Mountain and valley wind circulation
The mountain and valley wind circulation is a diurnal local wind system that occurs in the mountains or hill country on days of fine weather when there is little supraregional air flow and thus a typical phenomenon for mountain climates . The valley wind that blows upwards during the day is driven by the stronger warming of the slopes as a result of solar radiation compared to the open atmosphere. The engine of the mountain wind blowing downwards from late evening to morning is the stronger radiation and cooling of the air over the plateaus and slopes compared to the valley . The mountain and valley wind circulation is similar to the daily cycle of land and sea wind , but is more complicated because of the diversity of the relief and the overlap with the up and down winds .
Valley wind
On a sunlit mountainside sets after sunrise a strong heating of the bottom one, whereby the heated air near the ground faster than the hang farther air. Like all gases, air can practically not be heated by radiant heat, in contrast to the ground. The air layers close to the ground are heated from the ground by conduction . Due to the reduction in air density ( thermal lift), an upward wind sets in in the morning . Its maximum speed is usually 2 to 3 km / h. The rising air cools down over the mountain ridge and sinks back down over the original or a neighboring valley or over the foreland. In order to replace the air masses that have risen, a balancing current gradually sets in from the foreland up the valleys, the valley wind. During the afternoon this wind outstrips the slope updraft at around 20 km / h. At lunchtime, depending on the weather ( layer stability and humidity ), cumulus clouds can form over the mountain slopes . Above the valleys, however, the sky remains cloudless because of the downward tendency of the air. The valley wind can be increased or decreased by supra-regional wind as a result of the pressure equalization between high and low pressure areas.
Mountain wind
After sunset, slope and valley winds slowly come to a standstill. After a short standstill, the wind system is reversed. The air close to the ground above the plateaus and the mountain slopes cools down faster than the air above the valley at a comparable height due to the stronger radiation of the ground. The now cooler, denser and therefore heavier air flows down the slopes and provides fresh air on the valley floor. Usually the nocturnal mountain wind is weaker than the valley wind during the day. As a cold air lake forms in the valley floor at night , you hardly feel the mountain wind sweeping over it. In the middle of the valley, a reverse thermal can form and the flow convergence occasionally causes the air to rise to the level of condensation and form clouds, in this case mostly smaller, flat cumulus clouds ( stratocumuli ).
Individual evidence
- ↑ The Brockhaus. Weather and climate . Page 39, Brockhaus, Leipzig / Mannheim 2009, ISBN 978-3-7653-3381-1
- ↑ a b c Berthold Wiedersich: Pocket Atlas Weather. 2nd edition, page 233f, Klett-Perthes Verlag, Gotha 2003, ISBN 3-12-828122-X
- ↑ Weather service: mountain and valley wind