Weird weather – is cold wind heavier than warm wind?

Paul Lees, managing director of Crusader Sails

Paul Lees, managing director at Crusader Sails:

Windsurfers and racing yacht skippers have long been aware that the heeling moment exerted on a yacht’s sail varies according to the relative density of the wind. Many a windsurfer has puzzled over why a 5.2m sail is quite manageable during the summer holidays in the Mediterranean but seems impossibly overpowered for the same wind speed in early December in the UK. The denser wind does exert a greater heeling force on the rig. Add to this its effect on sea state and the general human perception that things are nicer and easier when it’s sunny, and it does make a noticeable difference. This poses the interesting question: are wind farms in colder climes more efficient than those situated in the sunnier parts of our world?

Frank Singleton was a Met Office chief forecaster and director of operations

Frank Singleton, meteorologist, franksweather.co.uk:

The ‘weight of the wind’ is an old saying that, perhaps unusually, has more than a grain of truth, although it is a fairly small grain.

The force on a sail varies directly with air density and the square of the wind speed. According to Boyle’s Law – for a given pressure, the density of a gas varies inversely with the temperature – cold air is denser than warm air and so creates a greater force.

The effect will be compounded by the fact that moist air is less dense than dry air. This is because water vapour is a relatively light gas compared with oxygen and nitrogen – the main constituents of air. When water vapour content increases, the amount of oxygen and nitrogen decreases. Air density decreases because the mass decreases.

For a fairly extreme example, in the tropics with a temperature of 32°C and 90 per cent humidity, the air density is 1.14 kg/m³. In a northerly airstream around the UK, we might have temperatures of 10°C and 10 per cent humidity giving an air density of 1.25 kg/m³. So with cold dry air, the force of the wind on a sail would be about 10 per cent higher than with warm humid air at the same wind speed. For dry air at both temperatures there would be a 7 per cent increase; the water vapour effect is fairly small.

However, the expectation may lead to some misinterpretation of what is actually happening. An increase in wind speed from 14 to 15 knots will give an increase in force on a sail of more than 15 per cent. In cold and unstable air, convection mixes air between 1,000 to around 4,000 metres, causing stronger gusts than with warm stable air in which the mixing is much more restricted in depth. Compare the steady winds in the warm sector of a depression with the much more variable wind behind the cold front.

So, the reality is that there is ‘more weight’ in cold dry air than in warm moist air. However, around the UK during the sailing season in one area the variation in the ‘weight’ of the wind must be fairly small and masked by larger effects due to variations in wind speed and especially when there are convective gusts. The effect may only be detectable to a far better helmsman than I ever was.

A practical corollary of the above is that wind turbines can generate more power at a given wind speed in cold climates than they do in warm ones.