Strong, cold, northerly wind that blows offshore with great frequency along the Mediterranean coast from northern Spain to northern Italy, and that is particularly frequent in the lower Rhône valley. The wind may persist for several days, and is best developed when a depression is forming in the Gulf of Genoa to the east of a ridge of high pressure. The airstream that feeds the mistral is commonly derived from polar air of maritime origin. In the Rhône valley and similar areas of occurrence, the airflow is strengthened by katabatic and funnelling effects producing speeds of up to 75 knots (139 km/h), compared with the typical 40 knots (74 km/h) experienced along the coast.