Geoscience Reference
In-Depth Information
energy). The energy is transferred in a variety of forms
and, during these transfers, it undergoes numerous
transformations.
While the general principles of flow are known, the
figures quoted are, in most cases, best estimates. Measure-
ments have been taken at a number of places, but in
insufficient quantities to give a reliable global figure. It is
little use giving a global average based on a few clustered
observations. This has been one of the problems in
determining the magnitude of any 'enhanced' greenhouse
effect. Satellite observations have helped (and led to
appreciable changes in estimates of Earth's albedo) but
there are still numerous flows which are imperfectly
known. Long-wave emission by the atmosphere, the
separation into direct and diffuse radiation and sensible
and latent heat transfer are the main problems, as
conditions vary quickly, and, until measurements become
more comprehensive, some of the figures are little more
than intelligent guesses. The actual value of the flows will
depend, in part, on the nature of the assumptions made
about them. What we can be sure about, both theoretically
and from satellite measurements, is that
what energy comes
into the earth/atmosphere system must eventually leave
.
1000
20
800
Temperature
Radiation
input
600
10
400
200
0
0
0
6
12
18
24
Hours
and temperature on a clear day.
occur in response to the daily progress of the sun. As we
shall see later, the associated changes in temperature may
lead to significant changes in humidity, and they often
spark off major atmospheric processes such as vertical
movements of air and even heavy storms.
It is also apparent that this daily pattern of insolation
and temperature change itself varies according to
atmospheric conditions. The effects are most obvious
when the air is clear and still, for then heating and cooling
proceed uninterrupted. If the sky is cloudy or very
hazy, however, the daily pattern of temperature is much
more variable (
Figure 3.5
).
Similarly, the pattern varies
spatially. It is less marked over the sea, for much more of
the incoming energy is used to heat up and evaporate the
water, and less is returned directly to heat the atmosphere.
During the night the sea cools slowly, with the result that
temperatures do not fall so much as on land - one reason
why coastal areas are less prone to night-time frost (
Figure
climates. There incoming radiation is large, and little
energy is used for evaporation, so temperatures are high,
while radiational cooling at night is intense, giving rise at
times to low air temperatures.
EFFECTS UPON TEMPERATURE
Let us now consider briefly the effects that these energy
inputs and outputs have upon temperature. Firstly we will
look at the daily pattern, then the seasonal one.
Daily pattern
If we consider a clear spring day in an area of, say, London,
sunrise will be at about 6.00 a.m. local time. Temperatures
then are low, for during the night the ground has been
losing heat by radiational cooling. Slowly, as the sun rises,
the ground warms up and, in turn, the air in contact with
the surface is heating too (
Figure 3.13
).
Between about
2.00 and 3.00 p.m. the ground and air are usually at their
warmest, the maximum temperature at the surface being
earlier than that in the air because that is where the heat
conversion takes place. From then on, as the sun gradually
sinks, the ground surface and the overlying air will
cool. The sun sets at about 6.00 p.m.; cooling continues
throughout the night until minimum temperatures are
reached just before dawn.
This daily variation in insolation and temperature is
one of the most basic components of our climate. So
obvious is it and so regular that we take it for granted. And
yet quite marked differences in atmospheric conditions
Seasonal pattern
A very similar pattern of variation takes place on a
seasonal scale. The cause in this case is not Earth's rotation
but its changing relationship with the sun: the variation
within its orbit that produces the apparent seasonal
progress of the sun from the Tropic of Cancer to the
Tropic of Capricorn and back.
This change in the position of the sun leads to
changes in the angle of the incoming rays and in the
duration of daylight. Both factors influence the amount
