Wind and Pressure
Weather & Meteorology Fundamentals
Course 1 · Chapter 4 · Wind and Pressure
Wind is air moving from where there is more pressure to where there is less. That sentence is the whole of the idea, but the details are what make weather charts readable: the Earth's spin bends the wind, so it circles around highs and lows instead of rushing straight between them, and on the largest scale the same ideas arrange the winds of the planet into belts. This chapter explains the forces at work, shows how to read wind from a pressure chart and describes the great circulation. Figures are given from general knowledge and flagged where they should be checked.
The Push from Pressure
Where air is heated, it expands and rises, leaving lower pressure at the surface; where air cools and sinks, pressure at the surface is higher. The difference in pressure between two places creates a force that pushes air from high toward low, called the pressure gradient force. On a chart, pressure is drawn as lines of equal value, called isobars, and the closer together they are, the steeper the gradient and the stronger the wind.
The Spin of the Earth
Air does not flow straight from high to low pressure, because the Earth rotates beneath it. Seen from the ground, moving air appears to be pushed sideways: to the right in the Northern Hemisphere and to the left in the Southern. This apparent deflection is called the Coriolis effect, after the French engineer and mathematician Gaspard-Gustave Coriolis, who described it in the 1830s (check the date and the details). It is zero at the equator and strongest at the poles, and it matters for large, slow movements such as weather systems, not for a bath plug or a small stream (check this common misconception).
Low pressure
In the Northern Hemisphere, air circulates anticlockwise around a low and spirals slightly inward. Air converges, rises, and cloud and rain are common.
High pressure
In the Northern Hemisphere, air circulates clockwise around a high and spirals slightly outward. Air sinks, and the weather is usually settled and dry.
Buys Ballot's rule
Stand with your back to the wind in the Northern Hemisphere, and low pressure is to your left (check the name and date: a Dutch meteorologist, in the 1850s). It is a quick way to locate a low.
Friction
Near the ground, friction slows the wind and lets it cross the isobars toward low pressure at an angle. Higher up, the wind runs nearly along the isobars.
The Great Circulation
On a planetary scale, the sun heats the equator more than the poles, and the atmosphere carries heat toward the poles. The Earth's spin splits this flow into belts, in each hemisphere.
| Belt | Latitude (roughly) | Character |
|---|---|---|
| Trade winds | 0 to 30 degrees | Steady winds blowing toward the equator, from the east |
| Subtropical highs | About 30 degrees | Sinking air, dry weather; many deserts lie here |
| Westerlies | 30 to 60 degrees | Winds from the west; the belt that brings changeable weather to the British Isles |
| Polar easterlies | 60 to 90 degrees | Cold winds from the east near the poles |
The pattern is conventionally described as three cells in each hemisphere, named the Hadley, Ferrel and polar cells (check the names and the details, since the Ferrel cell is a simplified idea). Near the equator, the trade winds from the two hemispheres meet in a band of rising air, heavy cloud and heavy rain called the intertropical convergence zone (check).
Local Winds
On smaller scales, uneven heating creates local winds. The sea breeze is the classic example. By day the land heats faster than the sea, air rises over the land, and a cooler breeze flows in from the sea to replace it. At night the land cools faster, and the flow reverses into a gentler land breeze. Winds in valleys and mountains follow similar logic.
Measuring Wind: The Beaufort Scale
Before instruments were common, the wind was described by its effects. The Beaufort scale, devised by a British naval officer, Francis Beaufort, in the early nineteenth century, ranks wind from 0 (calm) to 12 (hurricane force) according to what it does to smoke, trees and the sea (check the date and the details). A light breeze, force 2, is enough to rustle leaves; a gale, force 8, breaks twigs and makes walking difficult; force 12 is destructive (check the thresholds). Modern forecasts give speeds, but the Beaufort scale is still used in shipping forecasts.
Exercises
In the Northern Hemisphere, you stand with your back to the wind and note that it is blowing steadily. On which side is the low pressure, and how does the circulation around a low differ from that around a high?
Name the three main wind belts between the equator and the north pole, and say which of them gives the British Isles most of its weather.
Explain why a sea breeze blows from the sea to the land by day, and what you would expect at night.
Quick Reference
- Wind: air pushed from high to low pressure; closer isobars mean stronger wind
- Coriolis effect: deflects moving air right in the north, left in the south
- Lows (north): anticlockwise, converging, rising, cloudy
- Highs (north): clockwise, diverging, sinking, settled
- Belts: trade winds, subtropical highs, westerlies, polar easterlies
- Jet streams: fast ribbons of wind near the tropopause that steer storms
- Beaufort scale: 0 (calm) to 12 (hurricane force)
Next chapter: Fronts and Depressions, which covers air masses and the storms of the mid-latitudes.