Bài giảng khí hậu học chương 2

G304 – Physical Meteorology and Climatology

Chapter 2
Solar radiation and
the seasons

By Vu Thanh Hang, Department of Meteorology, HUS


2.1 Energy
• Energy is defined as “the ability to do work.”
• The standard unit of energy in the International System
(SI) used in scientific applications is the joule (J).
• 1 joule = 0.239 calories
• Power is the rate at which energy is released, transferred,
or received.
• The unit of power is the watt (W), which corresponds to 1
joule per second
• About one two-billionth of the energy emitted by the Sun is
transferred to Earth as electromagnetic radiation, Æ
absorbed by the atmosphere and surface Æ provide
energy for the movement of the atmos., growth of plants,
evaporation of water, ect.


2.1 Energy (cont.)
• All forms of energy fall into the general categories of
kinetic energy and potential energy.
• Kinetic energy (energy in use) is often described as the
energy of motion.
• Potential energy is energy that has not yet been used,

such as a cloud droplet that occupies some position
above Earth’s surface Æ the droplet is subject to the
effect of gravity Æ as it falls toward Earth’s surface, the
PE is converted to KE.
• The higher the droplet’s elevation, the greater its
potential energy.


2.1 Energy (cont.)


2.1 Energy (cont.)
• Energy can be transferred from one place to another by
three processes: conduction, convection, and radiation.
• Conduction is the movement of heat through a substance
without the movement of molecules in the direction of heat
transfer.
• Conduction is most effective in solid materials, but it also
is an important process in a very thin layer of air near
Earth’s surface.


2.1 Energy (cont.)
• Convection is the transfer of heat by the mixing of a fluid,
is accomplished by displacement (movement) of the
medium
• During the daytime, heating of Earth’s surface warms a
very thin layer of air in contact with the surface Æ air
heated from below expands and rises upward because of
the inherent buoyancy of warm air.

• The

atmosphere

can

undergo

convection

without

buoyancy Æ forced convection Æ the vertical mixing
happens as the wind blows


2.1 Energy (cont.)
Free convection due
to strong heating from
below

Forced convection due
to turbulence created
by horizontal wind flow
over rough surface


2.2 Radiation
• Radiation is the only one that can be propagated without a
transfer medium.

• The transfer of energy by radiation can occur through empty
space.
• Virtually all the energy available on Earth originates from the
Sun.
• Radiation is emitted by all matter.
• Different types of radiation have different effects, all are
transmitted as a sequence of waves
• Radiation consists of both an electrical and a magnetic wave
• When an object emits radiation, both an electrical field and a
magnetic field radiate outward
• The electric and magnetic waves are perpendicular to

one another; rise and fall in unison


2.2 Radiation (cont.)
• Quantity is associated with the height of the wave, or its
amplitude
• Everything else being equal, the amount of energy carried
is directly proportional to wave amplitude
• The quality, or “type,” of radiation is the distance between
wave crests or wavelength (crest - to - crest, trough - to trough)
• All forms of electromagnetic radiation travel through space
at the speed of light (~300,000km/s) Æ takes 8 minutes
for energy from the Sun to reach Earth
• The energy received from the other, more distant stars
take even longer to arrive at Earth


2.2 Radiation (cont.)

Fig. 2-5: Electromagnetic radiation
consists of an
electric wave (E) and a magnetic wave
(M). As radiation travels, the waves
migrate in the direction shown by the pink
arrow. The waves in (a) and (b) have the
same amplitude, so the radiation intensity
is the same. However, (a) has a
shorter wavelength, so it is qualitatively
different than (b). Depending on the
exact wavelengths involved, the radiation
in (a) might pass through the
atmosphere, whereas that in (b)
might be absorbed.


2.2 Radiation (cont.)
Specify wavelengths using
small units called micrometers (or microns).
1 micrometer equals one-millionth of a meter.


2.2 Radiation (cont.)

Electromagnetic Spectrum Chart from: Berkeley Lab. Berkeley
• All

objects radiate energy not merely at one single wavelength but
over a wide range of wavelengths

2.2 Radiation (cont.)
• Perfect emitters of radiation, so-called blackbodies are purely
hypothetical bodies that emit the maximum possible radiation at
every wavelength.
• Earth and the Sun are almost blackbodies.
• The single factor that determines how much energy a
blackbody radiates is its temperature Æ Hotter bodies emit
more energy than do cooler ones.
• Stefan-Boltzmann law: The intensity of energy radiated by a
blackbody increases according to the fourth power of its
absolute temperature.
I = σT4
where I is the intensity of radiation (Wm-2), σ is a constant (5.67
x 10-8 Wm-2K-4) and T is the temperature of the body (K)


2.2 Radiation (cont.)
• Blackbodies do not exists in nature Æ most liquids and solids
can be treated as graybodies Æ they emit some percentage of
the maximum amount of radiation possible at a given
temperature
• The percentage of energy radiated by a substance is referred
to its emissivity, range from just above zero to just below 100%
(ε):
I = ε σT4


