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 To See Them. Unfortunately, the best time to witness the Northern Lights is in the dead of winter, from 11:30 p.m. to 3:30 a.m., with peaks around 1:30 or 2:30 a.m. However, some say you can catch a brief glimpse of the Northern Lights 240 nights a year, especially during March, September and October. Your best hope is for a clear, cloudless night. Other ideal times include the equinoxes around September 22 and March 22.