|
|
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
in which case the
Nighttime conditions are Stable.
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|
