Is the Alpine warm enough for sub-freezing weather?

Yes for typical winter cold. The polyester fleece lining is insulated, and owners report warm feet on cold commutes and while shoveling. For extreme cold or long periods standing still, add wool socks.

Is it fully waterproof?

It has a waterproof membrane and a seam-sealed inner bootie, so it handles snow, slush and rain. It is not a rubber wading boot; deep puddles above the collar or prolonged soaking can still let water in.

Owners commonly describe it as running narrow, and some size up. If you wear thick winter socks or have wide feet, consider a half size larger.

Can I use it for hiking or mountaineering?

For light trails in snow, yes. It is a flexible insulated boot, not a stiff mountaineering boot, so it is not built for crampons or steep technical terrain.

Does it work in milder weather?

It is best in cold conditions. The insulation and membrane make it warmer and less breathable than a regular shoe, so feet can feel clammy on mild days.