The formula is used to measure the speed of ocean waves, like tsunamis, that have very long wavelengths relative to the depth of the water. Tsunamis can have wavelengths greater than 700 kilometers (the average ocean depth is 3-4 kilometers). The formula estimates the tsunami's speed while it is in deeper waters (as it approaches shallower coastal waters, the tsunami slows down, its wavelength shortens, and its height increases).
Head lice and their eggs (nits) soon perish if separated from their human host. Adult head lice can live only a day or so off the human head without blood for feeding. Nymphs (young head lice) can live only for several hours without feeding on a human.
The Life of Lice. About the size of a sesame seed, head lice, six-legged parasites that live on the human head, are hard to see. And nits -- eggs that females glue onto hairs near the scalp -- are even more difficult to spot. But, luckily, lice can't live more than a few days away from the warmth and food the human head provides. And though they are hardy in some ways -- they can survive submersion for up to six hours (that's why swimming and showers don't kill them) -- they can't jump, hop, or fly.