dense fog; heavy fog; thick fog
A weather and traffic-report term for fog thick enough to greatly reduce visibility.
濃霧のため、列車が遅れた。
The train was delayed because of dense fog.
Dense fog occurred around the airport.
空港は濃霧で一時閉鎖された。
The airport was temporarily closed due to heavy fog.
The flight was cancelled due to dense fog.
The whole town was enveloped in dense fog.
Visibility is poor due to dense fog, so caution is needed when driving.
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.