The Heat Source of the Foehn

Twelve years of upper air data and surface observations across Iceland are used to investigate the connection between the difference of surface air temperature upstream and downstream of the Icelandic highlands and several parameters of the airflow, such as precipitation, static stability and wind speed. The data does not support the well-known concept of the heat of the foehn being a result of latent heating. In the winter, warm air, descending from above the upstream boundary layer appears to be responsible for the downslope heating. There is little correlation between the upstream wind speed and the upstream-downstream temperature difference. This is explained by weak winds contributing to low level upstream blocking and descent from upper levels in the lee, while strong winds contribute directly to vertical mixing through mechanical turbulence. The annual cycle of temperature difference between the upstream and the downstream slopes indicates that in the summer, solar heating over the downstream slopes is responsible for a substantial part of the heating of the foehn.