What is Wind?

It comes at you as a breeze.  As a gust.  As a gale.  Or in the scariest of situations as a hurricane or a tornado with wind speeds of up to 400 kilometres an hour.  But what is wind?

Moving air can be delightful or it can be frightening.  But what is wind?  And how does it all happen?  In one sentence, wind is caused by air flowing from an area of high pressure to an area of low pressure.  The long answer involves a bunch of variables that affect wind formation, speed, and direction, including the Sun and the Earth‘s rotation.

Broadly-speaking, wind is gas in motion.  Here on Earth, the air that surrounds us is mostly made of nitrogen molecules (~ 70 %), oxygen molecules (21 %) and water vapour (1-4 %).

 

Atmospheric Pressure

When we talk about air pressure, what we really mean is the amount of force that these molecules exert on a given area.

In the diagram, we see that the mass of air in the column right above the head of the observer is pulled down by the gravity of the Earth and exerts a force (and pressure) on the observer.

As masses of air warm, it rises, thus exerting less pressure on the surface of the Earth.

At the surface of the Earth, the pressure is 1000 N m^-2.  This is roughly equivalent to 14.7 pounds per square inch (1 atmosphere = 101,000 Pascals = 1 kPa = 1 bar).

At higher altitudes, less air sits above an observer and so there is a smaller atmospheric pressure.

 

Air is moist

Air is moist.  And as it rises, air can become unstable and create cloudy, rainy, or even stormy conditions.

That is why low pressure systems are often associated with bad weather.  However, that low pressure area you can see on weather maps is only low in relation to the air around it.  There is always a massive cooler drier air coming in from the polar regions waiting to rush in and push that low out of the way, and sit right on top of a landmass.

That is a high pressure system.  When it dominates the weather, clear skies usually come with it.

Think of the wind as the atmosphere‘s eternally feudal struggle to find a balance  between these two types of competing systems.  Air flows from a high pressure to a low pressure system to try and reach a state of equilibrium.  In doing so, air masses create wind.

 

Under the Influence…

Unsurprisingly, the Sun has a broad influence on the Earth’s atmosphere.  As it heats the surface of the Earth, about two percent of all the Sun’s energy that reaches our planet is eventually converted into wind energy – a small portion of which we, humans, try to harness with wind turbines.

The spinning motion of Earth also plays an important role in the production of global winds.  Indeed, if the Earth did not rotate on itself, global winds would pretty much flow in a straight line, with cold air coming from the poles, heating up the equator, and then going back again.

 

The Coriolis Effect and Global Wind Direction

Instead, the Earth’s rotation produces a force on everything that is moving relative to the Earth, including air masses.  A phenomenon known as the Coriolis effect.

That rotation deflects the direction of the winds to the right in the Northern hemisphere, and to the left in the Southern hemisphere.  Put together, these forces can do amazing things.

The direction of a wind is reported by the direction from which it originates.  A northerly wind flows from North to South, and so on.  The direction can be determined using a windsock, or a weather vane.

Wind direction can also be reported using azimuth degrees.  A wind coming from the south is given as 180 degrees; one from the east as 90 degrees.

 

Wind Strength: The Beaufort Scale

The closer the high and low pressure areas are to each other, the stronger the wind.

The Beaufort scale is an empirical measure for quantifying wind force.  Whether at sea or on land, the Beaufort scale relates the wind speed to the observed conditions:

