An advertisement said “the whole island is air conditioned!” Iceland's cool, oceanic climate is quite mild for its latitude of 63-66° North, thanks to the warm Gulf Stream. The summers are short but bright and the best time to visit is late May to early September – that is, if you’re planning a summer holiday. The average daytime temperature around the coast during May to September is 10-12°C (50-55°F). A warm summer day would have temperatures around 20°C. Average daily sunshine in July and August is 5-6 hours and during the summer months the nights are bright. On clear days you have 24 hours of daylight and even the midnight sun near the Arctic Circle. However, the weather is extremely changeable and unpredictable so you should always be prepared for the unexpected.
If you are looking to see the Northern Lights, the time to visit is from September through March. Icelandic winters are not as cold as you might expect and the winter is in many ways a more interesting time to visit Iceland. There is a saying in Iceland, as in many other countries: “If you don´t like the weather, just wait 5 minutes”. A typical weather experience would be something like this:
Last June I went with a group of friends to the Hekla volcano. Our journey started with beautiful clear skies on a very still and sunny morning. When we reached the summit around noon, it was foggy and started to snow, which turned to rain when we came down the mountain and in the afternoon there was a refreshing wind and rain in our face.
In Iceland, you can expect all kinds of weather in a single day.
You can check out the weather forecast on the website of the Icelandic Meteorological Office.