One helpful tool in the quest for the northern lights is the Iceland northern lights forecast published by the Icelandic Meteorological office. This article walk you through the site and how to use it.
There are three things that affect the northern light visibility: Northern lights activity, brightness and cloud cover (being the most important factor).
The first thing to look at in the Iceland northern lights forecast
Below is a snapshot of the aurora forecast website. On the cloud cover map white color represents clear sky and green color represents cloudy sky. Normally the forecast is most accurate for current day/night. Below the map is a time axis to view the cloud cover forecast for the next five days. Be aware of the five-day forecast, it can get inaccurate due to quick changes in the Icelandic weather. So do not plan a trip based on the five-day forecast without following up on the forecast every day.
Next thing is to check out the northern lights activity
On the right site of the aurora forecast website is a Kp-scale from 0 – 9 that represents the northern lights activity. 0 means no activity and 9 means extreme activity. Common values for northern lights activity are between 0 – 3. Northern lights can be seen when the Kp-value is as low as 2 but are very visible when Kp-value is 3. Note that the Kp-value gets very seldom to the upper part of the scale so there is no point waiting for Kp-value as high as 7, 8 or 9.
The final thing is to look at the brightness (or the darkness to be exact)
Iceland is located between 63° and 66° north just south of the arctic circle. Due to the incline of the earths axis of rotation the day get very long during summer and very short during winter.
The longest day of the year is during summer solstice june 21st when it will not get dark at all. So during summer it is not possible to see the northern lights in Iceland. The shortest day is during winter solstice december 21st when it is only bright for few hours a day. So during winter the brightness is not big problem.
Below the Kp-scale on the aurora forecast website are info about sunset and sunrise as well how long it is dark (and when it get dark). Normally the northern lights activities peak around midnight.
The moonlight can impact the exposure of the northern light due to it brightness when it is full moon but the northern lights will be visible.
Let´s summarize it
By applying the steps described in the Iceland northern lights forecast in the example above the likeliest place to see the northern lights is the south cost of Iceland, with moderate northern lights activity (Kp-value = 2) from 8:28 pm to 6:26 am and very little moonlight.