Beaufort Number	Description	Wind Speed	Waves Height	On Land	At Sea
0	Calm	< 1.1 km/h (< 0.7 mph) < 0.6 knot	0 m	Calm. Smoke rises vertically.	Flat
1	Light air	1.1-1.5 km/h (0.7-3.4 mph) 0.6-3 knot	0-0.2 m	Smoke drift indicates wind direction. Leaves and wind vanes are stationary.	Ripples without crests.
2	Light breeze	5.5-11.9 km/h $ ($3.4-7.4 mph$ )$ 3-6.4 knot	0.2-0.5 m	Wind felt on exposed skin. Leaves rustle. Wind vanes start to move.	Small wavelets. Crests of glassy appearance, not breaking.
3	Gentle breeze	11.9-19.7 km/h (7.4-12.2 mph) 6.4-10.6 knot	0.5-1 m	Leaves and small twigs constantly moving. Light flags extended.	Large wavelets. Crests begin to break. Scattered whitecaps.
4	Moderate breeze	19.7-28.7 km/h (12.2-17.9 mph) <10.6-15.5 knot	1-2 m	Dust and loose paper raised. Small branches begin to move.	Small waves with breaking crests. Fairly frequent whitecaps.
5	Fresh breeze	28.7-38.8 km/h (17.9-24.1 mph) 15.5-21 knot	2-3 m	Branches of moderate size move. Small trees in leaf begin to sway.	Moderate waves of some length. Many whitecaps. Small amounts of spray.
6	Strong breeze	38.8-49.9 km/h (24.1-31 mph) 21-26.9 knot	3-4 m	Large branches in motion. Whistling heard in overhead wires. Umbrella use becomes difficult. Empty plastic bins tip over.	Long waves begin to form. White foam crests are very frequent. Some airborne spray is present.
7	High wind, moderate gale, near gale	49.9-61.8 km/h (31-38.4 mph) 26.9-33.4 knot	4-5.5 m	Whole trees in motion. Effort needed to walk against the wind.	Sea heaps up. Some foam from breaking waves is blown into streaks along wind direction. Moderate amounts of airborne spray.
8	Gale, fresh gale	61.8-74.6 km/h (38.4-46.3 mph) 33.4-40.3 knot	5.5-7.5 m	Some twigs broken from trees. Cars veer on road. Progress on foot is seriously impeded.	Moderately high waves with breaking crests forming spindrift. Well-marked streaks of foam are blown along wind direction. Considerable airborne spray.
9	Strong/severe gale	74.6-88.1 km/h (46.3-54.8 mph) 40.3-47.6 knot	7-10 m	Some branches break off trees, and some small trees blow over.	High waves whose crests sometimes roll over. Dense foam is blown along wind direction. Large amounts of airborne spray may begin to reduce visibility.
10	Storm, whole gale	88.1-102.4 km/h (54.8-63.6 mph) 47.6-55.3 knot	9-12.5 m	Trees are broken off or uprooted. Structural damage is likely.	Very high waves with overhanging crests. Large patches of foam from wave crests give the sea a white appearance. Considerable tumbling of waves with heavy impact. Large amount of airborne spray reduce visibility.
11	Violent storm	102.4-117.4 km/h (63.6-72.9 mph) 55.3-63.4 knot	11.5-16 m	Widespread vegetation and structural damage likely.	Exceptionally high waves. Very large patches of foam, driven before the wind, cover much of the sea surface. Very large amounts of airborne spray severely reduce visibility.
12	Hurricane force	>=117.4 km/h (>=72.9 mph) >= 63.4 knot	>=14 m	Severe widespread damage to vegetation and structures. Debris and unsecured objects are hurled about.	Huge waves. Sea is completely white with foam and spray. Air is filled with driving spray, greatly reducing visibility.

 

Wind Speed

Wind speed is measured using an anemometer – a device invented in its simplest form by Italian Renaissance Man Leon Battista Alberti in 1450, which is still commonly used today in weather science.  On a four-cup anemometer, it is easy to see that since the cups are arranged symmetrically on the end of the arms, the wind always has the hollow of one cup presented to it and is blowing on the back of the cup on the opposite end of the cross.

The wind speed unit is the knot, corresponding to one nautical mile per hour (1.852 km h^-1 or about 1.151 miles per hour).

According to most sources, the highest wind speed ever recorded was at the top of Mount Washington in New Hampshire.  In 1934, during a particular brutal storm, wind gusts topped out at 372 kilometres per hour.

In 1996, however, 10 metres above the Earth’s surface, winds of 408 kilometres per hour were recorded during cyclone Olivia, off the coast of Australia.

 

Global Winds as a Geological Force

Wind is also a factor of erosion.  Along with precipitations (rain, snow, etc.) and gravity, wind can literally shape the surface of the Earth.