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.
When is the Best Time to See the Northern Lights? The northern lights are visible under dark skies between the months of September to April, preferably under a clear, cloudless sky. Usually seen between 5pm and 2am, it is important to be away from artificial light.