Global Winds and the Coriolis Effect – The Ever Changing Atmospheric System

A photograph showing blue sky and the wind blowing through a field of oats.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

A diagram illustrating what is meant by atmospheric pressure at the surface of the Earth. On average, a column of air one square centimetre [cm2] (0.16 sq in) in cross-section, measured from sea level to the top of the atmosphere, has a mass of about 1.03 kilograms (2.3 lb) and weight of about 10.1 newtons (2.3 lbf).
Atmospheric pressure arises as a result of the weight of the air sitting above the observer.

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.

 

A black and white photograph showing a man struggling against the wind with his open umbrella in very windy and rainy weather conditions. Image: Adrian SommelingAir 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.

 

An animation explaining how the Coriolis effect works: the red ball always travels in a straight line over the revolving Earth, but its track over the spinning globe makes it appear to loop around the North Pole.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 NumberDescriptionWind SpeedWaves HeightOn LandAt Sea
0Calm< 1.1 km/h
(< 0.7 mph)
< 0.6 knot
0 mCalm. Smoke rises vertically.Flat
1Light air1.1-1.5 km/h
(0.7-3.4 mph)
0.6-3 knot
0-0.2 mSmoke drift indicates wind direction. Leaves and wind vanes are stationary.Ripples without crests.
2Light breeze5.5-11.9 km/h
$ ($3.4-7.4 mph$ )$ 3-6.4 knot
0.2-0.5 mWind felt on exposed skin. Leaves rustle. Wind vanes start to move.Small wavelets. Crests of glassy appearance, not breaking.
3Gentle breeze11.9-19.7 km/h
(7.4-12.2 mph)
6.4-10.6 knot
0.5-1 mLeaves and small twigs constantly moving. Light flags extended.Large wavelets. Crests begin to break. Scattered whitecaps.
4Moderate breeze19.7-28.7 km/h
(12.2-17.9 mph)
<10.6-15.5 knot
1-2 mDust and loose paper raised. Small branches begin to move.Small waves with breaking crests. Fairly frequent whitecaps.
5Fresh breeze28.7-38.8 km/h
(17.9-24.1 mph)
15.5-21 knot
2-3 mBranches of moderate size move. Small trees in leaf begin to sway.Moderate waves of some length. Many whitecaps. Small amounts of spray.
6Strong breeze38.8-49.9 km/h
(24.1-31 mph)
21-26.9 knot
3-4 mLarge 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.
7High wind, moderate gale, near gale49.9-61.8 km/h
(31-38.4 mph)
26.9-33.4 knot
4-5.5 mWhole 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.
8Gale, fresh gale61.8-74.6 km/h
(38.4-46.3 mph)
33.4-40.3 knot
5.5-7.5 mSome 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.
9Strong/severe gale74.6-88.1 km/h
(46.3-54.8 mph)
40.3-47.6 knot
7-10 mSome 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.
10Storm, whole gale88.1-102.4 km/h
(54.8-63.6 mph)
47.6-55.3 knot
9-12.5 mTrees 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.
11Violent storm102.4-117.4 km/h
(63.6-72.9 mph)
55.3-63.4 knot
11.5-16 mWidespread 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.
12Hurricane force>=117.4 km/h
(>=72.9 mph)
>= 63.4 knot
>=14 mSevere 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.

 

An animation showing a cup anemometer surmounted by weather vane.
A basic cup anemometer surmounted by a weather vane. Source: Gifmania

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.

 

A photograph showing one of the most famous geological features of the Arizona desert, called The Wave.
Surreal, otherworldly landscapes like The Wave in Arizona, were shaped over time by the effect of swirling wind erosion on sandstones. Source: Pinterest

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.