Another possible reduction, one that will remain a matter of subjective judgment (Van Wagner 1972b), is for incomplete combustion. It is quite variable and very difficult to measure. Forest fires produce smoke which consists mainly of unburnt tars and carbon (Luke and McArthur 1978). Smoke color provides a good visible indication of the completeness of combustion. Lowvigor surface fires are typically more efficient than crown fires (i.e., emitted smoke is lighter in color).