2.2 Radiation (cont.)
• For any radiating body, the wavelength of peak emission

(in micrometers) is given by Wien’s law:
λmax = constant/T
where λmax refers to the wavelength of energy radiated with
greatest intensity; the constant rounds off to the value 2900
for T in Kelvins and λmax in micrometers
• Wien’s law tells us that hotter objects radiate energy at
shorter wavelengths than do cooler bodies.
• Shorter wavelengths correspond to higher energies


2.2 Radiation (cont.)
• Solar radiation is most intense in the visible portion of
the spectrum. Most of the radiation has wavelengths less
than 4 micrometers which we generically refer to as
shortwave radiation.
• Earth’s surface and atmosphere radiations consist
mainly of that having wavelengths longer than 4
micrometers. This type of electromagnetic energy is
called longwave radiation.
• Hotter bodies radiate more energy than do cooler
bodies at all wavelengths
• Weather satellites (infrared imagery) measure radiation
intensity to determine the cloud top temperature and also
the cloud thickness


2.2 Radiation (cont.)

Fig. 2-7: Energy radiated by
substances occurs

over a wide range of wavelengths.
Because of its higher temperature,
emission from a unit of area of the
Sun (a) is 160,000 times more intense
than that of the same area on Earth (b).
Solar radiation is also composed of
shorter wavelengths than
that emitted by Earth.


2.3 The solar constant
• The Sun is extremely hot and we are protected from its
great heat by the distance from the solar surface
• Radiation traveling through space carries the same
amount of energy and has the same wavelength as
when it left the solar surface
• At greater distance from the Sun, it is distributed over a
greater area Æ reduces its intensity
• Consider a sphere completely surrounding the Sun,
whose radius is equal to the mean distance between
Earth and the Sun (= 1.5 x 1011m)
• As the distance from the Sun increases, the intensity of
the radiation diminishes in proportion to the distance
squared Æ the inverse square law


2.3 The solar constant (cont.)
• By dividing total solar emission (3.865 x 1026 W) by the
area of our imaginary sphere surrounding the Sun (4πr2)
Æ determine the amount of solar energy received bya

surface perpendicular to the incoming rays at the mean
Earth-Sun distance
• This incoming radiation is:
3.865 ×10 26 W

(

4π 1.5 ×10 m
11

)

2

= 1367 W/m 2

• This value is refered to as solar constant


2.3 The solar constant (cont.)

Fig. 2-9 The intensity of a beam
of solar radiation does not
weaken as it travels away from
the Sun. However, its intensity is
reduced when it is distributed
over a large area.


2.4 The causes of Earth’s seasons

• Although the Sun emits a nearly constant amount of
radiation, on Earth we experience significant changes in
the amount radiation received during a year Æ the
seasons
• We know that the low latitudes received more solar
radiation per year at the top of the atmosphere than do
regions at higher latitudes
• Earth orbits the Sun once every 365 1/4 days as if it
were riding along a flat plane Æ refer to this imaginary
surface as the ecliptic plane and to Earth’s annual trip
about the plane as its revolution


2.4 The causes of Earth’s seasons (cont.)
Fig. 2-10 Earth’s orbit around the Sun is not perfectly
circular but is an ellipse

• Earth is nearest the Sun (perihelion) on or about January 3
(147,000,000 km).
• Earth is farthest from the Sun (aphelion) on or about July 3
(152,000,000 km).


2.4 The causes of Earth’s seasons (cont.)
• Earth also undergoes a spinning motion called rotation.
• Rotation occurs every 24 hours around an imaginary line
called Earth’s axis, connecting the North and South Poles.
• The axis is not perpendicular to the plane of the orbit of Earth
around the Sun but is tilted 23.5° from it.
• No matter what time of year it is, the axis is always tilted in

the same direction and always points to a distant star called
Polaris (the North Star).
• The constant direction of the tilt means that for half the year
the Northern Hemisphere is oriented somewhat toward the
Sun, and for half the year it is directed away from the Sun Æ
cause the seasons (not the varying distance between Earth
and the Sun)


2.4 The causes of Earth’s seasons (cont.)
• The Northern Hemisphere has its maximum tilt toward the
Sun on or about June 21, (June solstice).
• Six months later (on or about December 21), the Northern
Hemisphere has its minimum availability of solar radiation
on the December solstice.
• Intermediate between the two solstices are the March
equinox on or about March 21, and the September
equinox on or about September 21.
• On the equinoxes, every place on Earth has 12 hours of
day and night and both hemispheres receive equal
amounts of energy.


2.4 The causes of Earth’s seasons (cont.)
• The 23.5° tilt of the Northern Hemisphere toward the Sun
on the June solstice causes the subsolar point (where the
Sun’s rays meet the surface at a right angle and the Sun
appears directly overhead) to be located at 23.5° N.
• This is the most northward latitude at which the subsolar
point is located (Tropic of Cancer).

• On the December solstice, the sun is directly overhead at
23.5° S (Tropic of Capricorn).
• On the two equinoxes, the subsolar point is on the
equator.