Noise - Environmental - Weather - Wind Strength
For environmental noise, the weather plays an important role; the greater the separation distance, the greater the influence of the weather conditions; so, from day to day, a motorway some half a mile away can sound very loud, or can be completely inaudible. This very large variation in daily noise levels means that it is not unusual for residents not to notice the effects of a Noise Control programme; for example if "before noise control" noise levels varied from say 40 to 60 dBA, and "after" from 32 to 52 dBA, many residents would not notice any change; perhaps after time a proportion would appreciate that in general noise levels had fallen, but probably many residents would still not have noticed the change.
There is a well used model for predicting weather effects - "The Propagation of Noise from Petroleum and Petrochemical Complexes to Neighbouring Communities"; this is report No.4/81 published by the Oil companies international group for CONservation of Clean Air and Water - Europe; this is simply known as the CONCAWE model and is used for many noise sources. The full report can be ordered from www.concawe.be.
Wind Strength and Direction
The most well known weather effect is wind strength and direction; naturally winds blowing from the noise source towards the noise sensitive location will increase levels, and the stronger the wind the greater the effect, until the wind itself becomes the dominant noise source or is so turbulent that it disperses the "problem" noise. Standards such as BS.4142 and "Calculation of Road Traffic Noise" place limits on acceptable wind strength and, in some instances, on the wind speed in a particular direction - vector wind speed.
Wind speed is most commonly categorised by The Beaufort Scale the main categories relevant to noise measurements appear as follows