Table 9.1):
Sea Level: 1,013.25 mb, 15.2°C
1 km: 898.76 mb, 8.7°C
5 km: 540.48 mb, –17.3°C
10 km: 265.00 mb, –49.7°C
**Vertical pressure gradient force** is much larger than horizontal pressure gradient
However, it is **balanced by equal but opposite gravitational force**
Therefore, strong upward winds are not experienced
Horizontal Distribution of Pressure
**Isobars:** Lines connecting places having **equal atmospheric pressure** at constant levels
Used to study sea level pressure distribution
**Pressure pattern interpretation:**
**Close isobars** = Strong pressure gradient = Faster winds
**Distant/spaced isobars** = Weak pressure gradient = Slower winds
**Pressure systems:**
**Low-pressure system (depression/cyclone):** One or more isobars with **lowest pressure at centre** — surrounded by higher pressures
**High-pressure system (anticyclone):** One or more isobars with **highest pressure at centre** — surrounded by lower pressures
World Distribution of Sea Level Pressure
**Permanent pressure belts** (from Figures 9.2 and 9.3 showing January and July distributions):
1. **Equatorial Low (0°):** Near the equator — continuous belt of low pressure
Result of high insolation and thermal convection
2. **Subtropical High (30°N and 30°S):** Known as **horse latitudes**
High pressure zones at approximately 30° latitude
Result of air convergence and subsidence
3. **Subpolar Low (60°N and 60°S):** Low-pressure belts
Located between subtropical high and polar high
4. **Polar High (90°N and 90°S):** Near the poles
High pressure due to cold, dense air
**Important characteristics:**
These belts are **NOT permanent** — they oscillate seasonally
In Northern Hemisphere winter: pressure belts shift **southward**
In Northern Hemisphere summer: pressure belts shift **northward**
This seasonal migration follows the **apparent path of the Sun** (along tropical zones)
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FORCES AFFECTING WIND VELOCITY AND DIRECTION
**Wind definition:** Air in horizontal motion caused by differences in atmospheric pressure
**Three main forces control surface wind:**
1. Pressure gradient force
2. Coriolis force
3. Frictional force
Pressure Gradient Force (PGF)
**Generated by:** Differences in atmospheric pressure between locations
**Mechanism:** Causes air to move from high pressure to low pressure areas
**Strength determination:**
**Strong PGF:** When isobars are **close together** — pressure changes rapidly over short distance
**Weak PGF:** When isobars are **far apart** — pressure changes slowly
**Direction:** Always perpendicular to isobars (points from high to low pressure)
**Effect:** Primary force initiating wind movement
Coriolis Force
**Definition:** The deflecting force caused by **rotation of the Earth** about its axis (described by French physicist Gustave-Gaspard de Coriolis in 1844)
**Characteristics:**
**Direction of deflection:**
Northern Hemisphere: Deflects wind **to the right**
Southern Hemisphere: Deflects wind **to the left**
**Magnitude varies with:**
**Latitude:** Maximum at poles, **zero at equator** (critical for cyclone formation)
**Wind velocity:** Larger deflection with higher wind speeds
**Mathematical relationship:** Directly proportional to latitude and wind velocity
**Angle of operation:** Acts perpendicular to both pressure gradient force and wind direction
**Critical implication:** **Why tropical cyclones do not form near equator?**
At equator, Coriolis force = 0
Wind blows perpendicular to isobars without deflection
Low pressure gets **filled** rather than **intensified**
Cyclonic circulation cannot develop
Frictional Force
**Affected surfaces:** Earth's surface and atmosphere interface
**Magnitude:** **Greatest at surface**, decreases with altitude
**Effective range:** Generally extends up to **1-3 km elevation**
**Over different surfaces:**
Over land: High friction (due to roughness)
Over sea: **Minimal friction** (smooth surface)
**Effect:** Reduces wind speed by **slowing air movement**
**Upper atmosphere winds:** Above friction layer (2-3 km), winds are faster
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GEOSTROPHIC WIND
**Definition:** Wind that blows **parallel to isobars** when pressure gradient force is balanced by Coriolis force
**Conditions for formation:**
Straight isobars with no curves
No friction effects (upper atmosphere 2-3 km above surface)
Pressure gradient force ⊥ perpendicular to isobars
Coriolis force ⊥ perpendicular to wind direction
Both forces act perpendicular to each other with equal magnitude
