Matters of state and motion - Physical Processes in Earth and Environmental Sciences - page 22

by only 1 m 2 area of the outer atmosphere and equivalent
to the output of a small domestic electric bar heater. The
heat energy available to drive plates is thus minuscule
(though quite adequate for the purpose) by comparison
with that provided to drive external Earth processes like
life's metabolism, hydrological cycling, oceanographic
circulations, and weather.

GEOTHERMALHEAT
65 mW m -2

2.2.3 How does heat travel?

Radiative heat energy is felt from a hot object at a
distance, for example, when we sunbathe or bask in the glow
of a fire, in the latter case feeling less as we move further
away. The heat energy is being transported through space
and atmosphere at the speed of light as electromagnetic
waves.
Conductive heat energy is also felt as a transfer process
by directly touching a hot mass, like rock or water, because
the energy transmits or travels through the substance to
be detected by our nervous system. In liquids we feel the
effects of movement of free molecules possessing kinetic
energy, in metals the transfer of free electrons, and in the
solid or liquid state as the atoms transmit heat energy by
vibrations.
Convection is when heat energy is transferred in bulk
motion or flow of a fluid mass (gas or liquid) that has been
externally or internally heated in the first place by radiation
or conduction.

HEAT ENERGY is required
for life, plate motion, water
cycling, weather, and
convectional circulations

SOLAR HEAT
1,367 W m -2

Fig. 2.4 Heat energy available to drive plates is minuscule
when compared with that provided by solar sources for life, the
hydrological cycle, weather, etc.

average magnitude of which on an imaginary unit surface
placed at the uppermost surface of Earth's atmosphere
facing the sun is now approximately 1,367 W m 2 . This solar
constant is the result of a luminosity which varies by
>0.3 percent during sunspot cycles, possibly more during
mysterious periods of negligible sunspots like the Maunder
Minimum (300-370 years BP ) coincident with the Little Ice
Age. At any point on Earth's surface, seasonal variations in
received radiation occur due to planetary tilt and elliptical
orbit, with longer term variations up to 1 percent due to the
Croll-Milankovitch effect (Section 6.1).
Internal heat energy comes from two sources. A minority,
about 20 percent, comes from the “fossil” heat of the
molten outer core. The remainder comes from the radioac-
tive decay of elemental isotopes like 238 U and 40 K locked
up in rock minerals, especially low density granite-type
rocks of the Earth's crust where such elements have been
concentrated over geological time. However, the total
mass of such isotopes has continued to decrease since the
origin of the Earth's mantle and crust, so that the mean
internal outward heat flux has also decreased with time.
Today, the mean flux of heat issuing from interior Earth is
around 65 mW m 2 (Fig. 2.4), though there are areas of
active volcanoes and geothermally active areas where the
flux is very much greater. The mean flux outward is thus
only some 4.8

2.2.4 Temperature through Earth's atmosphere

The mean air temperature close to the land surface at sea
level is about 15 C. Commonsense might suggest that the

mean temperature increases the further we ascend in the
atmosphere: like Icarus, “flying too close to the sun,”
more radiant energy would be received. In the lower
atmosphere, this commonsense notion, like many, is soon
proved wrong (Fig. 2.5) either by direct experience of
temperatures at altitude or from airborne temperature
measurements. The “greenhouse” effect of the lower
atmosphere (Sections 3.4, 4.19, and 6.1) keeps the surface
warmer than the mean - 20

C or so, which would result in
the absence of atmosphere. Although a little difficult to
compare exactly, since the Moon always faces the same way
toward the Sun, mean Moon surface temperature is
of about this order (varying from 10 5 of the solar constant. To make this

contrast readily apparent, the total output of internal heat
from the area enclosed by a 400 m circumference racetrack
would be about 1 kW, of the same order as that received

130 C on the sunlit

side to C on the dark side). Due to the declining

greenhouse effect, as Earth's atmosphere thins, tempera-
ture declines upward to a minimum of about

158

55 C above