may extend to around 60,000 feet. This oblong nature of the  troposphere is illustrated in  Figure 16-38 . Most civilian aviation takes place in the troposphere in  which temperature decreases as altitude increases. The rate  of change is somewhat constant at about –2 °C or –3.5 °F for  every 1,000 feet of increase in altitude. The upper boundary  of the troposphere is the tropopause. It is characterized as a  zone of relatively constant temperature of –57 °C or –69 °F. Above the tropopause lies the stratosphere. Temperature  increases with altitude in the stratosphere to near 0 °C before  decreasing again in the mesosphere, which lies above it. The  stratosphere contains the ozone layer that protects the earth’s  inhabitants from harmful UV rays. Some civilian flights  and numerous military flights occur in the stratosphere.  Figure 16-39  diagrams the temperature variations in different  layers of the atmosphere.  When an aircraft is flown at high altitude, it burns less  fuel for a given airspeed than it does for the same speed  at a lower altitude. This is due to decreased drag that  results from the reduction in air density. Bad weather and  turbulence can also be avoided by flying in the relatively  smooth air above storms and convective activity that  occur in the lower troposphere. To take advantage of these  efficiencies, aircraft are equipped with environmental  systems to overcome extreme temperature and pressure  levels. While supplemental oxygen and a means of staying  warm suffice, aircraft pressurization and air conditioning  systems have been developed to make high altitude flight  more comfortable.  Figure 16-40  illustrates the temperatures  and pressures at various altitudes in the atmosphere.  Pressurizing an aircraft cabin assists in making flight  possible in the hostile environment of the upper atmosphere.  The following terms should be understood for the discussion  of pressurization and cabin environmental systems that  follows:  16-23 Figure 16-39.  The atmospheric layers with temperature changes  depicted by the red line.  Figure 16-38.  The troposphere extends higher above the earth ’ s  surface at the equator than it does at the poles. Pressurization Terms Pressurization Issues 3. Cabin rate of climb—the rate of change of air pressure  inside the cabin, expressed in feet per minute (fpm)  of cabin altitude change.  2. Cabin differential pressure—the difference between  the air pressure inside the cabin and the air pressure  outside the cabin. Cabin pressure (psi) – ambient  pressure (psi) = cabin differential pressure (psid or  Δ psi). 1. Cabin altitude—given the air pressure inside the  cabin, the altitude on a standard day that has the  same pressure as that in the cabin. Rather than saying  the pressure inside the cabin is 10.92 psi, it can be  said that the cabin altitude is 8,000 feet (MSL).  Temperature Height (km) 90 80 70 60 50 40 30 20 10 –100 –80 –60 –40 –20  0  20  40 50 °C –140 –100 –60  –20 0 20  60  100 120 °F  Mt. Everest Temperature Ozone layer Thermosphere Mesosphere Stratosphere Troposphere Mesopause Stratopause Tropopause 38,000 ft 160,000 ft 160,000 ft N S Equator T r o p o s p h e r e S t r a to s p h e r e $^{M}$e s o s p he r e T h e r m os ph e r e 55,000–65,000 feet 25,000–30,000 feet 137,000–153,000 feet 100,000–116,000 feet