The FOGcast map shows where areas of widespread fog are expected for the current day. Fog is defined as a cloud in contact with the ground that reduces visibility to less than 1km. Fog forms when the difference between the air temperature and dewpoint is 5°F (3°C), or less. Fog is a cloud in contact with the ground. Fog differs from other clouds only in that fog touches the surface of the Earth. The same cloud that is not fog on lower ground may be fog where it contacts higher ground such as hilltops or mountain ridges. Fog is distinct from mist only in its density. Fog is defined as cloud which reduces visibility to less than 1 km, whereas mist is that which reduces visibility to less than 2 km. The current fog forecast is updated every 24 hours.