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Noise - Environmental - Weather - Atmosphere

Atmospheric Stability
The second weather condition to influence received noise levels is the stability of the atmosphere or the temperature gradient.  Note that in this section wind speed is the speed irrespective of direction, i.e. not vector wind speed.  For our purposes the stability of the atmosphere can be considered as having three main states or conditions as follows-

Unstable - U
When hot air rises, noise is also carried upwards and less of it reaches the noise sensitive location; the greater the incoming solar radiation the greater the likelihood of the atmosphere being unstable.  This effect is more likely to happen, the less cloud  cover there is, the closer to summer and the closer it is to the early afternoon.  Naturally it only happens during the daytime.

It is a complex judgment as to how much incoming radiation there is. As a rough "rule of thumb" the following table can be used, for the period of late morning to mid-afternoon, where "cc" is the percentage of sky obscured by lower, denser cloud, as opposed to high thin cloud.

 

Percentage Cloud Cover "cc"

Incoming Solar Radiation mW/cm2

Summer

Spring/Autumn

Winter

cc=0

>60

>60

>60

0<cc<25

>60

>60

30-60

25<cc>50

>60

30-60

30-60

50<cc<75

30-60

30-60

<30

75<cc<100

30-60

<30

<30

Similarly for up to mid-morning and after mid-afternoon

Percentage Cloud Cover "cc"

Incoming Solar Radiation mW/cm2

Summer

Spring/Autumn

Winter

cc=0

>60

>60

30-60

0<cc<25

>60

30-60

30-60

25<cc>50

30-60

30-60

<30

50<cc<75

30-60

<30

<30

75<cc<100

<30

<30

<30

Stable - S
When a layer of cold air is trapped close to the ground, under warmer air, this is the reverse of normal conditions, and this reversal is known as a temperature inversion; any noise generated in the cooler layer is also trapped within it and unusually high noise levels can be experienced. Inversions are more likely to occur at night when there is little cloud cover; the ground itself cools and this also cools the layer of air close to it. If there is significant cloud cover, this tends to radiate heat back towards the ground and inhibits an inversion; equally if the winds are significant the turbulence mixes the layers and again inhibits the formation of an inversion layer.  Naturally it only happens at night.

Normal - N
Finally normal conditions where the temperature slowly decreases with height such as overcast conditions and/or when the wind is high enough to cause mixing of any atmospheric layers.  These conditions can occur day or night; they will always prevail when it is fairly windy, overcast or at the beginning or end of the day.


According to the CONCAWE model, these conditions occur as follows

 

Wind Speed m/s

Day Time
Incoming Solar Radiation mW/cm2

1 hour before sunset or after Sunrise

Nighttime
Cloud Cover (octas)

>60

30-60

<30

Overcast

0-3

4-7

8

1.5

U

U

U

N

N

S

S

N

2.0-2.5

U

U

N

N

N

S

N

N

3.0-4.5

U

N

N

N

N

N

N

N

5.0-6.0

N

N

N

N

N

N

N

N

>6.0

N

N

N

N

N

N

N

N

Note: It is important to stress that the wind speeds used are the general wind speeds between the source and receiver, e.g. Beaufort Scale speeds NOT the wind speed at the measurement position and NOT vector wind speeds.

Summary to Atmospheric Stability
All the above can be reduced to the following

1. The Conditions are Neutral if any of the following statements are true

  • the wind speed is greater than 5.0 m/s, or

  • the sky is overcast, or

  • it is within one hour after sunrise or one hour before sunset

If any of the above is correct, do not consider the following.

2. For Daytime, conditions are Neutral, unless any of the following is true

  • the wind speed is 1.5 m/s or less, or

  • there is > 60 mW/cm2 of incoming solar radiation and a wind speed < 5 m/s

  • the wind speed is < 3.0 m/s and there is >30 mW/cm2 of incoming solar radiation

in which case, the Daytime conditions are Unstable.

3. For Nighttime, conditions are Neutral, unless any of the following is true 

  • the wind speed is 1.5 m/s or less and there is not full cloud cover

  • the percentage cloud cover is less than 50% and the wind speed is less than 2.5 m/s.

in which case the Nighttime conditions are Stable.

<<< Back to Wind Strength

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