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.
Northern Lights in Iceland. Iceland is a scarcely populated arctic island, with many wild, unpolluted places to see the Northern lights. Even though the lights of Reykjavik can't hide them, then they may be even brighter outside the city limits.