I may well have offered a wrong description,
Medico, but if you click
here, that will link you to Michael Quinion - the noted etymologist/lexicographer's - web-page about Cloud Nine, which also mentions cumulo-nimbus as the cloud concerned...though for somewhat different reasons from mine! Though I haven't read it, I understand Bill Bryson's �
The History of Nearly Everything' - or some such title - also offers cumulo-nimbus as the appropriate cloud-type. So, I could have got hold of the wrong end of the stick, but I'm in good company!