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.
View tour. The Northern Lights tour in Iceland takes you to see a spectacular natural phenomenon which happens during the winter months. The Northern Lights, also known as Aurora Borealis, are caused by the interaction of particles from the sun with the upper atmosphere near the North Pole.