Considering the northerly location of Iceland, its climate is much milder than might be expected, especially in the southern coastal areas, that are strongly influenced by the warm waters of the atlantic Gulf stream. The mean annual temperature for Reykjavνk is 5°C, the average January temperature being -0.4°C and July 11.2°C. The annual rainfall on the south coast is quite high, about 3000 mm, whereas in the highlands north of Vatnajökull have only 400 mm or less.
The weather in Iceland is very changeable and is mostly influenced of the atmospheric depressions crossing the North Atlantic, the so-called "Iceland-lows". The passage of a depression some distance south of Iceland causes relatively cold and dry weather, especially in southern districts, while one passing north-eastward between Iceland and Greenland brings relatively mild and dry weather, especially in the north. Gales are common among the coastal areas of Iceland, especially in winter. Thunderstorms are extremely rare.
Iceland's southern and western coasts experience relatively mild winter temperatures thanks to the warm waters of the Gulf Stream. July and August are the warmest months and, in general, the chances of fine weather improve as you move north and east. While they're more prone to clear weather than the coastal areas, the interior deserts can experience other problems such as blizzards and high winds that cause sandstorms. The weather in Iceland can change very rapidly so don't hesitate to check the weather forecast to see what kind of weather to expect. In the winter you can never be too cautious if you are planning to travel into the interior.
For two to three months in summer there is continuous daylight in Iceland, the so-called midnight sun. During summer the nights are bright throughout Iceland and in June the sun in the north never fully goes down. The winter darkness (three to four hours' daylight) lasts from about mid-November until the end of January. Another special Atmosferic Phenomena is Aurora Borealis, the so-called northern lights. The northern lights can mostly be seen in the autumn and winter months.
Required clothing: Lightweights in warmer months, with extra woollens for walking and the cooler evenings. Medium- to heavyweights are advised in winter. Waterproofing is recommended throughout the year. Be prepared for rapid and extreme weather changes, no matter where and when you go.
The climate of iceland can be classified as Cf Climate; a warm temperated humid climate with the warmest month lower than 22°C over average and four or more months above 10°C over average.The climate of the northern parts of Iceland and the mountainous regions can be classified as E Climate; an Ice climate with the warmest month under 10°C.