The never-ceasing movement of the world's air and oceans is driven by the heat from the sun. It happens because nearly all fluids, and certainly air and water, change density with any change of temperature. As the land warms in the equatorial sunlight, the air in contact also warms and becomes less dense; it then rises like a hot air balloon. When the sea surface warms it becomes lighter and floats on top of the cool waters below; it does not mix with these cooler waters and consequently a stratified warm upper layer forms. Frorn the warm ocean surface water evaporates continuously and mixes with the air, lowering its density and providing what is called insensible heat, so called because it is a measure of the heat content of the air, not its temperature. It takes an astonishing amount of heat to evaporate a gram of water - about 600 calories - and this heat is retrievable when the water condenses again. As a parcel of warm, wet air rises it cools and the water vapour condenses, releasing its latent heat and providing more heat energy for the air parcel to rise further. This is part of the force that energizes the tropical thunder storms.

The next consequence of all this rising warm damp air is movement away from the equator to the north and to the south. The vertical motion of the warmed air cannot easily penetrate into the stratosphere, a layer of the atmosphere that exists, in north temperate regions, above about ten kilometres from the surface (where most of us have been without knowing it, as passengers traveling in jet aircraft). The stratosphere is warmer than the air mass immediately below; and lies above it much as the warm water of the surface layer of the ocean lies above the cooler water beneath. The boundary between these two separated parts of the air is called the tropopause, and it forms an invisible barrier to upward air motion. It is higher, at about seventeen kilometres, in equatorial regions. The warm wet tropical air rises through the troposphere - the lower atmosphere where we and the clouds exist and as it rises sheds its water as rain. When the dried air reaches the tropopause it turns north or south and moves as a pair of flat cylinders girdling the planet. When the northerly or southerly motion takes the air to about latitudes 30° north or south it begins to descend; the downward movement now heats the air by compression and makes the surface regions in the descending air the hottest and driest parts of the world: the deserts of Australia, Chile, Sahara, Texas and Mexico, and the Persian Gulf.

George Hadley first proposed this form of planetary air motion in a paper to the Royal Society in 1735 entitled 'Concerning the cause of the General Trade Winds', and the cellular zones of the Earth where it happens are now called Hadley cells. His name was rightly chosen for one of the foremost climate centres of the world, now part of the Meteorological Office at Exeter, in the UK, which was established by the then Prime Minister Margaret Thatcher in 1988. Global warming tends to make the Hadley cells grow larger and extend further north and south. The hot dry regions could then extend into the temperate zone. The return flows of dry air to the tropics are the north-east and south-east trade winds so welcomed by sailors. Because of the expansion of the Hadley cells as global heating intensifies, it would be unwise for Europeans to assume that the now prevailing westerly winds will continue to blow at the same latitude.

Air motion driven by heat is the source of the wind, but the motion of a fluid is rarely ever simple; the water in your kitchen sink never flows uniformly into the waste pipe when the plug is pulled. It often forms a vortex, spinning as it goes. Sometimes this vortex is powerful enough to have a central empty core, down which air is noisily dragged along with the water. So it is with the atmosphere: the large-scale heat-driven motions that spin as hurricanes or cyclones are the source of the wind. There is no simple rational answer to the question 'why does a vortex form?' All that we can say is: whenever there is a flow of energy through matter, interesting things like vortices, flames and life emerge. Erich Jantsch, in The Self-organizing Universe (1980), observed that we seem to live in a universe where orderly structures form whenever there is a flow of energy.

Humankind has used wind energy from its earliest times, mainly to move wooden sailing ships across the sea. The green movement has been the advocate for this constant clean source of energy, and in the long run it might have promise. But at present, wind energy as a whole system is in the early stages of development and is not much more efficient than were those early biplanes held together with wire that were the first form of air transport. We still have much to learn about using wind power, and most of all about storing the energy it produces when the wind blows. The wind is restless and blows only some of the time. Anticyclones with little or no wind bring the hot days of summer when air-conditioning may be needed, and they bring the cold frosty days of winter when energy is required to keep warm. But when we can store the wind energy, all could be well. There is no reason in principle why we can't; there is, for example, in Wales a high reservoir built in the 1950s into which water is pumped using electricity when it is in surplus, such as in the middle of the night. When electricity is needed such as at rush hour, water is drawn from this reservoir through water turbines and used to generate the needed extra supply of electricity. This is potentially a fine and reliable way of storing energy but it needs a suitable mountainous region close to a windy place. Other ways of storing energy, such as using compressed air, can be thought of and planned, but the loose talk of enthusiasts for wind energy that the energy could be stored as hydrogen and that this could then be used as fuel for cars, ignores the decades of engineering development needed to make this a practical option. At present none of them is immediately available on the scale that is needed.

There are many parts of the world; the Great Plains of the United States and Russia, for example, where wind farms could coexist with fields of agribusiness and be welcome; wind farms offshore sound good, too, as the wind is more powerful and reliable than on land and they could be out of sight. Unfortunately; the costs of maintenance are much higher than-for land-based turbines. Each individual turbine would have to be serviced by small boats; unfavourable tides and rough seas would often delay or prevent them from docking at the turbine. The placement of efficient, that is, huge (100 metres or higher) wind turbines in the densely populated parts of Europe is proving highly unpopular. On aesthetic grounds these are not suitable places for harvesting wind energy on a large scale.

Aesthetics alone is an insufficient reason for rejecting what might be a clean and valued energy resource, and if wind power was truly capable of providing a serious proportion of our energy needs in Western Europe, most of us would grit our teeth and accept it, even though it is to many an unpleasant and intrusive power system. It is sometimes claimed by wind enthusiasts that all our electricity could come from wind; I doubt if many of them have calculated the number of 100 metre, one megawatt turbines needed. To supply the UK's present electricity needs would require 276,000 wind generators, about three per square mile, if national parks, urban, suburban and industrial areas are excluded; also needed would be an efficient way of storing the electricity they produced. But in no way is it efficient and economic; the intermittency of the wind means that, at best, energy is available from wind turbines only 25 per cent of the time. During the remaining 75 per cent, electricity has to be made in standby fossil fuel power stations; worse still, the power stations have to be kept idling when wind energy is available, an inefficient way for them to operate. The most recent report from Germany put wind energy as available only 16 per cent of the time, and in Denmark, which has pioneered their development, Niels Gram of the Danish Federation of Industries said, 'In green terms windmills are a mistake and economically make no sense. . . Many of us thought wind was the 100 per cent solution for the future but we were wrong. In fact, taking all energy needs into account it is only a 3 per cent solution.'

According to the Royal Society of Engineers 2004 report, onshore European wind energy is two and a half times, and offshore wind energy over three times, more expensive per kilowatt hour than gas or nuclear energy.1 No sensible community would ever support so outrageously expensive and unreliable an energy source were it not that the true costs have been hidden from the public by subsidies and the distortion of market forces through legislation. Enthusiasm for renewable energy coupled with a politics in which each nation tries to gain brownie points for its diligence in meeting the Kyoto limits is an unhappy mixture. It will fail and bring discredit both to the greens and to the politicians foolish enough to adopt renewables as a major source of energy before they have been properly developed.

Wind energy, through crude and unsustainable industrial development, is already devaluing some unusually beautiful countryside. That countryside, although already damaged by agribusiness, still has a few areas that are an example of how to live in a decent and seemly way with the natural world. I think that the responsibility for city dwellers with a romantic, impractical dream of clean, renewable energy coupled with a misplaced fear of nuclear energy but no real empathy with Gaia of the natural world.