Q: Does The Air Get Colder or Warmer at Higher Altitudes?

A: It is reasonable to think that the air closer to the sun should be warmer. But it's not closeness to the sun that influences air temperature, it's absorption of its radiant energy by matter. The earth's atmosphere is layered, like a cake. In some layers the air gets colder and in others it gets warmer. Most of the energy of sunlight passes through the atmosphere and is absorbed by the earth's surface. The surface heats up and warms the air above it. This warm air rises and mixes the lower atmosphere. This layer, about seven miles thick, is called the troposphere. Above the troposphere is the stratosphere where the air begins to get warmer as altitude increases. Unlike the troposphere which is heated from the bottom up, the stratosphere is heated from the top down. The atmosphere is transparent and transmits most of the sun's radiant energy to the surface. But air, like most substances, is not completely transparent. For example, clear window glass selectively absorbs a small amount of red light. The transmitted light is slightly green, as seen on the edge of a glass louver. Like window glass earth's atmosphere selectively absorbs radiant energy except it absorbs ultraviolet and transmits the visible and much of the infrared. In the stratosphere oxygen molecules absorb ultraviolet radiation as they are torn apart into individual oxygen atoms. This heats the air at the top and has two other effects. It absorbs the ultraviolet frequencies and prevents this potentially damaging radiation from reaching the surface and it creates ozone. The absorption and associated heating begins about thirty miles up and continues down to the top of the troposphere until little of the ultraviolet remains. The earth's atmosphere is heated by the sun's energy from both the top and the bottom as it absorbs ultraviolet and transmits visible and infrared radiation to the surface. Through the process called selective absorption, sunlight and atmosphere exist in a thermal balance which creates and maintains an environment that is both suitable and necessary for the earth's life forms, including ourselves.

Richard Brill is assistant professor of science at Honolulu Community College where he teaches earth and physical science. Send questions on any science topic to him at Honolulu Community College, 874 Dillingham Blvd., Honolulu, HI 96817 or email to rickb@hcc.hawaii.edu

Colder or Warmer ©1995 Richard C. Brill