**Result:** Wind flows **parallel to isobars** (neither toward nor away from them)
**Importance:** Represents theoretical upper-wind pattern; actual surface winds deviate due to friction
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WIND CIRCULATION PATTERNS IN PRESSURE SYSTEMS
Cyclonic and Anticyclonic Circulation
**Wind circulation patterns differ by hemisphere and pressure system type** (Table 9.2):
**Cyclones (Low-pressure systems):**
Northern Hemisphere: **Anticlockwise/counterclockwise** circulation
Southern Hemisphere: **Clockwise** circulation
Mechanism: Air converges toward low pressure and spirals inward
**Anticyclones (High-pressure systems):**
Northern Hemisphere: **Clockwise** circulation
Southern Hemisphere: **Anticlockwise/counterclockwise** circulation
Mechanism: Air diverges outward from high pressure in spiraling pattern
Vertical Motion: Convergence and Divergence
**Above Low-pressure areas:**
Surface air **converges** (moves toward center)
Air **rises** (vertical motion upward)
Creates **lifting mechanism** for cloud formation and precipitation
Associated with **disturbed weather**
**Above High-pressure areas:**
Air **subsides** (sinks from aloft)
Surface air **diverges** (moves away from center)
Associated with **clear, dry weather**
**Other causes of air rise:**
Convection (thermal rising due to heating)
Orographic uplift (forced rising over mountains)
Frontal uplift (warm air forced up by cold front)
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GENERAL CIRCULATION OF THE ATMOSPHERE
**Definition:** The pattern of planetary-scale wind movement determined by large-scale factors
**Controlling factors:**
1. Latitudinal variation of atmospheric heating (unequal solar radiation)
2. Emergence of pressure belts (equatorial, subtropical, subpolar, polar)
3. Seasonal migration of belts following the Sun's apparent path
4. Distribution of continents and oceans
5. Rotation of Earth (Coriolis force)
Three-Cell Model: Hadley, Ferrel, and Polar Cells
**Hadley Cell (Tropical circulation, 0°–30°):**
**At ITCZ (0°):** Air rises due to convection from intense insolation
High temperature causes low pressure
Converging trade winds create updraft
Reaches top of troposphere (~14 km altitude)
**Upper-level motion:** Air moves toward poles (poleward)
**At 30°N and 30°S:** Accumulated air **subsides/sinks**
Cooling causes high pressure (subtropical high)
Creates descending dry air and **arid zones**
**Surface-level motion:** Air returns equatorward as **easterlies/trade winds**
Deflected by Coriolis force
Blow from northeast (NH) and southeast (SH)
**ITCZ location:** Converged trade winds from both hemispheres
**Ferrel Cell (Middle latitudes, 30°–60°):**
**Surface winds:** **Westerlies** blowing from subtropical high (30°) toward subpolar low (60°)
Deflected westward by Coriolis force
Blow from southwest (NH) and northwest (SH)
**Mechanism:** Circulation between sinking cold air from poles and rising warm air from subtropical high
**Upper-level:** Air moves poleward from subtropical high
**Polar Cell (High latitudes, 60°–90°):**
**At poles:** Very cold, dense air **subsides** continuously
Creates persistent polar high pressure
**Surface winds:** **Polar easterlies** flowing from poles toward subpolar low (60°)
Deflected eastward by Coriolis force
Blow from northeast (NH) and southeast (SH)
**Upper-level:** Air moves equatorward from poles toward 60° latitude
**Heat transfer mechanism:**
Continuous transfer of heat energy from **lower (tropical) to higher (polar) latitudes**
Maintains persistent circulation and temperature gradients
Prevents poles from becoming extremely cold and tropics from overheating
General Circulation Effects on Oceans
**Ocean-atmosphere interaction:**
Large-scale planetary winds **initiate ocean currents** (slow-moving, large-scale)
Oceans provide **energy and water vapor input** to atmosphere
Interactions occur slowly over extensive ocean areas
Feedback loops regulate global climate
**Pacific Ocean warming and ENSO phenomenon:**
**El Niño:**
**Definition:** Appearance of warm water off South American coast (coast of Peru)
**Mechanism:** Warm water from central Pacific slowly drifts toward South America
**Displacement:** Replaces **cool Peruvian Current** (cold-water current normally flowing northward along Peru coast)
**Associated feature:** Pressure changes in Central Pacific and Australian region
**Southern Oscillation:**
**Definition:** Change in pressure conditions over Pacific Ocean (Indian Ocean–Australia region)
**Mechanism:** Related to pressure reversal between Australian and Pacific regions
**ENSO (El Niño–Southern Oscillation):**
**Combined phenomenon:** El Niño + Southern Oscillation
**Intensity:** Strongest El Niño events associated with strong ENSO
**Global weather impacts:** When ENSO is strong, large-scale weather variations occur worldwide:
**South America:** Arid west coast receives **heavy rainfall** (floods)
**Australia:** Experiences **droughts**
**India:** Sometimes affected by **droughts** (reducing monsoon rainfall)
**China:** Subject to **floods**
**Monitoring:** Closely monitored globally
**Applications:** Used for **long-range weather forecasting** in major regions worldwide
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SEASONAL WINDS
