A considerable proportion of föhn days are not accompanied by any precipitation south of the Alpine chain, so that the thermodynamic föhn theory does not explain the warm air of the Alpine föhn. The föhn phenomenon on the northern side of the Alps can, however, be explained by the fact that the air which is detectable as a föhn in the northern Alpine valleys does not come from the southern foot of the Alps, but from higher up; the windward air beneath it forms a layer of stable air and is prevented from crossing the barrier. Through the deeply incised Alpine passes, some of this relatively cool, trapped windward air reaches the north as a moderate föhn.