These storms occur from the moisture, warm air, and lift caused by cold fronts, mountains, the sun’s heat, and other factors. Of course, you should always seek shelter before thunderstorms, and if you hear the rumbling of thunder, go indoors. Fully enclosed buildings offer the best protection. Their electrical wiring and any metal piping offer a grounding effect in the event of a lightning strike. And, of course, the building itself provides shelter from the wind and blowing debris. Just stay away from conductive things, like wiring, corded telephones, plumbing pipes and fixtures, during the storm. Continue to avoid these items for 30 minutes after the storm, in case of lingering lightning. Although thunderstorms are relatively small compared to hurricanes, they still average 15 miles (24 km) in diameter and carry significant destructive force. All thunderstorms are dangerous, producing deadly lightning, and often high winds and hail. Each year in the United States, there are approximately 100,000 thunderstorms. Roughly 10 percent of these are classified as “severe” by the National Weather Service, having damaging winds of at least 58 mph (50 knots) and/or having hail at least 3/4 inches (19 mm) in diameter.