Page 106 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

70 part I The energy–atmosphere System
the amount of heat will be small. The thermosphere is not “hot” in the way we are familiar with, because the den- sity of molecules is so low there that little actual heat is produced. The thermosphere would actually feel cold to us because the number of molecules is not great enough to transfer heat to our skin. (Scientists measure tempera- ture indirectly at those altitudes, using the low density, which is measured by the amount of drag on satellites.) Closer to Earth’s surface, the atmosphere is denser. The greater number of molecules transmits their kinetic en- ergy as sensible heat, meaning that we can measure and feel it. Further discussion of heat and temperature is in Chapters 4 and 5; discussion of heat as it relates to den- sity is in Chapter 7.
Mesosphere The mesosphere is the area from 50 to 80 km above Earth and is within the homosphere. As Figure 3.3 shows, the mesosphere’s outer boundary, the mesopause, is the coldest portion of the atmosphere, averaging –90°C, although that temperature may vary considerably (by 25–30 C°). Note in Figure 3.2b the ex- tremely low pressures (low density of molecules) in the mesosphere.
The mesosphere sometimes receives cosmic or meteoric dust particles, which act as nuclei around which fine ice crystals form. At high latitudes, an observer at night may see these bands of ice crystals glow in rare and unusual noctilucent clouds, which are so high in altitude that they still catch sunlight after sunset. For reasons
▼Figure 3.3 Temperature profile of the atmosphere, highlighting the troposphere. [naSa.]
480 400
200
80
50
20 10
Thermopause
Thermosphere
Equatorial tropopause
∆ = Temperature change
Midlatitude tropopause
Polar tropopause
–50 –40 –30 –20 –10 0 10 20°C
Temperature profile
–90°C Mesosphere
Stratosphere
Auroras
Mesopause Noctilucent clouds
18 16 14 12 10
8 6 4 2
0
–57°C Troposphere
Normal lapse rate Equatorial tropopause
Mount Everest
Mesosphere
0°C:–90–30 015324008001200 (a) Temperature profile plots temperature changes with altitude.
Decreasing temperature
0°C
Stratopause
(b) Temperature decreases with increased altitude at the normal lapse rate.
(c) A sunset from orbit shows a silhouetted cumulonimbus thunderhead cloud topping out at the tropopause.
Thunderhead
Normal lapse rate ∆6.4 C° per 1000 m (1km)
Increasing altitude (km)
Kilometres