How gusty the wind

Gusts and their formation

Lately it has been mostly windy or stormy in Germany. Extreme hurricane gusts (> 140 km / h) were repeatedly measured on the Brocken in the Harz Mountains last Wednesday (November 18), between 5 and 6 a.m. CET it was even a maximum of 170.3 km / h. But also in other regions on Wednesday night there were widespread gusts of wind or heavy gusts of wind, and gale-force gusts or gusts of gale force in some peaks.

In #Erfurt 107 km / h, on the #Brocken 170 km / h !! #HEINI took care of and ensures powerful #wind. / V

- DWD (@DWD_presse) November 18, 2015

Gusts or gusts are short-term fluctuations in wind strength and direction. The tricky thing is that gusts can sometimes be very surprising, even if there was almost no wind a short time before. Usually it only takes a few seconds until the "spook" is over again.

According to the definition, one speaks of a gust (by the way, taken from the word painting Middle Dutch word "bui" in the 17th century) if the measured ten-minute mean value of the wind speed increases within a few seconds (at most 20, lasting at least three seconds) by at least 5 m / s (18 km / h) is exceeded. If no gusts occur, one speaks of a so-called "laminar" flow and the air particles move in an orderly manner. In a wind tunnel, this laminar flow can be recognized by the parallel course of the flow lines made visible, for example, with smoke or fog.

However, if air hits an obstacle placed in the wind tunnel, it swirls behind it. It then flows completely chaotically at different speeds in different directions. This type of turbulence can also be observed in nature when the air flow is disturbed by obstacles such as houses, trees, hills or mountains. In the process, eddies with a horizontal axis of rotation are formed. These eddies vary depending on the nature of the obstacles - the so-called roughness. The gusts that result from it are correspondingly different.

However, obstacles do not necessarily have to have a large vertical extent. The air flow close to the ground is already slowed down by the nature of the earth's surface due to friction. Above this, however, the air flow is higher due to the lower friction, which creates eddies - the flow becomes turbulent.

But gusts can also have thermal causes: especially in the summer half-year, the ground and, as a result, the air close to the ground, heats up quite markedly due to the solar radiation. The heated air rises. To compensate for this, colder air from higher air layers is transported downwards. Because, as mentioned above, the wind speed increases with altitude, air of higher speed thus descends. It accelerates the slower air parcels stored there briefly, which is also beneficial to the gustiness.

Furthermore, it is possible that in the free atmosphere - i.e. above about 1500 meters - the speed of the intrinsically laminar flow may exceed a value that is critical for it. Then the laminar flow becomes susceptible to the smallest disturbances. If it then becomes turbulent, gusts can sometimes reach the ground.

From the current perspective, it will only be windy in the mountains and on the coasts in the next few days. Nevertheless, there is no boredom, because winter initially pays us a visit at higher, but temporarily also at lower altitudes.