The Foehn wind
The foehn wind is a downslope wind at the lee side of a mountain that is accelerated, warmed and dried as a result of the orographic disturbance on the prevailing flow (Elvidge, 2016). There are 3 possible mechanisms for foehn warming and drying: the isentropic drawdown (fig 2.1b), the "thermodynamical" foehn (fig. 2.1a) and the mechanical mixing (fig. 2.1c) (Elvidge, 2016). The radiative mixing (fig. 2.1d) is not explicitly considered as foehn mechanism, but is often associated with the previous ones. However, the causes of foehn warming and drying are not fully understood.
Figure 2.1: Foehn warming mechanisms. From (Elvidge, 2016)
In Madeira, the most probable dominant mechanism for the foehn event promoting the ignition and the rapid spread of wildfires is the isentropic drawdown. This is here analysed in more detail. However, the foehn warming can be the result of more than one mechanism at the same time, depending on larger‐scale meteorological conditions and orographically forced flow dynamics.