The train was delayed because of dense fog.
The airport was temporarily closed due to heavy fog.
Dense fog occurred around the airport.
The flight was cancelled due to dense fog.
The whole town was enveloped in dense fog.
もや describes a light haze or thin mist, whereas 濃霧 is much denser and implies dangerous visibility conditions.
A Sino-Japanese compound written with 濃, meaning dense or thick, and 霧, meaning fog; the spelling transparently matches the meaning.
Visibility is poor due to dense fog, so caution is needed when driving.