This article is part of our collection on the 'Golden Globe Ocean Race' where former Open University student Antoine Cousot is circumnavigating the globe for 30,000 miles, alone. Our academics are championing this by getting you to explore information, science and ideas about the ocean with their explainer articles.
The power of the wind
Why is the sea so angry at 40oS?
The unequal heating of different parts of the Earth by the Sun drives the weather. One very visible effect is the flow of warm and cold air around the planet known as winds. Winds then drive the waves that Antoine is currently experiencing.
But how does the wind make waves?
So the wind provides one of the main forces that move the surface of the oceans. When the wind moves across water, surface friction transfers energy from the wind to the water through wind stress.
- Wind stress has the Greek symbol τ (tau) and is proportional to the square of the wind speed, W:
- τ = cW2 where c is the constant of proportionality.
- So if the wind speed increases from 1 m s−1 to 4 m s−1 then wind stress will increase by a factor of 16.
- This large jump in the wind stress happens because when the winds are weak, the surface of the ocean is relatively flat and there are few wave tops for the wind to push against.
- As energy is transferred from the wind to the ocean, the surface becomes rougher and 'stretched', so more of the surface is in contact with the wind. The increased surface area leads to more energy being transferred to the ocean and larger surface waves.
Quick question: So if the wind speed increases from 2 ms-1 (Beaufort Scale 2) to 16 ms-1 (Beaufort Scale 7) by how many times will wind stress increase?
Select here to reveal answer...
The wind stress will increase by a factor of 64. 16 ms-1 is eight times 2 ms-1 so the wind stress will increase by eight squared which equals 64.
- The wind speed, and consequently the wind stress driving the waves, varies around the globe and because of the long land-free belt of ocean around the globe from 40 to 65oS (so no interruptions for the wind) the wind speeds are very high and consequently the waves in the sea are very ‘vigorous’!
- This area of strong west to east winds from 40 to 50oS is commonly referred to as the ‘Roaring Forties’. You can see this belt as the red and purple shades on this map. Sailing ships have used the speed of the wind in this area over many centuries to speed their passage from Europe to Asia and Australia. Antoine has definitely arrived in the ‘angry seas’!
When the worst weather arrives we can go indoors but Antoine, and wildlife, have to brave it!
Scientists can now examine the conditions in some of the wildest seas in the laboratory.
Power from the wind
Can the energy in the wind be harnessed by man?
- The energy contained in the winds Antoine is experiencing is driving the waves, and his sails, but is also being used in other parts of the world to provide usable energy.
- Wind energy was the fastest growing power source at the start of the 21st century, yet wind-driven mills and pumps, and nautical sails for transport were, along with waterwheels, the first mechanical devices to power industrial production.
- The advantages of harnessing wind energy are obvious; it is free, clean and widely available. Although a favoured source of 'green' energy, the increasing deployment of wind turbines where they are most efficient, on hilltops, coasts and at sea, together with their increasing size has led to outcries because of their impact on landscapes.
Here are some of the sites around the UK where wind energy is being harnessed to provide power:
Have a look at how wind turbines capture the force of the wind:
and the latest developments in offshore turbines.
Finally, if you want to experiment yourself and create some wild weather at home, try some of these!
Like oceans? Take it further