**Causes of seasonal variation:**
Shifting of regions of maximum heating (following Sun's apparent annual path)
Migration of pressure belts northward (summer) and southward (winter)
Displacement of wind belts accordingly
**Most pronounced effect:** **Monsoons** in southeast Asia
Dramatic seasonal reversal of wind direction
Associated with dramatic rainfall changes
Detailed study in "India: Physical Environment" chapter
**Definition:** Local winds are modifications of general circulation pattern caused by:
Differential heating and cooling of Earth surfaces
Daily or annual heating/cooling cycles
Regional topography and geography
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LAND AND SEA BREEZES
**Mechanism:** Differential heating and cooling of land and ocean
**Day-time (Sea Breeze):**
**Land heating:** Land surface heats faster than ocean (lower specific heat capacity)
**Temperature gradient:** Land becomes warmer than adjacent sea
**Pressure formation:** Over warm land, air rises → **low pressure**; over cool sea, high pressure
**Pressure gradient:** From sea to land
**Wind direction:** **Sea breeze** — winds blow from sea toward land
**Duration:** Develops during day, strongest in afternoon
**Distance:** Usually extends 10-20 km inland
**Night-time (Land Breeze):**
**Land cooling:** Land loses heat faster than ocean (lower heat storage capacity)
**Temperature reversal:** Land becomes cooler than adjacent sea
**Pressure formation:** Over cool land, high pressure; over warm sea, low pressure
**Pressure gradient:** From land to sea
**Wind direction:** **Land breeze** — winds blow from land toward sea
**Duration:** Develops at night, strongest before sunrise
**Characteristics:**
Regular, predictable pattern
Local phenomenon near coastlines
More pronounced in tropical regions with larger temperature contrasts
Important for coastal microclimates and human activities
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MOUNTAIN AND VALLEY WINDS
**Valley Wind (Upslope wind):**
**Day-time formation:** Mountain slopes receive direct solar radiation, heat rapidly
**Air expansion:** Heated air on slopes becomes less dense
**Air movement:** Air rises up the slope (upslope)
**Replacement:** Air from valley flows upward to fill the resulting gap
**Direction:** Blows up the valley during day
**Characteristics:** Warm, typically afternoon phenomenon
**Mountain Wind (Downslope wind/Katabatic wind):**
**Night-time formation:** Mountain slopes cool rapidly by radiation loss
**Air densification:** Cool, dense air develops on slopes
**Air movement:** Dense air flows downward under gravity
**Direction:** Flows down valley into lowlands at night
**Characteristics:** Cold, develops before sunrise
**Mechanism:** Gravity-driven drainage of cold air
**Katabatic Wind (Specific type):**
**Definition:** Cool air of high plateaus and ice fields draining into valleys
**Mechanism:** Gravity-driven descent of very cold, dense air
**Locations:** Most intense in polar regions (Greenland, Antarctica)
**Characteristics:** Extremely cold, strong, persistent
**Example:** Falls and Bora winds in Mediterranean region
**Foehn/Föhn Wind (Leeward warming):**
**Mechanism:** Warm wind on leeward (downwind) side of mountain ranges
**Process:**
Wind rises on windward slope, moisture condenses, precipitation occurs
Dry air descends leeward slope
Adiabatic warming (pressure increases, temperature rises) as air descends
Reaches leeward base much warmer than when it started
**Characteristics:** Warm, dry, often strong
**Effect:** Can melt snow rapidly in short time
**Examples:** Föhn in Alps, Chinook in North America
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AIR MASSES
**Definition:** A large body of air having **little horizontal variation in temperature and moisture**; air with distinctive characteristics acquired from its source region
**Source regions (homogenous surfaces):**
Must be large, uniform areas where air remains stationary sufficiently long
Air acquires temperature and moisture characteristics of the surface below
