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Noise - Environmental - Weather - Total Effect 

Predicting the Effect of Meteorological Categories
The CONCAWE model combines the vector wind speed "v" (m/s) and the three atmospheric categories to produce six Meteorological Categories numbered one to six as follows-

Met Category

Atmospheric Category

Unstable

Normal

Stable

1

v<-3.0

2

-3.0<v<-0.5

v<-3.0

3

-0.5<v<+0.5

-3.0<v<-0.5

4

+0.5<v<+3.0

-0.5<v<+0.5

-3.0<v<-0.5

5

v>3.0

+0.5<v<+3.0

-0.5<v<+0.5

6

v>3.0

+0.5<v<+3.0

Note: In the above table a negative vector wind speed denotes wind blowing from the receiver towards the source, i.e. reducing noise levels.

As the Met Categories increase, the received noise levels increase (attenuation decreases), and category No.4 is defined as having a zero meteorological influence. In other words categories 1 to 3 reduce noise levels, and categories 5 and 6 lead to higher noise levels. It can be seen that the highest likely noise levels occur either with Beaufort "Light air" winds under "stable" atmospheric conditions [temperature inversion] or with stronger winds (Beaufort "Light Breeze" or more) under normal atmospheric conditions.

Predicting Noise Levels

The main CONCAWE model predicts noise levels at a distance based on the weather conditions and a knowledge of the source spectrum in octave bands; however there is a simplified model simply based on the likely dBA effect as below.

When predictions using this model were compared against actual measurements of broad band noise, the measured level was within a 14 dBA range centred on the predicted level [95% confidence limits]. This was only slightly worse than using the full octave band method. It just shows that predicting the effect of weather is not very reliable!! You should consult the full report available from www.concawe.be for full details.

Using the chart it is possible to estimate the likely noise levels under differing weather conditions; for example if a measurement of 50 LAeq was made 1000m from a source under category 1 conditions, the model indicates that under category 6 conditions, the level would have been around 10 dBA higher.

Graph of Weather Effect in dB terms

The directory contains details of suppliers of prediction software incorporating the full CONCAWE model and consultants experienced in allowing for the effects of weather conditions.

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