When dump air near the ground cools enough for the water vapour to condense into droplets of water, then fog and mist occur. Because fog is cloud at ground level the reason for the formation of fog and cloud is the same. There are several reasons for air near the ground to cool. If the surface of the Earth is cold, it will cool the air above it. If the air above a cold surface is almost still, then as it cools .the water vapour will condense and will lead to the formation of fog. Sea fogs are common at San Francisco, in California, because there is a cold sea current near the coast. Cold air is heavy and tends to sink. So fog may form on the cold tops of hills and mountains, and roll down into the valleys.
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