This parameter is the product of mixing depth and the average wind speed within that volume. In practice we find that values less than 6000m²/s indicate a high pollution potential.

In working out the ventilation coefficient we cannot use wind speeds measured at the standard 10m anemometer height because wind speeds are generally higher at altitude. This is because of frictional losses with the ground. In many calculations involving air pollution it is necessary to be able to predict wind speeds at various altitudes. This certainly applies to later work in this unit which will involve you constructing a simple computer model on a spreadsheet.

u₁ and u₂ are wind speed at higher and lower elevation , respectively.

Z₁ and Z₂ are higher and lower elevation respectively.

And

p is a dimensionless parameter that varies with atmospheric stability and with the terrain. Some workers suggest that at heights above 200m we use the wind speed at 200m.