Frosty mornings and noise effects

Our community engagement phone line has been receiving more calls than usual lately from residents on the subject of noise.

When we receive a noise complaint a technician goes to the property boundary with mobile monitoring equipment and takes a sample reading.*

Over recent weeks we are finding that noise effects are indeed more noticeable in the mornings for a time and are less later on. At this time of the year this is due to a phenomenon known as temperature inversion.

On a typical sunny day, air is warmest near the ground and temperature decreases at higher altitudes. This temperature gradient causes sound waves to refract upward, away from the ground and results in lower noise levels being heard at a listener's position.

* For more information about noise concerns or complaints relating to Newmont Waihi Gold's activities please read: www.marthamine.co.nz/PDF/Noise.pdf

In the evening and morning, particularly during winter and especially with clear, calm nights, this temperature gradient will reverse, resulting in cooler temperatures near the ground and a warmer upper layer.

This upper layer acts like a lid, trapping air beneath it and also causing sound waves to bend downward toward the ground. This results in louder noise effects being felt and can cause distant sources of noise to sound much closer than they really are. Inversion layers are usually dispersed by wind or by warm air rising as the ground heats up.

In winter the inversion layer (cap) traps sound and air, as demonstrated in the diagram by smoke from a home fire.

With sufficient humidity in the cooler layer fog is often present below the inversion cap.

An inversion is also produced whenever radiation from the surface of the earth exceeds the amount of radiation received from the sun, which commonly occurs at night, or during the winter when the angle of the sun is very low in the sky.

Very-high frequency (VHF) radio waves can be refracted by temperature inversion. This is why it is possible to sometimes hear FM radio broadcasts from otherwise impossible distances on foggy nights. The signal, powerful enough to be received at hundreds or thousands of kilometres, would normally be refracted up and away from the ground-based antenna but instead it is refracted down towards the earth by the temperature-inversion cap.

Inversion effects occur frequently where a locality is closely surrounded by hills and mountains. In cities where many pollutants exist temperature inversion has a big effect on air quality.

There are many websites that provide in depth information about sound and temperature inversion. You may find these interesting:

ww.mocpa.com/inversion.html
www.acousticsbydesign.com/acoustics-blog/weather-affects-noise-study.htm