Why does fog cause tripping of Transmission lines?

When a fog occurs, the air humidity is approaching 100% and condensation will tend to occur on surfaces exposed to the fog. The condensation will also occur on the insulators which are supporting the transmission lines, leading in some cases to voltage breakdown and tripping of circuit breakers. This happens despite the fact that typical insulators have an elaborate system of “skirts” to both increase the surface path length along which a leakage current might flow, and to keep portions of this surface area relatively dry, so as to minimize surface leakage currents.

Two conductors paired as transmission lines are at sufficient distance from each other to prevent arcing between them. Such distance can become ineffective when fog increase
Due to the Fog, moisture content between the two transmission conductors increases which provides the low resistance current flow path and hence conductivity increases between lines. That’s why flash-over occurs between the transmission lines and large current flow them and tripping occurs.

Two reasons. Water has a much higher dielectric constant than air. AC power lines are shorted by the excess capacitive leakage caused by the moisture. The second cause is salt collecting on lines in costal areas. the salt when wet also shorts the lines and causes excess current flow.
Actually in fog. There is moisture, and causing insulator to conduct. And tripping occurs.