Examples: Vast ocean surfaces, vast plains, ice fields, deserts
Classification of Air Masses
**Based on source region location and characteristics:**
1. **Maritime Tropical (mT):**
Source: Warm tropical and subtropical oceans
Characteristics: **Warm and moist**
Bring humidity and often precipitation to coastal areas
2. **Continental Tropical (cT):**
Source: Subtropical hot deserts
Characteristics: **Warm and dry**
Associated with arid conditions
3. **Maritime Polar (mP):**
Source: Relatively cold high-latitude oceans
Characteristics: **Cold and moist**
Bring cool, humid conditions
4. **Continental Polar (cP):**
Source: Very cold snow-covered continents in high latitudes
Characteristics: **Cold and dry**
Bring severe cold conditions
5. **Continental Arctic (cA):**
Source: Permanently ice-covered continents (Arctic and Antarctica)
Characteristics: **Extremely cold and dry**
Associated with extreme cold events
**General classification:**
**Tropical air masses:** Warm (both mT and cT)
**Polar air masses:** Cold (mP, cP, cA)
**Maritime:** Moist (mT, mP)
**Continental:** Dry (cT, cP, cA)
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FRONTS
**Definition:** The **boundary zone between two different air masses** with contrasting temperature and moisture properties
**Frontogenesis:** Process of formation of fronts; occurs when air masses of different origins meet and interact
Types of Fronts
**1. Stationary Front:**
**Condition:** Front remains relatively stationary (does not move)
**Wind pattern:** Winds blow parallel to the front from opposite sides
**Characteristics:** Boundary between cold and warm air that neither advances
**2. Cold Front:**
**Movement:** **Cold air mass moves toward warm air mass**
**Definition:** Contact zone where cold air displaces warm air
**Wind pattern:** Wind direction changes abruptly across front
**Characteristics:**
**Steep slope** in cross-section (cold air wedges under warm air sharply)
**Steep temperature and pressure gradients**
**Weather effects:**
Cumulus clouds develop along cold front
Heavy showers and thunderstorms
Rapid temperature drop after passage
Wind direction shift
**3. Warm Front:**
**Movement:** **Warm air mass moves toward cold air mass**
**Definition:** Contact zone where warm air moves over cold air
**Wind pattern:** Wind direction changes gradually
**Characteristics:**
**Gentle slope** in cross-section (warm air glides gradually over cold air)
**Gradual temperature and pressure gradients**
**Weather effects:**
Sequence of clouds (stratus, stratocumulus types)
Continuous, moderate precipitation ahead of front
Gradual temperature rise before passage
**4. Occluded Front:**
**Formation:** Develops when cold front overtakes warm front
**Definition:** Air mass completely lifted above land/water surface
**Mechanism:** Cold air from behind catches and raises warm air completely
**Weather effects:** Mixed characteristics of both warm and cold fronts
**Front characteristics in middle latitudes:**
Steep gradients in temperature and pressure
Bring abrupt changes in weather conditions
Cause air to rise → cloud formation → precipitation
Found primarily in **middle latitudes** (30°–60°)
Associated with extra-tropical cyclones (weather systems)
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EXTRA-TROPICAL CYCLONES
**Definition:** Weather systems developing in **mid and high latitudes, beyond the tropics** (beyond 35°–40° latitude)
**Alternative names:** Middle-latitude cyclones, temperate cyclones, frontal cyclones
Formation and Structure
**Origin:**
Form along the **polar front** — boundary between tropical and polar air masses
Initially, front is stationary
**Development sequence (Northern Hemisphere):**
1. **Initial setup:**
Cold air from north, warm air from south of stationary front
Pressure begins dropping along front line
2. **Cyclonic circulation initiation:**
Pressure drops trigger motion of air masses
Warm air moves **northward** (poleward)
Cold air moves **southward** (equatorward)
Creates **anticlockwise (counterclockwise) cyclonic circulation** (NH)
3. **Mature cyclone development:**
Well-developed system with **warm front and cold front**
**Warm sector:** Pocket of warm air wedged between cold sectors
Frontal system produces characteristic cloud and precipitation patterns
Cloud and Weather Patterns in Mature Cyclone
**Ahead of warm front:**
Air gradually rises as warm air slides over cold air
**Sequence of clouds:** Cirrus → Cirrocumulus → Stratus
**Precipitation:** Continuous, moderate rainfall
**Temperature:** Rises gradually before warm front passage
**Along cold front:**
Rapidly rising warm air (cold air pushes from behind)
