In 1998 a field campaign has been conducted in the north-south oriented Kali Gandaki valley in Nepal to explore the structure of its extreme valley wind system, where strong upvalley winds blow through kind of a gap towards a wide elevated basin. Pibal observations were made to map these upvalley winds as well as the weak nocturnal flows. Moreover numerical simulations were made which successfully simulated most of the wind observations. The model results suggest that the vigorous upvalley winds can be seen as supercritical flow induced by contractions of the valley. Here, the results of a second campaign are reported where remotely piloted airplanes were used to obtain vertical profiles of temerature and humidity up to heights of ~ 2000 m above the ground. This technique is novel in some respects and turned out to be highly reliable even under extreme conditions. In addition four automatic stations were installed along the valley's axis. Winds were observed via pibal ascents. These data complement the wind data of 1998 so that the diural wind system of the Kali Gandaki valley is now documented reasonable well. Moreover, a validation of the model results is possible on the basis of these data.

It is found that the fully developed upvalley flow is confined to a turbulent layer which tends to be neutrally stratified throughout the domain of observations. The stratification above this layer is stable. A capping inversion is encountered occasionally. This finding excludes explanations of the strong winds in terms of hydraulic theories which rely on the presence of strong inversions. Pairs of simultaneous ascents separated by 5 - 10 km along the valley axis reveal a remarkable variability induced by the topography and, perhaps, by an instability of the flow. The analysis of the surface data as well as that of the soundings shows that the flow above the neutral layer affects the surface pressure distribution and, therefore, the acceleration of the extreme upvalley winds.