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.
One of the best experiences you can have in life is to watch the northern lights dancing in the sk. Northern Lights Photography. One of the best reasons to visit Iceland during winter is to see the Northern Lights.