The Tomorrow.io REST Weather API
fog insight category will trigger for any of the events below.
Dense Fog (Advisory)
Issued when widespread fog is expected to reduce visibilities to 1/2 mile or less over a large area for an extended period of time (2 or more hours).
Freezing Fog (Advisory)
Issued when fog is expected to reduce visibilities 1/2 mile or less, freezing on cold objects with deposits resulting in an impact to commerce, transportation, people, etc.