S wave -- a shadow zone appears when the velocity suddenly decreases 103-180 degrees from the epicenter. Describe the differences between P and S waves. P waves expand and contract, can move through liquid solid and gas. S wave moves up and down or side to side moves through only solids.
Optimal conditions to see the northern lights in Iceland is when skies are clear and dark (away from the city lights, in cold temperature and ideally from mid-September to mid-April.
5 Things No One Ever Tells You About the Northern Lights. My quest to see the Northern Lights began in September 2011. I had literally just left Tim in Iceland and the very next night the Northern Lights danced for a week straight. They were so bright and so active, they even kept him up at night.