Here's the deal with chickens and weather. If it's not windy, and it begins to snow or rain, chickens will usually go about doing whatever they were doing before it started to snow or rain. But the minute the wind chill kicks in, they will head, post haste, for shelter.
As for snow on the ground, most chickens have an instinctive repugnance for being on it. I believe it comes from being aware, on some level, that they show up vividly to predators against the white background. That and chicken feet find snow icky.
A few years ago, I had a six-month old Black Cochin cockerel Darrel. Darrel had never seen snow before, and so he raced outside when I opened the run gate, and almost instantly came to a screeching halt. There was a stretch of snow between him and where he desired to be. Rather than walking over the icky stuff, he tried to fly over it. Grown chickens don't fly well, and Cochins don't fly at all, so Darrel ended up stuck head first in a snow drift, feathered feet fluttering helplessly. I had to quit laughing so hard and haul him out of the drift. He never ventured outside again if he knew there was snow.
His sister Morgan, same age, went out free-ranging one afternoon, didn't go back to the run when it began to rain, and she was caught out in a severe hail storm. I forgot she was still outside, and I raced indoors when it started to hail. Afterwards, I went out to check on the chickens, and here comes Morgan, totally drenched, in no big rush to get back into the shelter of the run, seemingly none the worse for it.
Four years ago, on New Year's Day, I let the flock out to free range. There were large patches of snow but plenty of bare ground left for scratching around. I went off to shovel the long driveway, and came back later to find a red-tail hawk sitting on a wood pile next to the body of my six-month old Buff Brahma pullet Cleo lying on the ground with her head destroyed. The rest of the flock were hiding under the coop in the run, scrunched up in the far corner, totally traumatized by the event.
None of them will venture out when they know there's snow. They learned it's not safe.