Atmospheric Pressure and Circulation

3.1 Pressure Belts

Unequal heating of Earth's surface creates pressure differences that drive atmospheric circulation. The ideal pressure belts (undisturbed by land-sea distribution) are arranged by latitude:

Equatorial Low Pressure Belt (0°) — Doldrums

• Intense heating → air expands and rises rapidly → low surface pressure
• Rising air cools → condensation → heavy convectional rainfall year-round
• Very little surface wind (historically sailors feared this calm zone — hence "Doldrums")
• Temperature: 26–28°C; humidity: very high; daily convectional thunderstorms

Subtropical High Pressure Belts (25°–35° N/S) — Horse Latitudes

• Descending air from the Hadley Cell; compression → warming → dry conditions
• Clear skies; low rainfall; the origin of most world deserts (Sahara, Arabian, Australian, Sonoran)
• Historical name "Horse Latitudes" — legend that sailors becalmed here had to throw horses overboard
• Temperature and humidity relatively low; subsidence inversion prevents cloud formation

Subpolar Low Pressure Belts (55°–65° N/S) — Polar Fronts

• Where warm subtropical (westerly) winds meet cold polar (easterly) winds → the Polar Front
• Air forced to rise at this frontal zone → low pressure; cloudy, rainy, windy conditions
• Cyclonic activity strong — extratropical/temperate cyclones form here
• Most intense in Southern Hemisphere where there is no land to break the flow

Polar High Pressure Belts (90° N/S) — Polar Highs

• Extreme cold → air contracts and descends → high pressure
• Very cold, dry, dense air; minimal precipitation (polar "deserts")
• Antarctica receives only 50–200 mm precipitation/year (ice accumulates because it doesn't melt)

3.2 Three-Cell Model of Atmospheric Circulation

Hadley Cell (0°–30°)

• Surface: Equatorial heating → air rises → trades blow equatorward from subtropical highs
• Upper atmosphere: Air moves poleward at altitude from equator
• Descent: At 30°, air cools and descends → subtropical highs
• Named after George Hadley (1735) who explained trade winds; direct thermally driven cell

Ferrel Cell (30°–60°)

• Surface: Westerly winds blow poleward (SW in NH, NW in SH)
• Indirect cell — driven mechanically by Hadley and Polar cells
• Named after William Ferrel (1856); less clearly defined in reality; temperature moderate, weather variable

Polar Cell (60°–90°)

• Surface: Cold polar easterlies blow equatorward from polar highs
• Upper atmosphere: Air rises at polar front (~60°) and moves poleward at altitude, cools and descends at poles
• Small, well-defined; drives Arctic/Antarctic weather

3.3 Global Wind Systems

Trade Winds

• Blow from subtropical high to equatorial low; deflected by Coriolis effect to become:
  • Northeast Trades (Northern Hemisphere: blow from NE to SW)
  • Southeast Trades (Southern Hemisphere: blow from SE to NW)
• Most consistent, reliable winds on Earth; crucial for historical maritime trade (hence "Trade Winds")
• Carry moisture to eastern shores of continents (windward), creating lush tropical coasts
• Drive equatorial ocean currents; responsible for upwelling on western coasts of Africa and Americas

Westerlies

• Blow from subtropical high to subpolar low (30°–60°); deflected to become:
  • Southwest Westerlies (Northern Hemisphere)
  • Northwest Westerlies (Southern Hemisphere)
• Southern Hemisphere has stronger, unobstructed westerlies (no landmasses to break flow):
  • "Roaring Forties" (40°S): Powerful westerlies used by sailing ships going east
  • "Furious Fifties" (50°S): Even stronger
  • "Screaming Sixties" (60°S): Extreme

Polar Easterlies

• Blow from polar high to subpolar low; cold, dry
• Northeast Polar Easterlies (NH); Southeast Polar Easterlies (SH)

Monsoon Winds

• Seasonal wind reversal driven by differential heating of land and sea
• Summer monsoon: Land heats faster → low pressure over land → moist ocean air drawn inland
• Winter monsoon: Land cools → high pressure → dry winds blow seaward
• Major monsoon systems: South Asian Monsoon, East Asian Monsoon, West African Monsoon

Local Winds

Wind	Location	Nature	Season
Foehn/Chinook	Alpine/Rocky Mountains (leeward side)	Hot, dry (orographic warming on descent)	Any
Mistral	Southern France (Rhône Valley)	Cold, dry NW wind	Winter
Simoom/Khamsin	Sahara/North Africa	Hot, dust-laden	Spring-Summer
Sirocco	N. Africa → Mediterranean	Hot, humid, dusty	Spring-Summer
Bora	NE Adriatic coast	Cold, dry NE wind	Winter
Loo	Northwestern India (Rajasthan, UP)	Hot, dry westerly	May-June
Mango Showers	Kerala, Karnataka	Pre-monsoon showers	May

3.4 The Coriolis Effect

Named after: Gaspard-Gustave de Coriolis (1835 — French physicist)

Cause: Earth rotates from west to east — a freely moving object appears to curve because the ground underneath it is rotating. The effect is stronger at higher latitudes (zero at equator) and proportional to wind speed.

Buys-Ballot's Law: "If you stand with the wind at your back in the Northern Hemisphere, low pressure is to your left; in Southern Hemisphere, to your right."

Applications of the Coriolis Effect

• Northern Hemisphere: Winds deflected to the right — cyclones rotate anticlockwise (counterclockwise); anticyclones rotate clockwise
• Southern Hemisphere: Winds deflected to the left — cyclones rotate clockwise; anticyclones rotate anticlockwise
• Explains why rivers slightly erode their right banks in NH (flowing rivers also experience Coriolis deflection)

Important: The Coriolis effect does NOT determine which direction water rotates when draining from a bathtub — the effect is too small at that scale; the initial conditions of the water determine drain direction.

3.5 Jet Streams

Definition: Fast-flowing, narrow air currents in the upper troposphere/lower stratosphere, typically at 9–16 km altitude, with core speeds of 120–400 km/h (sometimes exceeding 450 km/h).

Key Jet Streams

Jet Stream	Location	Altitude	Season	Significance
Subtropical Jet (STJ)	~25–30°N/S	~12 km	Year-round (stronger winter)	Separates tropical from mid-latitude air; influences subtropical weather
Polar Front Jet (PFJ)	~50–60°N/S	~9 km	Year-round (strongest winter)	Drives mid-latitude weather systems; meanders widely → affects regional temperatures
Tropical Easterly Jet (TEJ)	~15°N	~15 km	Northern Hemisphere Summer only	Drives the Indian Monsoon northward by creating upper divergence
Polar Night Jet	Stratosphere (winter)	~25 km	Winter only	When disrupted → Sudden Stratospheric Warming → extreme cold outbreaks in mid-latitudes

Jet Stream and Indian Monsoon

• Onset trigger: In early June, the Subtropical Westerly Jet retreats north of the Himalayas and the Tropical Easterly Jet establishes over India — this is associated with the southwesterly monsoon's arrival
• Withdrawal trigger: Subtropical westerly jet reinstates south of Himalayas → monsoon withdraws

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