"Atmospheric Optics" or http://atoptics.co.uk was an amazing reference website by Les Cowley, that was only from 1996 to 2020, at which point it switched to a new, AI-written blog site which was hard to navigate, and was removed from the Wayback Machine by the new owners.

In 2022, Les announced that the site had come back.

Months later, it quickly disappeared again, only to resurface at http://old.atoptics.co.uk.

That link stopped working in 2025 but Les announced that he had uploaded the site to a new address at https://www.atoptics.org.uk. I had copied the site here for my reference but recommend you access it through Les' new link instead.

Screenshot from 2020: