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 - Aurora Borealis in Iceland, Land of the Sagas. The Northern Lights, also called Aurora Borealis, and NorÃ°urljÃ³s in Icelandic, are one of the most spectacular shows on this earth and can frequently be seen in Iceland from September through March on clear and crisp nights.