BREEZES, GUSTS, GALES – call them what you will, winds are just bodies of fast-moving air. But what gets them going in the first place, and why do they blow at different speeds?
Winds are a response to differences in air pressure. They blow from high-pressure areas, where cooling air is sinking, to low-pressure areas, where warming air is rising, in an attempt to even out the pressure. Given the choice, winds would just get on with their job and flow directly from one area to the other. But the friction caused by the surface over which they travel, the rotation of the Earth and something called the Coriolis Force all interfere and deflect the wind’s direction.
As a result, in the Northern Hemisphere, a wind will swirl around, down and out in a clockwise – or anticyclonic – direction from a high-pressure area; and around, up and into a low-pressure area in an anticlockwise – or cyclonic – direction. South of the equator it’s the other way round. Generally, the greater the difference in air pressure, the stronger the wind will be. Easy, isn’t it?
‘When the wind is in the east
It’s good for neither man nor beast.
When the wind is in the north
The skilful fisher goes not forth.
When the wind is in the south
It blows the bait in the fish’s mouth.
When the wind is in the west
The weather is at the best.’
Traditional rhyme
Think of planet Earth like a spinning top, delicately balanced at a slight angle. Air movement between the hot tropics and the cold poles would normally flow directly from the centre to the north and south respectively, and back. But because of the planet’s spin, the natural flow is twisted off course to travel diagonally rather than directly up and down.
Winds generally follow the isobars (lines joining points of the same pressure). When the isobars on a weather chart are close together, it means that there is a steep change in pressure – so you can expect a blustery day. Close isobars plus low pressure mean it’s going to be wet too.
On 12 April 1934, a gust of wind – funnelled up between the top of Mount Washington, in New Hampshire in the United States, and the troposphere (the lowest layer of the Earth’s atmosphere) – reached a record speed of 372 kilometres (231 miles) per hour.
DO IT YOURSELF!
It’s easy to test the direction of the wind using a windsock and a compass. To make one yourself gather up the following items:
• Any plastic bottle
• Cloth ribbons
• Sellotape
• Cutting tool
• String
• Hole punch
Cut the top and bottom of the bottle, which will leave a ring of plastic – make sure it’s a few centimetres thick. Then you’ll need to punch two holes at the top of your plastic ring at opposite ends and put string through both holes so that you can tie up the windsock. Punch four more holes at the bottom of the ring at equal distances and tie some ribbons in these holes. The windsock will then need hanging up, probably on a pole so that the wind can get to it from all directions. When the wind starts to blow the sock and ribbons will lift, and if you check your compass you’ll see which direction it’s coming from.
Long before any talk of alternative green power, sailors were already using a source of natural energy to propel them across the seas: wind. However, the wind was an unpredictable, uncontrollable force and sailing ships were at its mercy. An accurate method of assessing its speed was necessary, but sophisticated, scientific equipment was not yet available.
In 1805, Rear-Admiral Sir Francis Beaufort of the British Royal Navy came up with a bright idea. By observing how frigate sails reacted in different wind speeds, he invented a scale for measuring the force of the wind – a wind might be so light, for example, that it gave only enough power for the ship to be steered, or so strong that it would shred the sails. Known as the Beaufort Scale, it was adopted by the Royal Navy in 1838.
When steamships appeared, the behaviour of the sea was also taken into account and, later, conditions on land. The Scale originally had thirteen categories from 0 (calm) to 12 (hurricane force). In 1955, the United States Weather Bureau added a further five, forces 13 to 17, to grade hurricane-force winds. The Beaufort Scale is still used today, alongside more scientific methods.
Here are the original categories of The Beaufort Scale, expressed in knots and miles (for landlubbers, a knot is a unit by which a ship’s speed is measured, and is equivalent to about 1.8 kilometres/1 mile per hour).
Force |
Average speed knots/mph |
Description |
Effects |
0 |
0/0 |
Calm |
Calm, glassy sea; smoke rises vertically |
1 |
2/2 |
Light air |
Wind direction shown by smokedrift but not wind vanes |
2 |
5/6 |
Light breeze |
Ripples on sea; wind felt on face, leaves rustle, vanes moved by wind |
3 |
9/10 |
Gentle breeze |
Smooth wavelets; small twigs in constant motion, light flags extended |
4 |
13/15 |
Moderate breeze |
Numerous whitecaps on sea; dust, leaves and loose paper raised, small branches move |
5 |
19/21 |
Fresh breeze |
Many whitecaps on sea, some spray; small trees sway |
6 |
24/28 |
Strong breeze |
Larger waves, whitecaps everywhere, more spray; large branches move, whistling in phone wires, umbrellas difficult to use |
7 |
30/35 |
Near gale |
White foam from breaking waves blown in streaks; whole trees in motion; walking inconvenient |
8 |
37/43 |
Gale |
Wave crests begin to break into spindrift; twigs break off trees, hard to walk |
9 |
44/51 |
Severe gale |
High waves, heavy swell, spray may reduce visibility; chimney pots and slates removed, large branches break |
10 |
52/60 |
Storm |
Very high waves with overhanging crests, sea white with blowing foam; trees uprooted, structural damage |
11 |
60/69 |
Violent storm |
Exceptionally high waves; widespread damage |
12 |
64+/74+ |
Hurricane |
Sea white, air filled with foam, visibility greatly reduced; widespread damage, rarely experienced on land |
WATCH YOUR LANGUAGE!
Gust A wind that lasts from a few seconds to a few minutes.
When the weather forecaster talks about a ‘northerly’ or ‘westerly’, this refers to the direction the wind is coming from – not where it is heading.