The weather is highly unpredictable in Iceland.
Average temperatures in Reykjavik:
Summer = 10 deg. C, or 50 F.
Winter = 0 deg. C, or 32 F.
Expect the worst and hope for the best.
Summer: Bring warmer clothes than what you think you'll need. It is always better to have too many and too warm clothes than not at all. We have some extra in case you don't bring the right clothes for the ride.
Winter: Bring a hat and gloves. Warm wind and rainproof clothes are recommended. We have some extra in case you don't bring the right clothes for the ride.
Check out this synoptic chart from the Iceland Met Office.
Live camera Lake Tjornin Reykjavik
Live camera Austurvollur Square (main square in Reykjavik).