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.
Hi Ella. Have a look at these sites, they do tours to Iceland in February. I like VJV the best, but the other might be more to your likings, since they usually have more family based and mixed groups. Allready in February you get a full day of daylight, and yes February is probably the best month to see the Nortern lights.