Mixing height is the depth of the layer above the surface within which air is being turbulently stirred, typically shallow at dawn and growing through the day as the sun heats the ground; transport wind is the average wind speed and direction through that layer. Together they describe the volume of air available to a fire and where its smoke will be carried. A high mixing height with a strong transport wind ventilates a fire, feeds it oxygen and pushes smoke downwind; a low mixing height traps smoke near the ground and holds fire behaviour down. Forecasters combine the two into a ventilation index, the product of mixing height and transport wind, which is issued routinely for smoke management and prescribed-burn windows. The confusion this resolves is equating transport wind with surface wind: transport wind is a layer average and is usually considerably stronger. After this Concept you can read a mixing height and transport wind and say how well a fire's smoke and heat will be ventilated.
Mixing height — the vertical extent to which surface-heated air parcels can rise through the atmosphere — is governed by the dry adiabatic lapse rate (the rate, 5.5°F per 1000 feet, at which unsatura…