A. When a layer of cold air close to the ground is covered by a layer of warmer air, sound waves traveling upward may be bent, or refracted, by the difference in temperature and redirected toward the ground. An observer standing where the descending sound is focused may therefore hear a sound he would not ordinarily have heard because of his distance from its source.

Q. Why do fluorescent lights give off more light than incandescent lights at the same wattage?

A. Much of the electrical energy in an incandescent light bulb is wasted as heat, while a fluorescent lamp converts electrical energy to light very efficiently with little heat loss. In an incandescent bulb, a filament is heated to white heat by an electrical current. In a fluorescent tube, the electrical current passes through a vacuum containing just enough mercury atoms in the form of vapor to produce invisible ultraviolet light. As this strikes the special coating on the inside of the tube, it is absorbed, causing the coating to emit cool visible light.

Q. In warm climates there are large numbers of plant and animal species, but relatively small numbers of individuals. In Arctic climates there are few species, but large numbers of individuals. Why?

A. In the Arctic region, the harsh climate limits the number of species that can survive. There is less competition for the available food, so a species that has adapted to the rigorous environment can easily expand. In the tropics, there is a longer growing season and fewer weather fluctuations. There are also many different niches in which different species develop, but the competition for food and space is more intense.