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.
I spent four nights in Iceland; four chances to see the northern lights. The first night, I booked a stay in the small town of Hveragerdi, about a 45-minute drive from the light pollution of Reykjavik. When conditions align, the outdoor thermal hot tubs of Frost and Fire Guesthouse are an ideal viewing spot.celand is one of the world's most accessible spots for viewing the northern lights, making it one of the country's top tourist attractions.