noun
BAD : Her flight was held up on account of the mist.
GOOD : Her flight was held up on account of the fog.
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mist = tiny drops of water in the air that make it difficult to see things that are far away: 'As the day wore on, the early morning mist quickly disappeared.'
fog = very thick mist: 'Traffic on several stretches of the M1 was slowed to a walking pace this morning as the result of thick fog.'