Formation of mid-level clouds due to an approaching trough over the Islands in a rare high under low 500mb weather synoptic. The cloud sub-type was very difficult to identify because the cloud on the picture looked like undulatus but resembled more the stratiform type as it seemed to be stretched. The weather sounding showed increasing wind speed with height at the cloud level of between around 2.5km and 3.7km hence conditions for undulatus formation was there. Also, weak frontal systems were present which would raise the cloud slowly in part encouraging the stratiformis type formation. The thumbnail attached confirms that at the same moment there was also the formation of altocumulus perlucidus. Although not an aesthetically beautiful cloud in itself, it conveys very important meteorological information as it indicates mid-level instability hence pointing to a possible thunderstorm development later in the day especially when seen in the morning. In fact, a very isolated mid-level thunderstorm did occur later on the same day. Side note – The highest MAX temperatures of the day were registered on Scandinavia near the Arctic Circle peaking at more than +30C.