Fog forms on or near the ground. Fog often forms when the ground cools at night after a warm, humid day. The ground cools the air just above the ground to the air's dew point. The next day the heat of the morning sun "burns" the fog off as its water droplets evaporate. Fog is more common in areas near bodies of water or low-lying marshy areas. In mountainous areas, fog can form as warm, moist air moves up the mountain slopes and cools.