Despite its location close to the Arctic Circle, Reykjavik's climate is similar to that of the northwestern U.S., although cooler and windier. The Gulf Stream helps keep the annual mean temperature at 40°F. Changes between summer and winter are not extreme. It is rarely very cold in winter or warm in summer. Winter temperatures below 20°F are unusual, as are summer temperatures above 60°F. The wind blows year round, however, and a wind chill factor between -15°F and 10°F is common in winter.
Cooler weather lasts from October through April. Snow may fall in Reykjavik as early as September and as late as June, but the normal season is between October or November and March or April. Even in midwinter, rain is as likely as snow. A large accumulation of snow is rare. Average annual rainfall is 31 inches in Reykjavik. During winter and spring, winds in the capital can reach hurricane force.
Iceland is so far north that the amount of daylight varies considerably throughout the year. An average daily gain of 6 minutes of daylight follows the winter solstice on December 21, and a daily loss of 6 minutes follows the summer solstice on June 21. December and January days have only about 4 hours of daylight; in February the days rapidly begin to lengthen; and by April they are as long as at midsummer in the U.S. From late May to late July, there is no darkness at all-20 hours of sun (or clouds) and 4 hours of twilight. Following this period of "white nights," the sun slowly retreats, and by October the days begin to shorten as rapidly as they lengthened in the spring.
Earthquakes are common in Iceland, but are rarely felt in Reykjavik. Volcanic activity is infrequent but rather spectacular when an eruption does occur. The underwater volcano that created the new island of Surtsey in the Westmann Islands off the south coast began erupting in November 1963 and remained active through mid-1967. In January 1973, a volcanic eruption on Heimaey Island in the Westmann Islands forced the evacuation of all 5,000 residents and destroyed more than 300 homes and buildings. In the Krafla area, near Lake Myvatn, an eruption took place in December 1975, lasting several days; this area subsequently has seen seven lesser eruptions, and further volcanic activity is expected there. The most famous of Iceland's volcanoes, Mt. Hekla, which had been expected to remain dormant for a 100 years or so after its spectacular 1947 eruption, produced eruptions in August 1980, April 1981, and January 1991. A volcano under the Glacier Vatnajokull erupted in November 1996, melting tons of ice and creating destructive flooding.