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.
In summertime Iceland is bright almost all the time, so that is the period when you can not see the Northern lights in Iceland. You don't have to drive far from ReykjavÃ­k city to see them, and if you donât feel like leaving Reykjavik, you can sometimes see them from downtown, just not quite as bright.