Weather Hazards and Atmospheric Systems

Atmospheric hazards — tropical storms, tornadoes, heatwaves, blizzards and other extreme weather events — affect more people globally than tectonic hazards. Understanding why they occur requires knowledge of the global atmospheric circulation, energy transfer mechanisms and the role of the jet stream. This lesson establishes the atmospheric science foundations that underpin all weather hazard topics in the AQA specification.

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The Global Atmospheric Circulation

The atmosphere is a giant heat engine. The fundamental driver of atmospheric circulation is the unequal heating of the Earth's surface by solar radiation:

• The tropics receive more incoming solar radiation (insolation) than they emit as outgoing longwave radiation — they have a radiation surplus
• The poles receive less insolation than they emit — they have a radiation deficit
• This imbalance creates a temperature gradient that drives the atmospheric (and oceanic) circulation, transferring heat from the equator to the poles

The Three-Cell Model

The global atmospheric circulation is conventionally described using the three-cell model, first conceptualised by George Hadley (1735), William Ferrel (1856) and others:

graph TD
    subgraph "Northern Hemisphere Circulation"
        A["Equator 0°<br/>ITCZ - Low pressure<br/>Rising air"] --> B["Hadley Cell<br/>Surface: NE Trade Winds<br/>Upper: SW flow"]
        B --> C["~30°N<br/>Subtropical High<br/>Descending air - Deserts"]
        C --> D["Ferrel Cell<br/>Surface: SW Westerlies<br/>Upper: NE flow"]
        D --> E["~60°N<br/>Subpolar Low<br/>Rising air - Polar Front"]
        E --> F["Polar Cell<br/>Surface: NE Polar Easterlies"]
        F --> G["90°N<br/>Polar High<br/>Descending cold air"]
    end

The Three Cells in Detail

Cell	Latitudes	Surface Winds	Key Features
Hadley Cell	0–30 degrees	Trade winds (NE in Northern Hemisphere, SE in Southern)	The largest and most powerful cell. Warm air rises at the ITCZ, flows poleward at altitude, cools and descends at ~30 degrees creating subtropical high-pressure belts and the world's hot deserts (Sahara, Arabian, Australian)
Ferrel Cell	30–60 degrees	Westerlies (SW in Northern Hemisphere, NW in Southern)	A thermally indirect cell driven by the Hadley and Polar cells. Surface airflows poleward and is deflected east by the Coriolis effect. Mid-latitude weather systems (depressions and anticyclones) dominate
Polar Cell	60–90 degrees	Polar easterlies (NE in Northern Hemisphere)	Cold, dense air sinks at the poles and flows equatorward. Where it meets the warmer Ferrel Cell air at ~60 degrees, the polar front forms — a zone of convergence that generates mid-latitude depressions

Pressure Belts and Wind Patterns

Latitude	Pressure	Surface Conditions
Equator (0 degrees)	Low (ITCZ)	Convergence of trade winds; intense convective uplift; heavy rainfall; thunderstorms
~30 degrees N/S	High (subtropical)	Descending air; clear skies; arid conditions; world's hot deserts
~60 degrees N/S	Low (subpolar)	Convergence of westerlies and polar easterlies; polar front; mid-latitude cyclones
~90 degrees N/S	High (polar)	Descending cold air; very cold; low precipitation (polar deserts)

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The Coriolis Effect

Key Definition: The Coriolis effect is the apparent deflection of moving objects (including air masses) caused by the Earth's rotation. In the Northern Hemisphere, deflection is to the right; in the Southern Hemisphere, to the left.

The Coriolis effect was mathematically described by Gaspard-Gustave de Coriolis (1835). It is not a true force but an apparent force arising from the Earth's rotation:

• The Earth rotates fastest at the equator (~1,670 km/h) and progressively slower toward the poles (0 km/h at the poles)
• Air moving from the equator toward the poles moves faster than the surface beneath it, causing it to appear to deflect eastward
• Air moving from the poles toward the equator moves slower than the surface, appearing to deflect westward

The Coriolis effect is responsible for:

• The curved path of the trade winds and westerlies
• The rotation of tropical cyclones (anticlockwise in the Northern Hemisphere, clockwise in the Southern)
• The rotation of mid-latitude depressions
• It is zero at the equator (which is why tropical storms cannot form within ~5 degrees of the equator — there is insufficient Coriolis force to initiate rotation)

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The Jet Stream

Key Definition: Jet streams are narrow bands of very fast-moving air (typically 150–300 km/h, occasionally exceeding 400 km/h) found in the upper troposphere, at altitudes of approximately 9–12 km.

Types of Jet Stream

Jet Stream	Location	Characteristics
Polar Front Jet Stream	~50–60 degrees N/S (but highly variable)	Located above the polar front, where cold polar air meets warm subtropical air. The temperature gradient drives the wind speed. Meanders north and south in a wave-like pattern (Rossby waves). Directly controls the track and intensity of mid-latitude weather systems
Subtropical Jet Stream	~25–30 degrees N/S	Found at the poleward edge of the Hadley Cell. More consistent in position than the polar jet. Influences the location of subtropical high-pressure systems
Tropical Easterly Jet	~15 degrees N (summer)	Seasonal jet stream associated with the Asian monsoon. Flows from east to west over the Indian subcontinent

Rossby Waves