Formation of altocumulus undulatus ahead of a mid-level thunderstorm probably due to an upper-level cold front as can be seen in the surface pressure chart (second thumbnail). The visible satellite image corresponds with that of the surface pressure chart but further east due to the eastward movement of weather systems in mid-latitudes. The action of the elavated thunderstorm might have been the main cause of this cloud undulations due to the storm initiating wave action between different layers of air as was depicted in the weather sounding (first thumbnail). Furthermore, lots of wind shear was present at the time indicating different layers of air underneath the thunderstorm.