The global circulation system

Uneven heating of the Earth drives a global circulation of the atmosphere, made of three cells in each hemisphere (Hadley, Ferrel and Polar). This creates belts of high and low pressure that fix the world's climate zones.

• At the Equator, intense heating makes air rise → low pressure → heavy rain → tropical rainforests.
• At about 30° N and S, air sinks → high pressure → dry, cloudless skies → hot deserts (e.g. the Sahara).
• Surface winds blow from high to low pressure, giving the trade winds and westerlies.

Key idea: where air rises you get rain (low pressure); where air sinks you get dry conditions (high pressure).

How tropical storms form

Tropical storms (hurricanes, cyclones, typhoons) form over warm tropical oceans. They need several conditions:

• Sea-surface temperature of at least about 26.5 °C, to a depth of ~50 m, providing energy through evaporation.
• Location roughly 5–30° from the Equator, so the Coriolis effect can spin the storm (they can't form on the Equator itself).
• Warm, moist air rises rapidly, condenses and releases latent heat, driving the storm; low wind shear lets it grow.

Their strength is measured on the Saffir–Simpson scale (categories 1–5 by wind speed). Their distribution is limited to tropical latitudes with warm seas.

Misconception: tropical storms are powered by warm ocean water, not by the land. Once they move over land or cooler seas they lose their energy source and weaken.

What powers a tropical storm?

Drought, El Niño and La Niña

Drought is a long period of below-average rainfall causing a water shortage. Causes include persistent high pressure (blocking anticyclones) and changes in ocean–atmosphere circulation.

• El Niño — warm surface water shifts east across the Pacific; this can bring drought to Australia and Indonesia and heavy rain to South America.
• La Niña — the opposite phase, with stronger-than-normal trade winds and cooler eastern Pacific water, reversing the pattern.

The distribution of drought includes the Sahel, southern Africa and Australia; its frequency may be changing with a warming climate.

When extreme weather becomes a hazard

OCR asks for two contrasting weather-hazard case studies — one UK and one non-UK. Two widely-studied examples:

• UK — Somerset Levels floods (winter 2013–14): weeks of Atlantic storms and record rainfall flooded ~600 homes and ~6,900 ha of farmland; villages such as Muchelney were cut off.
• Non-UK — Typhoon Haiyan, Philippines (Nov 2013): a category-5 tropical storm with a ~5–7 m storm surge; over 6,000 people died and Tacloban was devastated.

Exam tip: for each event learn the place-specific causes (the extreme weather), consequences (social, economic, environmental) and responses (short-term and long-term).

Match the term to its meaning

Tap a term on the left, then tap its correct description on the right.

Pressure and climate

The structure of the Earth

The Earth has a crust, mantle, outer core (liquid) and inner core (solid). Heat from the core drives convection currents in the mantle, which move the tectonic plates that make up the crust.

• Continental crust — thicker (~30–70 km), older and less dense.
• Oceanic crust — thinner (~5–10 km), younger and denser, so it subducts.

Key term — convection currents: circular movements of heat in the mantle that drag the plates above them, causing them to move apart, together or past each other.

Four types of plate boundary

• Constructive (divergent) — plates move apart; magma rises to form new crust and shield volcanoes (e.g. Mid-Atlantic Ridge).
• Destructive (convergent) — denser oceanic plate subducts under continental; causes strong earthquakes and explosive composite volcanoes.
• Conservative — plates slide past each other; earthquakes but no volcanoes (e.g. San Andreas Fault).
• Collision — two continental plates crumple upward to build fold mountains (e.g. the Himalayas).

Which boundary?

Case study: a tectonic hazard

A widely-studied contrasting pair shows how wealth affects the impact of an earthquake:

• Haiti, 2010 (LIDC): a magnitude 7.0 quake near Port-au-Prince killed around 230,000 people. Poor building quality, weak governance and no early-warning meant huge losses.
• New Zealand, Christchurch 2011 (AC): a magnitude 6.3 quake killed 185 people. Stricter building codes and preparedness reduced deaths despite a similar magnitude.

Exam tip: comparing an LIDC and an AC event shows that consequences depend on development, not just magnitude — a strong link to Topic 6 (Dynamic Development).

Can technology save lives?

Technology can mitigate tectonic hazards:

• Building design — earthquake-resistant buildings with deep foundations, cross-bracing and shock absorbers (e.g. in Japan).
• Prediction & monitoring — seismometers, gas sensors and satellites help forecast volcanic eruptions.
• Early-warning systems — earthquake alerts (seconds) and tsunami warning systems give people time to react.

Your turn — comparing impacts

Calculating change

Weather hazard or tectonic hazard?

Tap a hazard, then tap the box it belongs in.

Volcano or no volcano?

At each boundary, would you expect volcanoes?

Reading the scale

The key ideas to know

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