Time, AND weather?? I sat at one spot for 20 (real) minutes waiting for the time window to come up at 8:00, and the weather changed at the same time At least people have created guides so I don't have to decode the riddles myself.
Even then, given all the waiting required, I don't think I'll continue hunting these, especially since there's no reward.

I wish I had an addon that alerted me to nearby ones when the conditions were right.. Wait, no addons here..