Ocean currents may be divided into (a) those due to the wind, (b) those due to the distribution of water masses of varying temperatures, salinity and density and (c) tidal currents. Currents flow at all depths in all the oceans but mariners and shipbrokers are usually only interested in surface currents and their effect on ships. The main cause of currents in the open ocean is the direction of winds blowing on the sea surface. Warm currents flowing from the tropics, such as the Gulf Stream, not only ensure a temperate climate for the British Isles but even keep ports as far north as Narvik in Norway ice free all the year round. Conversely, currents from the poles such as the one sweeping down to the North of Japan result in severe winters on the Island of Hokkaido; even though it is several degrees of latitude further south than England. Sea fogs occur where warm moist winds blow over cold currents, lowering the temperature and causing condensation. One of the most famous areas for Sea Fog is off the coast of Newfoundland, where meets the warm North Atlantic drift and the cold Labrador Current (maximum frequency of the phenomena, May to September). Similarly this phenomena occurs off the coast of California (generally between June- December), in the Bering Sea (between June-August), the Baltic regions (between November-January) and the Hudson Bay (between June-September). Cold currents from the Polar Regions also carry one great hazard to shipping during summer thaws icebergs that have broken away from ice caps and glaciers and are carried away by currents. Icebergs carried by the Labrador Current are found around the Newfoundland area prior to breaking up when they meet warmer seas. The major hazard of an iceberg to ships is that seven-eighths is submerged and sometimes, because of irregularity of the iceberg, they have a tendency to overturn. Also a combination of sea fog and icebergs, such as is found off Newfoundland, makes navigation extremely difficult and only relatively recently has new technology helped Masters of vessels to detect such risks.