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The climate of Iceland is subarctic (Köppen climate classification: Dfc) near the southern coastal area and tundra inland in the highlands. The island lies in the path of the North Atlantic Current, which makes its climate more temperate than would be expected for its latitude just south of the Arctic Circle. This effect is aided by the Irminger Current, which also helps to moderate the island’s temperature. The weather in Iceland can be notoriously variable.
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The aurora borealis is often visible at night time during the winter. The midnight sun can be experienced in summer on the island of Grimsey off the north coast; the remainder of the country, since it lies just south of the polar circle, experiences a twilight period during which the sun sets briefly, but still has around 2 weeks of continuous daylight during the summer.
Winters
The Icelandic winter is relatively mild for its latitude. The southerly lowlands of the island average around 0 °C (32 °F) in winter, while the Highlands of Iceland tend to average around −10 °C (14 °F). The lowest temperatures in the northern part of the island range from around −25 to −30 °C (−13 to −22 °F). The lowest temperature on record is −39.7 °C (−39.5 °F).
Summers
The average July temperature in the southern part of the island is 10–13 °C (50–55 °F). Warm summer days can reach 20–25 °C (68–77 °F). The highest temperature recorded was 30.5 °C (86.9 °F) at the Eastern fjords in 1939. Annual average sunshine hours in Reykjavík are around 1300, which is similar to towns in Scotland and Ireland.
Winds and storms
The prevailing wind direction is easterly. Westerlies are very infrequent. Generally speaking, wind speeds tend to be higher in the highlands, but topographical features can aggravate winds and cause strong gusts in lowland areas. Wind speed in the lowlands reaches 18 m/s (40 mph) on 10-20 days per year but upwards of 50 days per year at places in the highlands. Heavy dust storms can be generated by strong glacial winds, and can be very strong. Up to 10 t (9.8 long tons; 11.0 short tons) of material can be in motion per transect per hour. These storms are very frequent in the early summer in the arid highland areas north of the Vatnajökull glacier.
Thunderstorms are extremely rare in Iceland, with fewer than five storms per year in the southern part of the island. They are most common in early or late summertime. They can be caused by warm air masses coming up from the continent, or deep lows from the southwest in wintertime. Lightning can usually be observed in connection with ash plumes erupting from the island’s volcanoes.
Atmospheric pressure
There is a persistent area of low pressure near Iceland known as the Icelandic Low, found between Iceland and Greenland. This area affects the amount of air brought into the Arctic to the east, and the amount coming out of the Arctic to the west. It is part of a greater pressure system known as the North Atlantic Oscillation.