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.
When lice start living in hair, they also start to lay eggs, or nits. Lice can survive up to 30 days on a person's head and can lay eight eggs a day. Lice attach their nits to pieces of hair, close to the scalp. If you see a small, oval blob on a strand of hair, that's probably a nit.