The weather in Iceland is the epitome of changeable. This is what makes for the beautiful scenery, the rainbows, and sweeping vistas. I once experienced perfectly clear and sunny conditions, followed by high wind and rain, then a quick big-fluffy-snow blizzard, followed by overcast and cloudy conditions, then another little sprinkling of rain (this time with considerably less wind), before the wind came in and blew the rain away, leaving partly sunny conditions and a noticeable but tolerable wind. All this was at Thingvellir National Park, and it spanned approximately 30-minutes during my group's visit there.
Since the weather is so unpredictable, I always like to prepare our guests with the following breakdown. I have separated the weather in Iceland into its two "pagan" seasons: summer and winter (the concepts of Spring and Autumn were only introduced during the imposition of Lutheranism, but having spent time on the island, there are only two seasons).
Temperatures vary by region. The temperatures for where you'll go with us average 10-13º C (50-55º F) in the South and 15-18º C (59-65º F) in the North. The Icelandic summer is relatively temperate in the South, and mild in the North. Expect wind and rain and on the rare occasion sleet and snow.
Winter temperatures where we visit in Iceland average around 0º C (32º F), but it can get as low as -10º C (14º F). The Icelandic winter is relatively mild for its latitude, but it can be very windy with rain, sleet, and snow to be expected.
Now, regardless of the season, pack an impermeable layer or two. I have waterproof hiking boots, impermeable jackets (medium-warmth for Summer, very-warm for Winter), and I carry gloves, scarves, and warm hats as well. Remember, Iceland is just north of the North Atlantic Drift, and just south of the Arctic Circle. It's where cold northern air does battle with warm southern air. I also strongly recommend thinking in terms of layers especially if you are coming from a warmer climate!
I also recommend bringing a pair of long underwear or two, especially for those who aren't used to colder weather. Better to have them and not need them, then to need them and not have them.
Click here for what to wear during any season in Iceland.