**Cumulus clouds** develop (vertical development)
**Weather:** Heavy showers, thunderstorms, sudden winds
**Precipitation:** Often heavy and violent
**Temperature:** Sharp drop after cold front passage
**In warm sector:**
Between advancing cold and retreating warm front
Generally clearing weather in sector itself
Occlusion and Dissipation
**Cold front moves faster** than warm front
Cold front eventually **overtakes warm front**
Warm air is **completely lifted** above surface
Front becomes **occluded front**
**Cyclone dissipates** — no more warm air to fuel the system
Energy source (temperature contrast) is eliminated
Movement and Scale
**Direction:** Move from **west to east** (driven by upper-level westerlies)
**Speed:** Typically 25-60 km/hour
**Affected area:** Much **larger area than tropical cyclones**
**Typical diameter:** 1,500-3,000 km
Comparison: Extra-Tropical vs. Tropical Cyclones
| Feature | Extra-Tropical | Tropical |
|---------|---|---|
| **Latitude** | 30°–70° (beyond tropics) | 5°–30° (tropical oceans) |
| **Frontal system** | **Clear warm and cold fronts** | No distinct fronts |
| **Origin** | Land and sea | Only over oceans |
| **Area affected** | **Much larger** | Smaller, more concentrated |
| **Wind velocity** | **Moderate** (40-100 km/hr) | **Very high** (150-300+ km/hr) — more destructive |
| **Duration** | 3-10 days | 1-2 weeks |
| **Movement** | **West to east** | **East to west** |
| **Dissipation** | Over land/progressive occlusion | Over land (moisture cutoff) |
| **Lifetime** | Relatively short | Can last 1-2 weeks |
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TROPICAL CYCLONES
**Definition:** **Violent storms originating over oceans in tropical areas** and moving toward coastal regions; bring large-scale destruction through intense winds, extremely heavy rainfall, and storm surges
**Characteristics:** One of the most devastating natural calamities
Regional Names
**Indian Ocean:** Cyclones
**Atlantic Ocean:** Hurricanes
**Western Pacific and South China Sea:** Typhoons
**Western Australia:** Willy-willies (or tropical cyclones)
**Generic term:** Tropical cyclones (international usage)
Conditions Favorable for Formation and Intensification
Five essential conditions must be present simultaneously:
1. **Large sea surface with water temperature higher than 27°C**
Energy source: Latent heat from ocean evaporation
Typical depth: Warm water extends down 60 m minimum
No formation in cooler waters
2. **Presence of Coriolis Force**
Required for circular rotation
**Cannot form at equator** (Coriolis force = 0)
Form poleward of 5°–6° latitude
Strengthens toward 15°–20° latitude (optimal)
3. **Small variations in vertical wind speed**
Wind shear must be minimal
Consistent wind profile vertically
Prevents disruption of circular structure
4. **Pre-existing weak low-pressure area or low-level cyclonic circulation**
Provides initial disturbance
May develop from easterly wave, monsoon depression, or weak trough
5. **Upper divergence above sea level system**
Upper-level winds remove air from storm top
Creates low pressure at surface
Allows continued rising of air parcels
Energy Source and Intensification
**Primary energy:** **Latent heat released during condensation** in towering cumulonimbus clouds
Moist air from warm ocean surface rises
Moisture condenses in clouds
Releases massive amounts of latent heat
Heat warms surrounding air, causing further rising
Positive feedback loop intensifies storm
**Maintenance requirement:** **Continuous moisture supply from sea**
As long as cyclone remains over warm ocean: strengthens
**Critical threshold:** Sea surface temperature of 27°C minimum
Dissipates if it moves over cooler water or land
Landfall and Recurvature
**Landfall:** The point where tropical cyclone **crosses coastline** and enters land
**Moisture cutoff:**
Land surface cannot supply continuous ocean-based moisture
Cyclone rapidly weakens and dissipates over land
**Recurvature (Tracking):**
Cyclones crossing **20°N latitude generally recurve** (change direction)
Begin moving from eastward to westward and then northward
Deflected by upper-level winds and planetary circulation patterns
**More destructive after recurvature** — may affect inhabited mid-latitude regions
Vertical Structure of Mature Tropical Cyclone (Figure 9.10)
**Eye:**
**Definition:** Central calm region of cyclone
**Characteristics:**
Region of **subsiding air** (sinking from aloft)
**Calm or light winds**
Clear skies or partly cloudy
**Highest temperatures** at surface
Diameter: 10-50 km typically
Can reach 150-250 km for largest storms
