Mid-level cloud seemingly in a band and with irregular small gaps hence the name perlucidus. The right-hand side photo thumbnail also shows individual bundles of the cloud `Ac floccus`. No particular cause of formation could be sighted but the weather sounding showed 70% at an altitude of 3.2km combined with level of free convection (LFC) at 3.1km altitude. That meant instability at that level hence the formation of small altocumulus rather than altostratus.