Page 107 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 3 earth’s Modern atmosphere 71
not clearly understood, these unique clouds are on the increase and may be seen in the midlatitudes. See, among several sites, lasp.colorado.edu/science/atmospheric/.
Stratosphere The stratosphere extends from about 18 to 50 km above Earth’s surface. Temperatures increase with altitude throughout the stratosphere, from –57°C at 18 km to 0°C at 50 km, the stratosphere’s outer bound- ary, called the stratopause. The stratosphere is the loca- tion of the ozone layer. Stratospheric changes measured over the past 25 years show that chlorofluorocarbons are increasing and ozone concentrations decreas- ing, discussed in Focus Study 3.1 later in this chap- ter. Greenhouse gases are also on the increase; a noted stratospheric cooling is the response.
Troposphere The troposphere is the final layer encoun- tered by incoming solar radiation as it surges through the atmosphere to the surface. This atmospheric layer sup- ports life, the biosphere, and is the region of principal weather activity.
Approximately 90% of the total mass of the atmo- sphere and the bulk of all water vapour, clouds, and air pollution are within the troposphere. An average tem- perature of –57°C defines the tropopause, the tropo- sphere’s upper limit, but its exact altitude varies with the season, latitude, and surface temperatures and pressures. Near the equator, because of intense heating from the surface, the tropopause occurs at 18 km; in the middle latitudes, it occurs at an average of 12 km and at the North and South Poles, it averages only 8 km or less above Earth’s surface (Figure 3.3b). The marked warm- ing with increasing altitude in the stratosphere above the tropopause causes the tropopause to act like a lid, generally preventing whatever is in the cooler (denser) air below from mixing into the warmer (less dense) stratosphere (Figure 3.3c).
Figure 3.3b illustrates the normal temperature pro- file within the troposphere during daytime. As the graph shows, temperatures decrease rapidly with increasing al- titude at an average of 6.4 C° per km, a rate known as the normal lapse rate.
The normal lapse rate is an average. The actual lapse rate may vary considerably because of local weather con- ditions and is called the environmental lapse rate. This
▲Figure 3.4 Absorption of wavelengths above Earth’s surface. as shortwave solar energy passes through the atmosphere, the shortest wavelengths are absorbed. Only a fraction of the ultraviolet radiation, but most of the visible light and shortwave infrared, reaches earth’s surface.
Animation
Ozone Breakdown Ozone Hole
SPECTRUM OF SHORTWAVE ENERGY
480 km
Gamma rays
X-rays 80 km
50 km
18 km 0 km
Ionosphere
Ozonosphere
Shortwave infrared
Ultraviolet Visible light
Earth
variation in temperature gradient in the lower tropo- sphere is central to our discussion of weather processes in Chapters 7 and 8.
Atmospheric Function Criterion
According to our final atmospheric criterion of func- tion, the atmosphere has two specific zones, the iono- sphere and the ozonosphere (ozone layer), which together remove most of the harmful wavelengths of incoming solar radiation and charged particles. Figure 3.4 gives a general depiction of the absorption of radiation by these functional layers of the atmosphere.
Ionosphere The outer functional layer, the ionosphere, extends throughout the thermosphere and into the meso- sphere below (Figure 3.1). The ionosphere absorbs cosmic rays, gamma rays, X-rays, and shorter wavelengths of ul- traviolet radiation, changing atoms to positively charged ions and giving the ionosphere its name. The glowing auroral lights discussed in Chapter 2 occur principally within the ionosphere.
CRiTiCALthinking 3.1 Where Is Your Tropopause?
On your next flight, as the plane reaches its highest alti- tude, see if you can learn what the temperature is outside the plane. (Some planes display it on video screens; or the flight attendant may have time to check it for you.) By defi- nition, the tropopause is wherever the temperature –57°C occurs. Depending on the season of the year, altitude, and temperature make an interesting comparison. is the tropo- pause at a higher altitude in summer or winter? •