Windward and Leeward Thermals
An extract from ‘Thermal Flying For Paraglider and Hang Glider Pilots’ by Burkhard Martens
W
hen flying in the mountains it is important to know where the wind is coming from, particularly when using thermals.
Normally we fly on the windward side, and if there’s no wind we aim for the sunny slopes. On no-wind days the thermals are easy to centre; they don’t get torn apart by the wind and they don’t drift Windward side thermals are almost equally nice as long as the wind remains weak Lee thermals are generally turbulent and thus best avoided Leeward thermals in the Alps are either on the
Illustration 3.19 Thermal formation in calm air
16 A i r b o r n
3.20 Lee thermal development in Pustertal, Sillian, Austria. We recognise the situation by the big clouds being pushed into the valley from their point of origin on the higher slopes. The day was flyable, as the thermals coming up the face of the mountain were strong enough to override the wind from behind. The launch thus had wind coming up it, but it was turbulent and difficult flying. south side of ridges near the northern boundary or in the leeward side of local valley wind systems. In the southern Alps the wind is generally local and south, since the mountains suck in so much air from the flatlands that the wind on the sunny south-facing slopes is almost always on. This means nice windward-side thermals! In strong winds lee flying must be avoided altogether. It can be extremely turbulent and dangerous. Standing on launch it is not always clear if the wind we’re feeling coming up the face is the ‘real’ macro-meteorological wind or just a thermal
Thermal coming off the windward side
EXPERIENCE Once while flying the Laber near Oberammergau in Germany I went into the well-known lee on the south side of the ridge. I was descending with a steady -9m/s and thinking I would surely have to land at the Ettal monastery when I hit the thermal coming off this monastery – from there I climbed out with a steady 6m/s thermal! This means that I went from –9m/s to +6m/s within a very short horizontal distance – a difference of 15m/s (3,000ft/min) or 54km/h! Since then I have always avoided getting low south of the Laber!
Thermal coming off the leeward side
