Energy and Changes in the Environment - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 40

power. This radiant power is known as the solar constant even though it
varies by a few tenths of a percent over 11-year-long sunspot cycles. Since
the sun does not shine all the time on every square meter of the earth, the
actual amount of incoming energy is about 340 watts per square meter.
Since the earth has temperature, it emits radiant energy called thermal
radiation or planetary infrared radiation. Measurements by satellites show
an average radiant emission from the earth of about 240 watts per square
meter. This is equivalent to the radiation that a black body would emit if
its temperature is at -19°C (-3°F). This is also the same energy rate as the
solar constant averaged over the earth's surface minus the 30% reflected
radiation. This shows that the amount of radiation emitted by the earth
is closely balanced by the amount of solar energy absorbed and since
the earth is in this state of balance, its temperature will change relatively
slowly from year to year.
If more solar energy is absorbed than infrared radiation emitted, the
earth would warm and a new equilibrium would appear. But, if the earth
had more clouds, it would reflect more solar radiation and absorb less.
This would have a cooling effect on the planet, lowering the amount of
infrared radiation that is escaping to space to balance the lower amount
of absorbed solar energy. The earth's radiant energy balance today is 240
watts per square meter.
The amount of energy the earth absorbs from the sun is the same
amount it radiates back to space on average over the 500 trillion square
meters of surface area. Satellites above the earth's atmosphere can measure
the outgoing thermal radiation and show this balance to a high degree of
precision.
An average of temperature records on the earth's surface over a
year indicates that the earth's average surface temperature is about 14°C
(57°F). But, the earth's 240 watts per square meter of thermal infrared
radiation as measured by satellite is equivalent to the radiation emitted
by a black body whose temperature is about -19°C (-3°F), not the 14°C
(57°F) average measured at the earth's surface. The 33°C (60°F) difference
between the apparent temperature of the earth as seen in space and the
actual temperature of the earth's surface is attributed to the greenhouse
effect.
The solar heat absorbed by the atmosphere and the earth's surface
goes back into the atmosphere through the evaporation of water. Thermals
are formed by the heating of air in contact with a warm surface and the
upward emission of energy.