Empirical relationships between extinction coefficient and visibility in fog.

Relationships between visibility and an extinction coefficient that is due to fog in optical windows that are free from molecular absorption are derived. The extinction coefficients in the visible (0.55 microm), the near IR (1.2 microm), and the mid IR (3.7 microm) are comparable to and roughly twice as much as that in the far IR (10.6 microm) when visibility is less than a few hundred meters. The advantage of far-IR radiation compared with shorter wavelengths grows as visibility exceeds 500 m. Correspondingly, the relationship between extinction coefficient and visibility becomes more sensitive to variations in the particle-size distribution of fog.