Here a typical example of a low stratus cloud is shown. This advective fog arises at the coast of San Francisco, when an air parcel flows horizontally over a cold surface and cools down until it reaches its dew point. Here the cooling surface is the sea near the coast. The main wind direction changes at the north-Californian coast around February/March, so that the winds come now from north-west. Due to the Coriolis-force the surface water which is driven by those winds is directed to the west. Thus cold deep-sea water with high nutrient content is "pumped" to the surface. This effect which endures until September is supported by trenches in the deep-sea. Close to the coast they reach up to the surface and thereby act like channels for the up-welling water.

This process occurs pulse-like with changing intensity. Especially in summer time the winds from north-west transport humid air into the region. As a result of the coincidence of humid air and cold surface water (around 12 to 16 deg C) fog banks are formed in front of the coast.

The land-sea-breeze-circulation moves the fog banks in direction to the coast. Thereby they pass the Golden Gate Bridge (GoldenGate1.jpg and GoldenGate2.jpg) and the coastal mountains (Mountain1.jpg and Mountain2.jpg). For the observer it looks as if the fog flows over mountains and valleys like water.

A temperature inversion closely to the surface causes a low-level upper fog limit. In the morning the fog can often still evaporate, but during the afternoon the air is heated over land and the ascending air pulls the low-level fog into the cities at the coast. This kind of fog is characteristic for the summer in north California. In September, however the general wind conditions change again and so the upside flow - also known as "coastal upwelling" - is reduced again.

Biological Implications

The cold deep-sea water is rich of nutrients. Getting more solar radiation from June on the phytoplankton starts to "bloom".

This results in an increase of the number density of the organisms by four orders of magnitude. This again drives a complex food chain and supports the reproduction of a lot of fish species. During this time the plants of the famous "Kelp"-forests, also known as seaweed of the type of macrocystis, grow up to 25 cm per day. Therefore the marine ecosystem in north California is characterized by a large biodiversity including sea otters, sea lions, big prawns, a variety of fishes and mussels.

J. Engel, S. Borrmann, San Francisco, September 1991