air currents within the layer. (Graedel 1998)The upper boundary of the layer ranges in height from 8 km in high latitudes, to 18 km above the equator. Its height also varies with the seasons, highest in the summer and lowest in the winter. A narrow zone called the tropopause separates the troposphere from the next highest layer called the stratosphere. (Graedel 1998) Air temperature within the tropopause remains constant with increasing altitude. Stratosphere The stratosphere is the second major stratum of air in the atmosphere. It resides between 10 and 50 km above the planet's surface. The air temperature in the stratosphere remains relatively constant up to an altitude of 25 km. (Graedel 1998) Then it increases gradually to 200-220 degrees Kelvin (K) at the lower boundary of the stratopause (~50 km), which is marked by a decrease in temperature. Because the air temperature in the stratosphere increases with altitude, it does not cause convection and has a stabilizing effect on atmospheric conditions in the region. (Graedel 1998) Ozone plays the major role in regulating the thermal regime of the stratosphere, as water vapor content within the layer is very low. Temperature increases with ozone concentration. Solar energy is converted to kinetic energy when ozone molecules absorb ultraviolet radiation, resulting in heating of the stratosphere. (Graedel 1998)The ozone layer is located at an altitude between 20-30 km. Approximately 90 % of the ozone in the atmosphere resides in the stratosphere. Ozone concentration in this region is about 10 parts per million by volume as compared to approximately 0.04 parts per million by volume in the troposphere. (Graedel 1998) Ozone absorbs the bulk of solar ultraviolet radiation in wavelengths from 290 nm - 320 nm. These wavelengths are harmful to life because they can be absorbed by the nucleic acid in cells. Increased penetration of ultraviolet radiation to the planet's surface would damage plant ...
