Why does skin wrinkle with age? How can you slow or prevent this process?

Suzan Obagi, assistant professor of dermatology at the University of Pittsburgh and director of the Cosmetic Surgery and Skin Health Center, offers this answer:

Wrinkles arise from physical shifts that occur naturally as we grow older—and they are exacerbated by outside influences, such as exposure to sun or tobacco smoke.

As we age, we gradually lose our collagen, a protein fiber that makes our skin firm; as a result, skin becomes thinner and more fragile. We also begin to lose the elastin that gives skin its elasticity and its glycosaminoglycans, or GAGs, which enable it to hold moisture. The result is drier skin with wrinkles that don't go away. Such changes, however, occur slowly and account for only a small amount of our furrows.

The effects of sun, tobacco smoke and pollution, for example, speed up the process. They can thicken parts of our skin, which is why we end up with lesions, skin cancer, freckles and sun spots. These factors can also exaggerate the normal loss of elasticity and firmness. The result is rough, uneven, patchy skin, with deeper creases than growing older by itself would cause.

Keeping skin smooth as long as possible means taking care of it. Every day apply a sunscreen of at least SPF 35, preferably one that contains zinc or titanium. After reaching the age of 25, use Retin-A, a vitamin A derivative with the generic name tretinoin, as an antiaging cream. It is a prescription agent that has been used for more than 30 years with excellent results. Although your skin might peel or flake at first, over time the tretinoin reduces fine lines, the size of pores and brown spots. If tretinoin treatment is not enough, peels and lasers can help build collagen and improve the skin's appearance. Because the regular sloughing off of skin tells the body to make more collagen, the laser treatments, which go a little deeper, boost that response in the skin.

What causes humidity? —W. CHOWANSKI, CAMARILLO, CALIF.

Jeffrey Hovis, a science and operations officer at the National Oceanic and Atmospheric Administration's National Weather Service in Charleston, W.V., explains:

Humidity describes the amount of water in the air, which varies as part of a never-ending cycle of evaporation and condensation. Water on the earth's surface—in lakes, rivers and oceans—evaporates as water vapor. The air continues to absorb the water until it cannot hold it anymore, at which point the water vapor can condense into clouds and return to the earth as rain or snow.

Relative humidity, which TV forecasters often mention, compares the amount of moisture in the air with the total that it could hold. Warm air can carry more water than cool air can. Regardless of temperature, air that bears half as much moisture as it could conceivably contain has a relative humidity of 50 percent. Weather reporters focus on this factor because people feel uncomfortable when high relative humidity accompanies high temperature. That discomfort results because the human body relies on evaporation for cooling. When the air is already saturated, our skin cannot shed sweat.

Meteorologists also measure the air's dew point, the temperature at which saturation occurs. For a good example of this effect, think of a glass of iced tea on a muggy day. Remember that warm air can hold more moisture than cold air. When the air around the tea cools, it reaches its dew point and leaves water on the outside of the glass. In the atmosphere, air that reaches the dew point forms clouds. Instead of condensing on a glass, however, water in air may form on dust particles. When enough air falls below the dew point, it results in a cloud of dust and water that is heavy enough, and precipitation falls.

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