Thingvellir is a popular destination for those wanting to enjoy Aurora Borealis, the northern lights. The Aurora Borealis are caused by the collision of energetic charged particles with atoms in the high altitude atmosphere (thermosphere).
It requires dark and partly clear skies to enjoy them. The northern lights are therefore unpredictable like the Icelandic weather but recently the Icelandic Met Office added Aurora forecast to their website.
The aurora forecast predicts the best places to enjoy this phenomena most tourists dream of experiencing when visiting Iceland in the autumn or winter.