I'm trying to build an ice farm in a mountain biome. According to the wiki:
As the climate is cold, rainfall changes to snowfall above approximately Y-level 92
So I've found an elevation in mountains where there is snow at Y=93 and build an ice farm there. As you can see, there are some ice patches on the farm and they freeze very quickly. The majority of the water, however, never freezes.
Once I went there during rain, and I noticed that there are some areas over the ice farm where it rains and others where it doesn't. You can see the huge rain patch of an irregular shape when looking straight up:
Then I've started to stack blocks in the places where it was raining, and noticed that at the certain height rain turns into snow. Then I elevated my farm to Y=97 as it looked like most of the blocks had snow on them. And it worked pretty well, but there were still two spots that won't freeze:
By putting water 1 block higher (at Y=98) I was able to quickly freeze it:
Is there any mechanic that defines these areas of snow and rain?