Fog — when the cloud settles on the ground

What is fog?

Fog is defined as condensed water droplets in the air near the ground that reduce visibility to below 1,000 metres. Between 1,000 and 5,000 metres visibility is called mist. Fog forms when air cools to the dew point near the surface and water vapour condenses around dust particles and other microparticles in the air.

Radiation fog

Radiation fog is the most common type in inland areas and valleys. It forms on clear, calm nights when the ground cools rapidly by radiating heat to the cold night sky. The air closest to the ground cools to the dew point and fog forms. This fog typically evaporates during the morning as the sun warms the ground again — hence the expression "morning mist".

Advection fog

Advection fog occurs when warm, moist air moves over a cold surface. This is particularly common along the Nordmøre coast in spring and early summer, when warm air from the south meets the cold Atlantic Ocean. This type of fog can be thick and long-lasting, and is a well-known challenge for shipping and aviation.

Fjord fog

In Norwegian fjords, a distinctive phenomenon occurs: fjord fog. The fjord water is often colder than the air in summer, and warm moist air condenses over the water surface. From the mountainside, the fjord appears to be filled with white cotton. Viewed from below, one can find oneself in thick fog while the sun shines just above.

Sea fog

Along the coast of Møre og Romsdal, sea fog can occur when warm, moist sea air meets cooler water temperatures. This fog can arrive suddenly and reduce visibility to just a few tens of metres. For boaters and fishermen, sea fog is a serious safety challenge.

Fog and traffic

Fog is one of the most dangerous weather conditions for road users. In fog, you should reduce speed significantly and increase following distance. Use front and rear fog lights where permitted. The Norwegian Meteorological Institute issues yellow warnings when visibility drops below 200 metres over major road sections.