Gustfront / Outflow Boundary

In the afternoon of 11 July 2010 showers and thunderstorms developed over the eastern Carpathian mountains. With a weak northeasterly flow they were advected to the Transylvanian Basin.


The storm to the northwest of Tirgu Mures produced thunder, but it didn't reach my location. However, a band of clouds with a rather flat base moved in my direction. (red dot = my position)



When it passed, the wind increased and it became cooler. This passage was the so called gustfront.
I also observed that the cloudband was curved and extended to the far west.





A gust front is the leading edge of cool air rushing down and out from a thunderstorm. When the downdraft and rain-cooled air reach the ground, the rain-cooled air spreads out along the ground and forms a gust front. Usually the winds associated with the gust front are not severe, but in extreme cases, a downburst can develop and produce severe wind gusts.
There are two main reasons why the air flows out of some thunderstoms so rapidly. One reason is the presence of relatively dry (low humidity) air in the lower atmosphere. This dry air causes some of the rain falling through it to evaporate, which cools the air. Since cool air sinks, this causes a down-rush of air that spreads out at the ground. The edge of this rapidly spreading cool pool of air is the gust front. The second reason is that the falling precipitation produces a drag on the air, forcing it downward.


The gustfront moved further south and I saw that on top of it the convection became deeper.


It developed to a nice towering cumulus, while the other clouds of the band almost dissipated.



Finally it became a shower, which also produced a few rumbles of thunder, thus officially a thunderstorm.
However, this thundery shower died soon, because the gustfront moved south faster than the shower, thereby cutting off it's energy.


Here's a satellite-animation where one can see the rapid movement of the gustfront (or outflow-boundary). Behind the boundary clearings can be seen!
The red arrow marks the new developing thundery shower near Tirgu Mures.



If the animation doesn't work click
here