The simple answer to that question is; on a dark night with a clear sky. The Northern Lights can be seen (should they appear) at anytime from dusk until dawn, and a display can last from a few seconds to a few hours. Knowing how long they will last, how strong they will shine, what colours will show or how much movement there will be, is pretty much impossible.
Icelandic Northern Lights guides are experienced in interpreting the various forecasts there are and then deciding which places are good on any given night, and the longer the night the better the chances. But having said that, we have seen them as early as August and as late as April, and a few good minutes of extraterrestrial fireworks in the starry dome can be the perfect aurora experience.
One of our guides put it this way: ”We are basically betting on odds any night we go out and sometimes we get lucky, sometimes not. But it is always worth the try. Whether we get the slow languid veils spread across the sky or jittery explosions of fantastic light overhead, it is always magic.”
Even though you can see the lights from anywhere, even downtown Reykjavik, if they are strong enough, it is always a good idea to get a little guidance if possible. Taking a tour is often about getting a little extra information and being in the capable hands of professionals. Also, there are safety issues to think of, the weather, for example, can change quickly during winter in Iceland. But that is also a part of the adventure.
Mankind has stood in awe and gazed at the Northern Lights since the dawn of time. They have been believed to be the souls of dead hunters, paths of the Valkyries and dancing gods. People have attached all sorts of meaning to those enigmatic shades of light including being bad omens, good omens and bringing good luck tochildren conceived beneath their magic. The scientific aspect is maybe a little less inspiring but nevertheless fascinating.
The predictions that can be found online are made by following measurements of activity on the surface of the sun, solar flares, and then calculating the amount of time it takes the solar winds to reach earth and ignite the aurora belt. So basically we are talking about space which is the main reason why it is so hard to say for sure exactly at what time a northern lights display mayor may not hit. However, given the relative frequency of said solar winds and the long winter nights in Iceland, your chances when visiting are pretty good.The fact is that as long as there is a reasonably clear sky the odds are with you that there will be some movement in the sky to enjoy.
Although there is no average with the northern lights they occur with enough regularity to be able to say that there is probably a 50/50 chance on any clear winter night in Iceland.