We find occasional descriptions—some of them from personal observation, no doubt—of winds peculiar to particular parts of the world. Gervase of Tilbury, as we have seen, tells of very violent blasts in the Rhone valley,[784] supposedly generated by the current of the river in a region now famed for the furious _mistral_ that sweeps across Dauphiny and Provence from the north. In another connection[785] he tells of a valley in the Kingdom of Arles, once so shut in by precipitous mountains that no winds at all entered it and that it consequently was sterile and useless. In the time of Charlemagne, however, Caesarius, the archbishop of Arles, filled his glove with sea breezes and let them forth in the valley; thus originated a wind known as _pontianum_, which wrought an immediate change in the character of the place and caused it henceforth to become fertile and healthy. This wind was doubtless the breeze now called _pontias_ that blows at Nyons in the Department of the Drôme; but as to its miraculous origin Gervase is merely repeating one of many popular medieval stories.[786] William of Tyre[787] describes in vivid terms the _simoom_ of the Arabian desert and how men have to lie flat on the ground at the time of its passing: equal to a storm at sea, it sweeps down upon the traveler waves of sand as huge as those of the sea and causes grave danger to persons who would cross the desert. _CLIMATOLOGY_ The most important factor in determining the atmospheric climate of any given region is the amount of sunlight and heat received. This, in turn, depends largely on geographical latitude. As we have already discussed the broad climatic divisions of the earth’s surface in zones, it remains here for us to deal merely with what was known of climatic conditions within the _oikoumene_. HOT AND COLD CLIMATES Climatic differences between northerly and southerly latitudes were well understood. Classical writers had told of the coldness of the regions beyond Thule, and in the _De imagine mundi_[788] we read that in those parts the sea is frozen and perpetual cold prevails. An interpolation into Solinus’ _Collectanea rerum memorabilium_ dating perhaps from our period contains a vivid and possibly exaggerated description of the cold of Iceland: “These people also are good Christians, but in winter they dare not leave their underground holes on account of the terrible cold. For if they go out they are smitten by such terrible cold that they lose their color like lepers and swell up. If by chance they blow their nose, it comes off and they throw it away” (Nansen’s translation).[789] Giraldus Cambrensis praises the temperate climate of Ireland, placed between the torrid warmth of Spain and the rigors of Iceland;[790] and the chroniclers and historians of the Crusades give evidence of first-hand knowledge of the terrific summer heats in the Holy Land.[791] Ambroise says, for example: