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.
4. Reykjavik, Iceland. Reykjavik is still the most affordable and accessible place to see the Northern Lights, but itâs getting more popular. In January 2015, 9,003 Britons visited Iceland, probably because, Lights aside, thereâs plenty to fascinate about this land of geysers, volcanoes and blue ice.