C along its southern Great Lakes margin to a full -40 C in parts of Alaska. Not only are winter temperatures low across most of the region, but winters are long. The average time between the last frost in the spring and the first in the fall is roughly 135 days at the southern margins of the area but little more than 14 days along parts of the Arctic Ocean. Because virtually all major food crops need a growing season of longer than 90 days, they can be grown in only a few small areas along the southern margins. Summers, generally short and cool, can have surprisingly warm days.
Maritime moderation is significant only along the peripheries, mainly in the east and west.
This dramatic seasonal variation in temperature results from the great shifts in length of day and the angle of incidence of the sun's rays. As the Earth follows its annual path around the sun, the North Pole is tilted toward the sun during what is summer in America and away from it in winter. Thus, everywhere north of the Arctic Circle is in darkness for at least one day at midwinter and experiences at least a 24-hour period without the sun setting at midsummer. Moreover, during the winter, the sun, when it rises, remains low on the horizon. Even southern sections of the region receive only six or seven daylight hours during the winter.