Slope wind
As Hangwind a locally influenced is wind referred whose direction by a mountainside is deflected up or down, or by sunlight results. Depending on the sun, regional wind strength and the shape of the slope, the vertical component of slope winds can reach several meters per second. The resulting updrafts are used to gain altitude during sailing , paragliding and hang-gliding flights.
Two types of creation
A slope wind can arise in two main ways:
- Due to a constantly blowing regional wind, which leads to an ascending air movement on the windward side of the mountain slope, but behind the mountain ridge (on the lee side ) to a sloping downdraft . The updrafts sometimes reach twice as high as the obstacle. If the humidity is sufficient, stationary clouds can form over the ridge , which dissolve again on the leeward side.
- When the sun is shining and the wind is low (“calm, radiant weather ”), it is caused by solar radiation . It warms the mountain slope and the air close to the ground, so that it rises along the slope. If the air flowing in from the valley is humid, clouds can form near the summit in the morning.
Conversely, after sunset there is a downwind because the air near the slope cools down more strongly than the open air at the same height due to the nocturnal radiation. The cooler soil air flows down the slope because of its greater density. In the mountains, the slope winds are part of the mountain and valley wind circulation .
Use in gliding, hang gliding or paragliding
For a flight without a motor, slope updrafts are the most important source of propulsion in addition to thermals . A special form of slope wind are the lee waves that occur occasionally at great heights on the leeward side of a mountain range , which are particularly important for long-distance and triangular flights .
In contrast to the locally limited thermal updraft, which can only be used by the glider making tight circles, a slope wind can also be "taken along" in cross-country flights. Since a high-performance glider has a sinking speed of only about 0.5–1 m per second when gliding , even a relatively weak upwind can lead to a noticeable gain in altitude if the flight route runs parallel to the slope on the sun or wind side. In the case of thermals, the most favorable route is often indicated by a series of small, fair-weather clouds lined up on the slope , because the forced ascent of the air goes hand in hand with its adiabatic cooling and promotes cloud formation.
Beyond the ridge , the upward wind turns into a downdraft for aerodynamic reasons , which, however, can also occur over larger bumps in the uphill slope .
history
While a thermal uphill wind (2.) can occur on any sunny slope, the aerodynamic slope winds (1.) are particularly pronounced on north-south mountain ranges . Along the Appalachian Mountains , the farmer Karl Striedieck was able to observe for years how the eagles whizzed south from Canada in autumn and vice versa along the mountain slopes without flapping their wings. Using the same principle - rapid gliding in a slope wind - on May 9, 1977 he succeeded for the first time in a motorless record flight of 1000 miles (over 1800 km).
Otto Lilienthal already made use of the slope wind in 1893, which blew against his practice hill “Fliegeberg” in Berlin. As a result, he was finally able to expand his air jumps to over 300 m and briefly fly higher than his take-off position - which, according to today's definition, distinguishes gliding from gliding.
DFS Hangwind was the name of a glider training aircraft developed in Germany in 1927 , a glider for training beginners. What is interesting about this construction is the forked tail - and that the wingspan (12 m) is twice the length (6.5 m).
See also
literature
- Jochen von Kalckreuth : Sailing over the Alps. Experience and technique of high mountain flight . Motorbuch-Verlag, Stuttgart 1972, ISBN 3-7168-1254-4 .
- Peter Riedel: From slope winds to thermals. Experienced Rhön history 1927–1932 . Motorbuch-Verlag, Stuttgart 1984, ISBN 3-87943-981-8 .