See INP Section 5
The INP intends that the noise levels used in assessing noise impacts at
the consent stage include the effects of any weather conditions that are a
feature of the area when the development operates. This means that the effects
of weather conditions such as temperature inversions and wind on the noise
level experienced at sensitive receivers should be adequately assessed at the
Wind can enhance noise propagation compared with calm conditions (where
there is no wind). When a wind blows, friction causes the air to move more
slowly close to the ground than at higher altitudes. This phenomenon of wind
speed increasing with height is termed 'wind shear'. The increase in noise
occurs because sound waves from the source are bent through this 'wind shear'
back towards the ground.
Unlike temperature inversions, wind can enhance propagation during any time of the
day, evening or night. Wind does not increase noise in all directions and can
also reduce noise. For example, wind blowing from the south to the north (termed a
'southerly' wind) increases noise to the north of an industrial premise and
also reduces noise to the south of that premise.
In some instances, where one or more significant weather conditions have
been identified as part of a noise assessment, noise levels from the industrial
premises under only these significant weather conditions have been assessed, but
noise levels under calm conditions have not.
The INP describes in Section 5 when weather is 'significant' (i.e. it
occurs more than 30% of the relevant time period) and how to apply this in the
noise assessment. This approach may result in noise levels at some receivers
being underestimated, as in the southerly prevailing wind scenario described
This application note clarifies that in all cases at each receiver
- noise levels from the premises under calm
conditions as well as any significant weather conditions as defined in the INP
should be predicted or measured
- the highest of the noise levels from Step 1 is to
be used in the assessment for that receiver
The intent of the INP is not to require that these conditions should be
applied exclusively where the significant weather conditions act to reduce
noise at a sensitive receiver.
For example, where a significant prevailing wind of speed less than
three metres per second increases noise levels at a receiver to the north of a
development (compared with those predicted under calm conditions), the noise
levels predicted under that prevailing wind should be used at that receiver.
For receiver(s) to the south of the same development, if the noise levels
predicted under calm wind conditions are higher than those predicted under the
significant prevailing wind, the noise levels predicted under calm wind
conditions should be used at the southern receiver(s).
The EPA has previously accepted (and will accept) noise predictions
based on modelling noise emissions using long term weather data, as it can
present a higher level of analysis than that required under the INP.
How calm is defined
See INP Section 5.1
In the assessment of wind effects, the INP requires the assessment of
wind speeds of up to 3 metres per second where these speeds are a feature of
the area (they occur for 30% of the time or more) but does not specify the
minimum wind speed that needs to be assessed. The calm condition is typically
represented by wind speeds less than or equal to 0.5 metres per second as this
is likely to be the lower limit of measurement.
Presenting predicted noise impacts
See INP Section 6.3
In carrying out noise impact predictions for a particular development,
predicted noise levels for calm conditions as well as any significant adverse
weather conditions should generally be provided. It is particularly useful to
provide predicted noise impacts for calm weather conditions where predicted
noise impacts under adverse weather conditions exceed the project-specific
noise levels. This allows for a better understanding of potential noise impacts
from the development.