Here are links to external webpages I particularly like, for a variety of reasons
General atmospheric optics and other phenomena
A similar concept to the Astronomy Picture of the Day, but OPOD posts tend to be more in depth than APOD and contain more than one photograph of the featured phenomenon, where APOD limits itself to just one image per post.
This is probably part of the reason OPOD is updated less consistently than APOD. I haven't checked if OPOD's even still being updated at all—but I wouldn't be surprised if it isn't, since the creator, Les Cowley (he may not have been the only one, though!), has retired from its upkeep and to my knowledge tried to remove the website from the internet entirely. This website, technically a clone of the original atoptics.co.uk, may even exist against his wishes. But, a lot of this is speculation based on only partial knowledge.
At some point I'll need to look more into the European atmospheric optics community. From the incredibly detailed websites I've stumbled on, there seems to be a thriving community–likely multiple communities in different countries–of perhaps amateur enthusiasts with a great deal of knowledge accumulated over decades of consistent study, including much fieldwork.
The METEOROS newsletter is a German monthly publication in regular print since 1998; those issues more than a year old are available for free. Each newsletter is many tens of pages describing topics as varied as meteor showers, observations of halos and auroras, and other relevant topics. The halo observations are collected by observers who fill out a detailed observation submission sheet and, from my cursory inspection, seem to be published for a given month a few months later. At the very end of each newsletter is an English-language summary of its contents.
There is a fantastic quantity of observational data and accumulated knowledge represented in this publication's decades-long history.
A website on atmospheric optics that, at a glance, appears roughly as thorough as Les Cowley's atoptics.co.uk site. That means a lot of wonderful photos and some really good information!
Halos
Rainbows
Auroras
Waves
Books or similar