A lot of people wonder how would they know that the faint streak that they sometimes see in the sky at night, is a cloud or a faint northern light.
The answer is : look for the stars. If you can see the stars through the faint light then it is the Aurora. If you can't then in all probability it is a cloud and in a cloudy sky, it is difficult to see the aurora anyway. You need a clear sky. The micro climate of Abisko actually promises a lot more clear nights than some other places.