Global Atmosphere & Extreme Weather
Why Does Climate Vary Around the World?
Climate is the long-term pattern of weather in a place. The same Earth has hot deserts and frozen poles, year-round rainfall and seasonal monsoons. Most of these patterns are explained by the global atmospheric circulation. The Sun heats the equator more strongly than the poles, and the atmosphere responds by moving warm air upwards and outwards. The way that moving air rises, falls and sweeps across the surface controls the climate zones we live in.
In each hemisphere there are three large circulation cells: the Hadley cell from the equator to about 30° latitude, the Ferrel cell from 30° to 60°, and the Polar cell from 60° to the pole.
How the Three Cells Work
Hot air rises at the equator, cools as it climbs, and the moisture inside condenses into towering clouds and heavy rain — creating the wet tropical rainforest belt. The dry air aloft drifts polewards and sinks back down at around 30° latitude. Sinking air warms and dries, producing the world’s great hot deserts: the Sahara, Arabian, Kalahari, Australian Outback and the dry south-west of the USA. Surface winds called trade winds blow back towards the equator, completing the Hadley cell.
Between 30° and 60° the Ferrel cell carries air polewards near the surface and forms the prevailing south-westerly winds that bring most of the UK’s wet, mild weather. From 60° to the poles the Polar cell sends cold, dense air sinking at the poles and rising again at around 60°, where it meets warmer air at the polar front and produces the storms that track across the North Atlantic.
The Main Climatic Regions
- Tropical (equatorial) — hot all year (around 27°C), high rainfall, dense rainforest in the Amazon, Congo and Indonesia.
- Hot desert — near 30° latitude, very low rainfall, big day-night temperature range.
- Temperate — mid-latitudes, moderate rainfall and clear seasons; covers most of the UK and Western Europe.
- Polar — cold all year, low rainfall, ice sheets in Antarctica and Greenland.
- Mediterranean — warm dry summers, mild wet winters, around southern Europe and parts of California.
By the time air reaches the descending limb of the Hadley cell at around 30° north and south, it has lost its moisture as rainfall over the equator. Sinking air warms by compression, so any cloud evaporates and high-pressure systems dominate. With clear skies and dry air, sunlight beats down on the surface most days. The result is the band of hot deserts that ring both hemispheres at this latitude.
Extreme Weather: Tropical Storms
Extreme weather conditions can cause natural hazards. One of the most damaging is the tropical storm. The same hazard is called a hurricane in the Atlantic and East Pacific, a typhoon in the West Pacific, and a cyclone in the Indian Ocean and around Australia.
How Tropical Storms Form
Tropical storms need a precise set of conditions to develop:
- Sea-surface temperatures of at least 27°C, usually in late summer when oceans are warmest.
- An ocean depth of around 70m of warm water, providing plenty of moisture and heat.
- Low wind shear (so winds at different heights blow in similar directions, letting the storm column build up rather than tear apart).
- A position roughly 5–30° north or south of the equator, where the spin of the Earth (the Coriolis effect) can twist the rising air.
Warm, moist air rises rapidly above the ocean. As it climbs, the water vapour condenses, releasing energy that drives the storm even harder. Air spirals inwards at the surface and outwards high in the atmosphere. The centre of the storm, the eye, is calm and clear, but the eyewall around it has the most extreme winds and rainfall.
A tropical storm needs sea temperatures of at least 27°C, around 70m of warm water, low wind shear and a location 5–30° from the equator. Warmer oceans because of climate change are widening the area where storms can form.
Tropical storms are concentrated in the Atlantic, the West Pacific, the Bay of Bengal and around northern Australia. Around 80–90 named storms form globally each year. Climate scientists are not certain that the total number is rising, but they have stronger evidence that the most intense storms (Category 4 and 5) are becoming more common because of warmer oceans, and that storms are dumping more rainfall.