This type of high-level cloud is really a build-up of various cirrus clouds hence is slightly more thicker than normal cirrus clouds. In this photo, it did not form from a former cumulonimbus cloud as there were none around. It seems, according to the weather sounding of the second thumbnail, that such cloud formation require a lot of upper-level moisture to develop so that the individual cloud bundles could accumulate. The first thumbnail show an earlier shot of the same cloud taken 30 minutes earlier. The cumulus in the background developed due to the usual solar heating of the ground under a slightly unstable airmass. Furthermore, such high-level clouds sometimes seem to precede the advancement of a warm air bubble from Africa.