#if you get pedantic on winter in the worldbuilding please be aware that the question was asked and grrm answered it well before you ever did

[nobodysuspectsthebutterfly]

GRRM, on the question of the agriculture in the North of Westeros

SJ: From what we’ve seen in the books so far, it looks like even in summer the snow covers most of the lands in the North, and it surely does cover all in winter, doesn’t it? GRRM: I wouldn’t say that snow “covers most of the lands” in summer. Rather than they have occasional summer snows. It never gets really hot in the north, even in summer, but it’s not icy and snowing all the time either. Winter is a different tale. SJ: But quite a lot of people are living there. What do they eat? GRRM: A lot of food is stored. Smoked, salted, packed away in granaries, and so on. The populations along the coast depend on fishing a great deal, and even inland, there is ice fishing on the rivers and on Long Lake. And some of the great lords try and maintain greenhouses to provide for their own castles… the “glass gardens” of Winterfell are referred to several times. But the short answer is… if the winter lasts too long, the food runs out… and then people move south, or starve… SJ: Are there some areas without snow, which are suitable for agriculture, or are there significant temperature changes inside the “bigger seasons”? To grow a harvest, at least a couple of months’ time of warm temperature (15-20 degrees by Celsius) is needed. Is it available in the North? GRRM: Sometimes. It is not something that can be relied on, given the random nature of the seasons, but there are “false springs” and “spirit summers.” The maesters try and monitor temperature grand closely, to advise on when to plant and when to harvest and how much food to store. SJ: And what happens when a winter comes - five, six years long? GRRM: Famine happens. The north is cruel. SJ: Surely, the import of grain from the South alone can’t cover the North’s needs. And, by the way, does it snow in the South during the winter? GRRM: Yes, some times, in some places. The Mountains of the Moon get quite a lot of snow, the Vale and the riverlands and the west rather less, but some. King’s Landing gets snow infrequently, the Storm Lands and the Reach rarely, Oldtown and Dorne almost never.

–June 21, 2001

For more on the seasons and the world of ASOIAF, please see this post.