What can cause light mist in a river valley on a cold morning?

Correct answer: Moisture from the river and pooling of cold air in the valley

Answer: A, moisture from the river together with pooling of cold air in the valley. A river or wet valley can supply moisture to the nearby air. During a cold morning, dense cold air may settle in the low valley, and contact with the cooled surroundings can lower the air temperature toward saturation. Some water vapour then condenses into tiny suspended droplets, producing mist. Option A is correct because it gives both the moisture source and the cooling effect of the relief. Option B is wrong because absence of river water would remove an important local moisture source. Option C is wrong because hot, dry air generally reduces the likelihood of saturation and mist. Option D is wrong because a river does not need to become a sea for mist to form. Memory cue: for valley mist, look for “moisture plus cold-air pooling.”