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 Northern Lights can appear at any time, but they usually grace the sky between 6 oâclock in the evening and 1 oâclock in the morning. It is rare to see the Northern Lights before 18. 00/6pm, even during the dark months.