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.
Where to see the Northern lights in Reykjavik. If you have a car, drive 15 minutes out to the President's residence, Bessastadir, in a town called Ãlftanes. Here you can watch the Northern lights from across a bay, as they dance above the cityscape of Reykjavik while reflecting in the sea.