Weather hazards: tropical storms and UK weather extremes

Atmospheric hazards are powered by the global atmospheric circulation — the system of pressure cells and prevailing winds that redistribute heat from the equator towards the poles. You should know the three-cell model (Hadley, Ferrel and Polar cells) and the resulting low-pressure (rising air, wet) and high-pressure (sinking air, dry) belts.

Global atmospheric circulation in 60 seconds

• At the equator, intense solar heating warms the air, which rises, cools and condenses → heavy rainfall (the rainforest belt).
• The risen air spreads out, sinks at about 30° N/S, creating a high-pressure belt → the world's hot deserts (Sahara, Atacama, Australian Outback).
• The sinking air flows back towards the equator as the trade winds — these are the engine of tropical storms.
• At ~60° N/S, warm tropical air meets cold polar air, rises, and produces low pressure (UK weather, mid-latitude depressions).
• Cold polar air sinks at the poles, completing the loop.

Tropical storms — formation, structure, distribution

Tropical storms have local names: hurricane (Atlantic, eastern Pacific), typhoon (western Pacific), cyclone (Indian Ocean and Australia). They share five formation conditions:

1. Sea surface temperature ≥ 26.5 °C — warm water evaporates rapidly, fuelling the storm.
2. Ocean depth at least 50 m of warm water.
3. Latitude 5°–30° N or S — Coriolis force is needed to spin the storm; too close to the equator and there's no rotation.
4. Low wind shear — strong upper-level winds tear storms apart.
5. Convergence of air at low levels — gives the initial uplift.

Structure

• Eye (calm, sinking air, clear sky, low pressure ~880 hPa)
• Eyewall (most intense rain and winds — up to 250 km/h)
• Spiral rain bands (curved arms of thunderstorms feeding the eyewall).

Pathway

Storms track westwards on the trade winds, then curve poleward as they intensify. They die when they pass over cold water, hit land (no fuel) or move into stronger wind shear.

Effects (use a real case study, e.g. Hurricane Maria 2017, Puerto Rico, Cat 5)

• Primary: 250 km/h winds destroyed 80 % of farms; storm surge flooded coast; rainfall caused inland floods.
• Secondary: power grid down for 11 months; ~3 000 excess deaths; out-migration to mainland US.
• Immediate response: federal aid criticised as slow.
• Long-term: stronger building codes, distributed solar grid pilots.

Climate change link

A warmer atmosphere holds more water vapour (Clausius–Clapeyron: ~7 % more per °C). Warmer seas extend the area where SST > 26.5 °C polewards. The result: tropical storms are not necessarily more frequent, but they are wetter, slower, more intense (more Cat 4/5). Maria, Harvey (2017) and Helene (2024) all show the trend.

Reducing impacts of tropical storms

• Prediction: satellites + reconnaissance aircraft give 3–5 days warning.
• Planning: evacuation routes, shelters; Bangladesh's network of cyclone shelters has cut deaths from ~500 000 (Bhola, 1970) to ~5 000 (Sidr, 2007).
• Protection: storm-surge barriers, hurricane straps for roofs.

UK weather hazards

The UK lies on the polar front jet stream, where polar and tropical air masses meet. This creates a parade of low-pressure depressions sweeping in from the Atlantic. Hazards include:

• Storms / strong winds (Storm Arwen 2021 — wind gusts 100 mph, three deaths, 1 million homes lost power for days).
• Heavy rain and flooding (Storm Desmond 2015 — Cumbria, 341 mm of rain in 24 h, £500 m damage).
• Heatwaves (UK 2022 — first 40 °C reading at Coningsby; 3 000 excess deaths; major wildfires).
• Drought (1976 — long high-pressure block; reservoirs at 26 %).
• Snow and freezing temperatures (the Beast from the East 2018 — Siberian air mass over UK; ~17 deaths; £1 bn cost).

Are UK weather extremes becoming more common?

Multiple lines of evidence say yes: the 10 hottest UK years on record have all occurred since 2002; named storms have increased; intense rainfall events have risen by ~17 % since the 1960s. Climate change is the primary driver.