2. Worldwide Marine Fog Occurrence and Climatology

Herein, an analysis is presented of the world’s marine fog distribution based upon the International Comprehensive Ocean-atmosphere Data Set (ICOADS) ship observations taken during 1950–2007. Fog, shallow fog, and mist are taken from routine weather reports that are encoded in an ICOADS ship observation with the “present weather” code. Occurrence is estimated by the number of observations of a type divided by the total present weather observations in a one-degree area. The bulk of the observations are in the northern temperate and tropical oceans, with decreasing numbers south of 20 °S and large data voids in the polar oceans. Marine fog is infrequent over most of the world’s oceans with the median occurrence 0.2 % while it is in isolated maxima for values larger than about 2 %. In a specific location, either fog or mist are the most frequent, followed with an order of magnitude lower occurrence by shallow fog.

The two major open ocean fog maxima in the world occur on the northwestern side of northern hemisphere oceans during the summer under atmospheric subsidence over a cold polar current. The distribution of the center of the maximum and highest values are over shallow water and follow the shape of the shallow bathymetry. For the highest occurrences, surface air is preconditioned by warming over a western boundary current followed by cooling over a negative SST gradient and stable lower atmosphere suppressing boundary layer exchange with the air above. The horizontal fog structure is set by surface ocean currents, sea surface temperature gradients and seasonal wind direction. Marine fog’s most frequent occurrence and largest areal coverage is in the NW Pacific in June–July–August, reaching its peak value of 59.8 % over the Kuril Islands on the western side of the Ohyashio current. The second largest marine fog maximum occurrence is in the NW Atlantic in June–July–August, reaching 45.0 % over the Grand Banks and the Labrador Current. The eastward extent of both of the NW ocean maxima is determined by the sub-polar ocean gyre.

Wind driven coastal ocean upwelling regions have a narrow zone of fog located against the coast, over the inner shelf and over the sea surface temperature minimum along the coast. A mist maximum occurs in a broader area beyond the temperature minimum. The lowest fog occurrence is in the cold season and the highest is in the warm season for all five areas except SW Africa which has its maximum in March–April–May. SW Africa has the highest single grid point fog occurrence and its upwelling, which lasts all year, and fog maximum, are both divided into two, separate areas. California–Oregon has the greatest along coast extent of fog occurrence and SST minimum as well at the lowest SST minimum. NW Africa, and Peru have significantly less fog occurrence, a shorter extent along coast of fog and a higher minimum SST. For all of the wind driven coastal upwelling zones, the Arabian Peninsula has the least fog occurrence, the shortest along coast extent as well as the highest SST minimum.

Significant fog and mist occurs at mid-latitudes in marginal seas and along the western side of northern hemisphere oceans. Over the NW Pacific, fog occurrence average of the 5 highest grid point values in the Sea of Okhotsk is 51 % in June–July–August, in the Japan Sea it is 27 % in June–July–August and the Yellow Sea it nears 15 % during March–April–May and June–July–August. The greatest fog and mist occurs along the southern China coast in December–January–March and March–April–May when the average of the 5 highest values are between 4 % and 6 %. On the NW Atlantic along the NE United States and Canada fog is most prevalent in June–July–August and least in December–January–February. During June–July–August, an elevated fog occurrence over the shelf extends along the coast from Cape Cod to SW Labrador that includes a maximum centered off the SE tip of Nova Scotia where the average of the five highest fog grid point occurrences is 41 %. The Nova Scotia maximum center is separate from that over the Grand Banks.

On the NE Atlantic appreciable fog and mist occurs around the N. European coastline in all seasons. In the North Sea, the average of the fog 5 highest grid point occurrences is greatest in March–April–May (8 %) and least in September–October–November (4 %). For the Baltic Sea, the average of the five highest fog grid point occurrences is most in March–April–May (15 %), and least is in September–October–November (6 %).

The polar seas have their greatest fog and mist occurrences during the warm season and the least during the cold season. The transitional seasons appear to have intermediate fog and mist values around the periphery while the interior is largely unsampled. Observations are mostly limited to the warm season, distributed unevenly and with vast areal data voids.

There are significant fog occurrence climate trend increases tested at the 0.05 significance level for June–July–August based upon the 1950–2007 record in three areas with high numbers of ship observations. The open ocean Kuril Island maximum occurrence in NW Pacific increased by 15.8 % and the Grand Banks maximum in the NW Atlantic increased by 12.8 %. The sea surface temperature (SST) over the same area and same period also increased which is consistent with published SST increases in the adjacent western boundary currents in both oceans. The third case is the increase of 7.4 % of the fog occurrence maximum along the California–Oregon coast over wind driven upwelling water. In contrast to the NW Ocean maximums, this coastal fog maximum is associated with a long term SST decrease.