The climate of the northern mountains can be quite brutal in winter, with snow, sub-zero temperatures and icy rain. All of this cause problems for the inhabitants of this area throughout the winter months.
The relentless cold makes it very difficult to heat housing, and can damage crops so badly that in the warmer months there is little to harvest, which can lead to critical food shortages. Water supplies
can also be frozen.
The snow is also constant and can fall for weeks on end without a break, and it is not surprising for snowfalls to get almost fifty feet deep in some places.
This makes getting around extremely difficult, can can lead to the mountains inhabitants being confined to their houses for days at a time, a problem not helped by the difficult mountain terrain. Icy paths makes the area treacherous to get around.
Eddard Stark says:
"The North is hard and cold, and has no mercy" which probably sums up the area and the climate perfectly.