Climbing out of Phoenix on an American Airlines flight to Des Moines, we passed through scattered cumulus clouds (visible below) before reaching what appeared to be a well defined altostratus layer. As we approached the cloud base it became apparent the cloud base was diffuse - not distinct. The altostratus looks like fog covering roughly the top half of the photo. The lower clouds look indistinct because we were climbing into a thicker cloud layer which eventually obscured the lower clouds.
The Weather Briefing Blog