[Fixed/featured in 3.70] 3.50 beta2 Human NPC unaffected by snow.

[Roman Lodestar]

Tested with robbers NPC on spring where the snow reachs the waist, when you run from then and they are hostile (not using skis) you tend to accumulate fatigue at a alarming rate while the robbers move at the same speed (even more) without getting any message of fatigue when I look at them. Njerpez are also inmune to the slow effects of the snow.

[PALU]

I believe this has been discussed before, but I can't find it.

As far as I can tell NPCs are unaffected by snow and move and tire at normal speed/rate for snow less ground as Roman Lodestar says.
However, it's not trivial to address the issue, as causing NPCs to move in snow using snow penalty would make them push overs for characters wearing skis as the NPCs would easily be run into the ground (or snow, rather) through fatigue, and would be unable to close the distance to the PC unless the PC allows it.
The villages would also get silly, as all the villagers would move around at a snail's pace or stand still due to fatigue when the ground is covered by significant amounts of snow.

The non trivial correction would be to get them to wear skis, but that would require them to also be taught how to handle ski sticks...

[Roman Lodestar]

I decided to move this to the bug section just to know what the developers would think, npc would be obligued to carry skis in order to function properly and villagers wouldn't be affected by the snow when they are inside the village limits.