What is a Smoke Control Forecast?
The SMOKE CONTROL FORECAST is designed to predict the capability
of the atmosphere to effectively disperse pollutants such as small
particulate matter (smoke).
Smoke Dispersion Factors
Two factors control the spread or dispersion of small particles
or gases. These factors are the wind speed (controls horizontal spread)
and the height of the "mixed layer". The mixed layer is the
portion of the atmosphere form the ground up to the level at which gases
and small particles freely mix. For example, smoke from a smokestack
will tend to rise to the top of the mixed layer and then level off.
Ventilation Index
The ventilation index is formed by multiplying the mixed layer
height by the average wind speed in this mixed layer. Stronger
wind speeds and thicker mixed layers will produce higher
ventilation index values. For convenience, the actual numbers
are converted to a scale of 0 to 100.
A ventilation index of '0' implies no ability of
the atmosphere to disperse pollutants (smoke) while a value
of '100' implies an excellent ability to disperse pollutants
(smoke)
The Environment Canada ventilation index in British Columbia
and Yukon is divided into the following categories;
0 - 33 - POOR
34 - 54 - FAIR
55 - 100 - GOOD
For most locations, ventilation index values are poor from sunset
until late morning
For locations within valleys, the ventilation index should be
lowered if the mixing
height is less than the height of the surrounding hills.
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