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.
The best time of year to see the Northern Lights depends on where  in the world you are. For example, in Iceland, the best time to see  them is during September, October, Fe â¦ bruary, and March. I am Flukey. 491,897 Contributions. I am an idealistic realist.