What Is Wind?
Wind is the horizontal movement of air in the atmosphere, caused by differences in air pressure. Air always moves from areas of high pressure to areas of low pressure — from where there is more air pushing outward to where there is less.
Wind is described by:
- Direction — the direction it comes from (e.g. a westerly wind blows from west to east)
- Speed — measured in km/h, m/s, or on the Beaufort Scale (0 = calm; 12 = hurricane)
Why Does Pressure Differ?
Temperature is the main cause of pressure differences:
- Warm air rises → creates low pressure at the surface (less air pressing down).
- Cold air sinks → creates high pressure at the surface (more air pressing down).
Near the equator, intense solar heating warms the air, causing it to rise (low pressure, rising air → clouds and rain). Near the poles, cold air sinks (high pressure, sinking air → dry, clear conditions).
Isobars are lines on a weather map connecting places with equal atmospheric pressure (measured in millibars, mb). When isobars are close together, the pressure gradient is steep and winds are strong. When isobars are far apart, the pressure gradient is gentle and winds are light. Meteorologists use isobar patterns to forecast wind speed and direction.
The Coriolis Effect
If the Earth did not rotate, wind simply blow straight from high to low pressure. But Earth's rotation causes winds to be deflected — to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is called the Coriolis effect.
As a result:
- Around a Low pressure system (cyclone): winds spiral anticlockwise in the Northern Hemisphere.
- Around a High pressure system (anticyclone): winds spiral clockwise in the Northern Hemisphere.
Global Wind Patterns
The combination of pressure differences and the Coriolis effect creates consistent global wind patterns:
| Wind belt | Location | Direction | Description |
|---|---|---|---|
| Trade winds | 0°–30° N/S | NE (N.H.) / SE (S.H.) toward equator | Reliable easterly winds; powered sailing ships |
| Westerlies | 30°–60° N/S | West to east | Weather systems across the British Isles |
| Polar easterlies | 60°–90° N/S | East to west | Cold, dry winds from the poles |
The trade winds were extremely important for sailing ships from the 15th–19th centuries. They blow reliably from east to west north and south of the equator, so sailors would sail south to pick up the trade winds to reach the Americas, then north to pick up the westerlies to return. This knowledge shaped global exploration and trade.
Local Wind Effects
Sea breeze: During the day, land heats faster than the sea. Warm air rises over the land; cool, dense air flows in from the sea → sea breeze (wind from sea toward land).
Land breeze: At night, land cools faster than the sea. Now the sea is warmer. Air rises over the sea; cool air flows from land to sea → land breeze.
These local breezes are important for coastal climate, sailing, and air quality in coastal cities.
Real world: Trade winds and the history of exploration
For centuries, the trade winds made transatlantic sailing possible. Christopher Columbus used the northeast trade winds to reach the Americas in 1492 — sailing southwest from Spain, picking up the reliable trade winds, and crossing the Atlantic in 36 days. Return journeys used the westerlies at higher latitudes. The whole pattern of European colonisation of the Americas was shaped by these wind belts discovered by sailors following pressure gradients.
Real world: The sea breeze and coastal resorts
Coastal towns around the Mediterranean, British seaside resorts, and tropical beach destinations are often cooler than inland towns on hot summer days — because of the sea breeze. As the land heats up, a refreshing onshore wind kicks in from the cool sea. This is why seaside temperatures are often 3–5°C cooler than cities 20 km inland on a hot summer afternoon. Sailors and windsurfers also rely on reliable sea breezes to practise their sports.
Real world: Storms and wind in weather forecasts
When meteorologists talk about a "low pressure system" approaching the UK or Central Europe, they are describing a cyclone — an area of low pressure where air spirals in anticlockwise, rising air brings cloud and rain. The isobars on weather maps show how tightly packed the pressure gradient is: closely-spaced isobars mean strong winds. Storm Ciaran in October 2023 brought gusts over 200 km/h to northern France and Italy, directly caused by very tight isobars around a deep low-pressure centre.
Where do you see this in real life? The next time you check a weather app, look at the wind direction and speed. Is it a westerly? Moist Atlantic air is likely bringing rain. An easterly in winter? Cold, dry continental air from Russia or eastern Europe.
Practice Problems
Why does wind blow from high pressure to low pressure rather than the reverse?
Show answer
Explain why a sea breeze blows toward the land during the day.