I have seen these lights for about 20 years now.
The local people call them swamp lights, as they are usually seen in swampy areas.
I live in Ontario Canada and they say that even in the spring they are seen on warmer nights. When there is a hot day and the evening seems to cool down quickly, you can be sure they will be there.
Sometimes there 2 or 3 of them. I estimate that they are about 2'to 3' in diameter. They will rise above the trees about 30'. The color is orange/amber or yellow.
They have been seen about one hour before sunset and I have watched them till about 2:00 a.m.
There are 2 places that I have personally seen them but I guess there are more. I find it facinating but I never had any luck in photographing them.