I am guessing it's a dial instead of a digital gauge, from your description.
Has it just started doing this?
If so, it might be because you are using the heaters on a higher, hotter setting in the cold weather. It shouldn't have quite as much an effect as you describe however the heaters will draw heat away from the engine.
I remember having to do this years ago when stuck in traffic in the heat of summer. Windows open, heaters on full blast, just to keep the engine from overheating
Try running it without moving, say on your drive and without the heaters on. Does it still not move up as much as it did?