Most lightning strikes occur at the beginning and end of afternoon storms. This is when positive and negative charges, which collide to produce the flash between clouds and the ground, build up the most electricity. Thunder (see sidebar), the sound waves produced by the explosive heating of air in the lightning channel, is the obvious omen we need to heed, but there are many other warning signs. Darkening skies, the buildup of anvil-shaped cumulonimbus clouds, and a sudden drop in temperature and increase in wind often presage the storms that are most likely to produce lightning. Immediately preceding a bolt, low levels of electricity fill the air, causing phenomena such as the hair on your body standing on end, a tingling sensation on the skin, or a metallic taste in your mouth. If you experience any of these, a strike is imminent.