Solar radiation is the driving power source for wind, waves, weather, river and ocean currents.
1. Radiation and Temperature
All
objects emit electromagnetic radiation as long as its temperature is greater
than absolute 0 degree.
Electromagnetic
radiation is a collection or spectrum of waves of a wide range of wavelengths
traveling quickly away from the surface of the subject. Wavelength describes the distance separating
one wave crest from the next wave crest.
The unit we measure wavelength is the micrometer.
A
micrometer is one millionth of a meter.
Light
is the radiation visible to our eyes and heat radiation, though not visible, is
easily felt when you hold a warm object.
There
is an inverse relationship between the wavelength of the radiation that an
object emits and the temperature of the object.
For example, the sun, a very hot object, emits radiation with short
wavelengths. In contrast, the earth, a much cooler object, emits radiation with
longer wavelengths.
Also
Hotter objects radiate more energy than cooler ones.
The
sun, with the surface temperature of 6000 C, emits radiation which can be
divided into three major portions, based on wavelengths: ultraviolet (0.2-0.4),
visible light (0.4-0.7), shortwave infrared (0.7-3)
The
sun does not emit all wavelengths of radiation equally,
the intensity of solar energy is strongest in visible wavelengths.
As
we said before, the wavelength of an object emits is inversely related to the
temperature of the object. Since the earth is much cooler than the sun, it
emits thermal infrared wave, which is also called longwave
radiation.. As we can see, the longwave
radiation emitted from the earth has three peaks at about 5, 10, and 20
um. Between these wavelengths,
atmospheric gases, like water vapor and carbon dioxide, absorb much of the
radiation leaving the surface.
2. Insolation
Insolation stands for incoming solar radiation.- the
flow of solar energy intercepted by an exposed surface for the case of a
uniformly spherical earth with no atmosphere.
Daily
insolation, which is the average insolation
rate over a 24-hour day, depends on two factors: (1) the angle at which the
sun’s rays strike the earth, and (2) the length of time of
exposure to the rays.
From
figure 2.7, we can see that daily insolation changes
over the year at all latitudes. At
equator, there are two periods of maximum daily insolation,
they occur on nearly equinoxes (March 21 & September 23), when the sun is
overhead at the equator. There are also
two minimum periods near the solstices (June 21 & December 23), when the subsolar point moves farthest north and south from the
earth. All latitudes between the tropics of cancer and the tropics of capricon (23.5 degrees) have two maximum and minimum
values. However, beyond these range,
become single peak which occur near summer (June) solstice in north hemisphere.
Annual
Insolation by latitude.
The
rate of insolation averaged over an entire year by
latitude. It decreases from equator to
poles.
3. Earth’s energy budget
In
a long run, there apparently is a perfect balance between the totle amount of insolation
received by Earth and its atmosphere on one hand, and the total amount of long
wave radiation from Earth and atmosphere.
Otherwise Earth would be getting progreesively
warmer or cooler. The annual balance
between incoming and outgoing radiation is the global heat budget.
Absorption. When insolation
strikes an object and is absorbed, the temperature of the object increases.
Different materials vary in their absorptive capabilities. Mineral materials
(rock, soil) are generally excellent absorbers; snow and ice are poor
absorbers.
Reflection Reflection is the ability of an object
to repel waves without altering either the object or the waves without altering
either the object or the waves. Thus in
some cases insolation striking a surface in the
atmosphere or on Earth is bounced away, unchanged, in the general direction
from which it came, much like a mirror reflection, where nothing is changed.
Scattering Particulate matter and gas molecules in
the air sometimes deflect light waves amd redirect
them in a process known as scattering.
This deflection involves a change in the direction of the light wave but
no change in wavelength. The percentage
of shortwave radiant energy scattered upward by a surface is called albedo.
Transmission
is the process whereby a wave passes completely through a medium, as when light
waves are transmitted through a pane of clear, colorless glass. There is
obviously considerable variability among mediums in their capacity to transmit
rays. For example, Earth materials are very poor transmitters of insolation; sunlight is absorbed at the surface of rock or
soil and does not penetrate at all. Water, on the other hand, transmits
sunlight well. The atmosphere transmits
a considerable amount of shortwave solar radiation, but it is not nearly as
effective a transmitter of long-wave radiation. In simplest terms, solar energy
readily penetrates to Earth’s surface, but reradiated the longwave
radiation is mostly trapped in the atmosphere and much of it is reflected back
toward the ground. This entrapment keeps Earth’s surface and lower atmosphere
at a higher average temperature than would be the case if there were no
atmosphere, so called greenhouse effect.
Conduction
The movement of heat energy from one molecule to another without changes in
their relative positions is called conduction.
Earth’s land surface warms up rapidly during the day because it si a good heat absorber, and some of that warmth is
transferred away from the surface by conduction. A small part is conducted
deeper underground, but not much because earht
materials are not good conductors. Most
of this absorbed heatr is transferred to the lowest
portion of the atmosphere by conduction from the ground surface. Air, however,
is a poor conductor, and so only the air layer touching the ground is heated
very much.
Convection In convection, heat is
transferred from one point to another by a moving substance. The difference between conduction and
convection is that molecules physically move away from the heat source in
convection, while in conduction not. Like the heated air in the lower
atmosphere rising and transfer heat upward, this is convection.
Latent
heat.we know water have three
phases: liquid, solid and gas. The change of phases involve either the storage or the release
of energy, depending on the process. The
two most common state changes are evaporation, in which liquid water is
converted to gaseous water vapor, and condensation, in which gaseous water vaporcondenses to liquid water. In evaporation, energy id stored as latent
heat (latent is from the Latin, “lying hidden”), in condensation, the latent
heat is released.
Now
let’s see the global energy budgets of the atmosphere and surface.
The
global energy budget for the earth’s surface and atmosphere is seeing in the
Figure 2.13. We begin with a discussion of shortwave radiation, shown in the
left part of the figure. We assuming
there are 100 units insolation reaching the top of
the atmosphere.
As shown, reflection by molecules and dust, clouds, and the surface totlas 31 percentage units. This is the albedo of the earth-atmosphere system. The remainer (69units) is absorbed by the ground and
atmosphere. Earth eventually reradiates all 69% into space as infrared
radiation: 21% (atmosphere heating) +45% surface heating and 3% ozone emission.
Furthermore,
let’s see (hydrology textbook)