In perfect conditions, the sky needs to be dark and clear of clouds. Clouds will block the view of the Northern Lights as the Lights are above the clouds at an altitude of 100km to 300km. And then there needs to be activity on the sun sending the charged particles to Earth. The activity on the sun is monitored and the particles take some time to get to Earth so the likelihood of Northern Lights can be predicted. A good website for Aurora forecast is the one at Gesphysical Institute. A level of 3 or more is favorable.One can choose a map for the appropriate area.In early fall the sky doesn't get dark until it's close to midnight, but as we get closer to winter, it will get dark earlier and Northern light tours are most often run for 4 hours in the period between 8pm and 2am."Hunting for the Northern Lights" is a term used when the sky is partially covered by clouds and one needs to find an opening in the clouds to see the Northern Lights. This may call for some travel.As seeing good Northern Lights can be hard to predict, it's advisable not to leave the Northern Light tour till the last night of your stay in Iceland.