Fog Stability Index
This is a stability index for fog formation. It is from 2WW/TN-79/008, "A New Technique for Forecasting the Occurence of Fog and Low Stratus Ceiling by Use of a Flow Chart".
The equation for the index is:
Fog Stability Index = 2*(ts - t850 + ts - tds ) + W850 =
=4ts - 2(t850 + tds) + W850
Fog risk (FSI)
high
middle
low
FSI < 31
31 < FSI < 55
FSI > 55