From AMS Glossary
(Also called altanus.) A strong southeast wind in south-central France, especially in Gascony and the upper Garonne River.
Near the Pyrenees the autan is very turbulent, growing in strength in the valleys. At Toulouse its average speed is 13 m s-1 (30 mph) with gusts of 20–22 m s-1 (45–50 mph); it tends to be strongest at midday. It increases in speed up to a height of 450 m (1500 ft), above which it weakens and veers to the south. North of Toulouse it loses its special character and becomes an ordinary southeast wind. There are two types. 1) The autan blanc brings fine dry weather, cold in winter, hot in summer, as a result of the downslope motion imposed by the Pyrenees and southern Cevennes. It occurs with an anticyclone centered near Denmark or moving northeastward from the Azores. It lasts for two to four days in winter, but may persist for more than a week in summer, bringing severe drought and desiccating the vegetation; in Catalonia (northeastern Spain) a similar wind is called the outo. 2) The autan noir is less frequent and rarely lasts for more than two days; it is more humid and cloudy, bringing fog, rain, or snow over high ground near the sea. As such, it is more like the marin, the name applied to the southeast wind out of the Cevennes where its maritime character predominates.