Understanding and Forecasting Alpine Foehn

This chapter focuses on the history, physics, climatology, forecasting and the broad effects of Alpine foehn on human populations. In the European Alps, foehn winds have been studied since the mid-1800s. The main focus of the investigations was the question of why foehn winds are so warm. While it soon became clear that adiabatic processes provide an explanation, the role of wet adiabatic rising on the upwind side of the Alps continued to be strongly debated. The so-called textbook theory for foehn – heat gain by wet adiabatic, forced lifting on the upwind side followed by dry adiabatic descent in the lee – represents only an extreme situation. Foehn occurs also with partial or complete blocking of the upwind air mass, i.e., with limited or no heat gain by wet adiabatic expansion. The second focus is on processes which lead to descending air masses after passing the mountain ridge. A discussion of the most important processes shows that there seems to be no theory which is applicable in all situations. Forecasting foehn is still a challenge to meteorologists. While the general foehn situation can be predicted reliably, today’s numerical models still often poorly simulate the sudden break in of potentially devastating foehn air in the lee. Efforts to improve this must continue because foehn storms have a significant societal impact (threat to transportation systems and massive increase of fire danger) as several recent incidents show.