**Eye wall:**
**Ring of clouds** immediately surrounding the eye
**Strongest convection** and updrafts occur here
**Steepest pressure gradient** between eye and eye wall
**Most violent weather:**
Strongest winds (150-300+ km/hr)
Heaviest rainfall
Most severe turbulence
Air rises in spiraling pattern to great heights
Cloud tops reach 12-16 km altitude
**Outer bands:**
**Spiral rain bands** extending outward from eye wall
Separated by areas of relatively lighter precipitation
Winds decrease toward outer margins
Extend 500+ km from storm center
Progressive weakening of convection and wind speed
**Vertical circulation:**
Rising air in eye wall and bands
Divergence aloft at tropopause level
Upper-level outflow removes air from system
Subsidence in eye completes circulation
Destructive Impacts
1. **Violent winds:**
Speed range: 150-300+ km/hr
Damage structures, uproot trees, cause flying debris injuries
2. **Extremely heavy rainfall:**
500-2,500 mm in 24-48 hours possible
Causes inland flooding, landslides
Most damaging over mountainous terrain
3. **Storm surge:**
Rapid rise in sea level (1-7 meters)
Driven by wind pushing water toward coast
Combines with high tide for maximum effect
Major cause of coastal deaths and property damage
4. **Tornadoes:**
Can form within outer bands
Add additional localized severe weather
Formation Zones and Seasonal Patterns
**Atlantic basin:** June-November peak (September maximum)
**Western Pacific:** Year-round activity, peaks May-November
**Indian Ocean:** April-May and October-November peaks
**Southern Hemisphere:** November-April
Global Distribution
Tropical cyclones form in all tropical oceans except South Atlantic and Southeast Pacific (cooler waters)
Typically 80-100 tropical cyclones form annually worldwide
Move generally westward and poleward (due to trade winds and upper-level circulation)
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INTER TROPICAL CONVERGENCE ZONE (ITCZ)
**Location:** At the equator (0° latitude)
**Formation mechanism:**
**Intense insolation** at equator causes high surface temperatures
**Thermal convection:** Warm air rises continuously due to heating
**Creates low pressure:** Ascending air reduces surface pressure
**Trade wind convergence:** Trade winds (northeasterlies from NH, southeasterlies from SH) converge at this low-pressure zone
**Air movement:**
**Surface:** Converging trade winds bring air toward ITCZ
**Upper levels:** Converged air rises to top of troposphere (≈14 km altitude)
**Poleward movement:** Air moves toward poles at upper levels (becomes part of Hadley circulation)
**Weather characteristics:**
**Heavy convection and cloudiness**
**Frequent, intense thunderstorms and precipitation**
**Humid conditions** (due to continuous moisture convergence)
Light and variable surface winds (convergence area, not strong winds)
**Seasonal shift:**
Follows the Sun's apparent annual path
Migrates northward in NH summer (June-September)
Migrates southward in NH winter (December-March)
Maximum displacement: ≈5°N during June, ≈10°S during December
**Global significance:**
**Wettest zone on Earth** in many locations
Critical for tropical agriculture and water supply
Influences monsoon patterns
Related to formation of monsoon troughs and depressions
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SUMMARY: KEY EXAMINATION POINTS
1. **Atmospheric pressure** fundamentals: definition, units (mb), sea level value (1,013.25 mb), vertical and horizontal variation
2. **Forces affecting wind:** Pressure gradient (main driver), Coriolis (deflection), friction (surface effect)
3. **Critical concept:** Coriolis force = 0 at equator → no tropical cyclone formation
4. **Pressure systems:** Low (cyclone) = converging, rising air; High (anticyclone) = subsiding, diverging air
5. **Three-cell circulation:** Hadley (tropics), Ferrel (mid-latitudes), Polar (high latitudes)
6. **ENSO phenomenon:** El Niño + Southern Oscillation; global weather impacts
7. **Local winds:** Sea/land breezes (daily), mountain/valley winds (daily), katabatic (gravity-driven)
8. **Air masses:** Classified by source region; 5 types (mT, cT, mP, cP, cA)
9. **Fronts:** Four types; cold/warm fronts produce abrupt weather changes
10. **Extra-tropical cyclones:** Mid-latitudes, with frontal system, move west-to-east, larger scale
11. **Tropical cyclones:** Low latitude, no fronts, move east-to-west, higher wind speeds, require 27°C+ water
12. **Cyclone formation requirements:** 5 conditions (warm water, Coriolis, vertical wind shear, initial disturbance, upper divergence)
13. **Map-based questions:** Pressure distribution (January/July), wind directions by latitude, ITCZ movement, cyclone tracks