Temperature Inversion
Temperature inversion is a meteorological condition which can increase the noise level heard near a wind turbine. Under temperature inversion conditions it is possible for wind at turbine height to continue blowing while all is still and calm at ground level. Turbine noise thus penetrates into a zone of low background noise where it becomes the dominant noise source.
What conditions cause temperature inversion?
Under normal Australian daytime conditions air at ground level is warm and temperature drops progressively with altitude. When the sun goes down the temperature of the ground drops and cools the layer of air next to it. Air close to the ground is cold, at moderate height it is warmer, then eventually at greater height the normal altitude effect takes over and the temperature drops with altitude as usual. This condition, the reverse of a normal daytime temperature profile, is temperature inversion.
Temperature inversion and wind
When air moves over the ground its speed varies with height. The layer of air directly against the ground is slowed by friction. Wind speed increases with height reaching a maximum well away from the surface.
It is useful to understand how wind behaves under normal, warm ground, daytime conditions:
Air against the ground is slowed to a stop by ground friction. It is heated by the ground (becomes less dense), rises to mix with the wind at greater height, and recovers its speed.
Contrast this behaviour with wind in a temperature inversion. Air against the ground is slowed to a stop by ground friction. It is cooled by the ground (becomes more dense) and stays against the ground. Meanwhile the wind continues to blow at height.
Wind Turbine noise under temperature inversion has been extensively studied by Prof Van Den Burg who published his seminal paper on the topic showing that wind and noise models used by manufacturers can underestimate wind turbine noise by as much as 15 dBA (ie. The noise is more than twice as loud) under temperature inversion conditions.
How common is temperature inversion.
A warm day with moderate wind followed by a crisp clear night will produce temperature inversion. These are the classic climate conditions of inland Australia during Spring, Autumn, and parts of winter, confirmed by the meteorological record.
Spring Range View
Those planning and approving wind turbines need to seriously consider Canberra's climate and its implications for wind turbine noise models. Development approvals to date (notably Taralga) have ignored the problem despite clear, honest, and compelling evidence submitted by residents. Refer to Paul Miskelly's Noise submission opposing Taralga wind plant.