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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Energy in our everyday life
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Energy in everyday life
Energy is the ability or capacity to do work on some form of matter
Any substance may have energy by virtue of its position or motion
A volume of air aloft in the atmosphere has more potential energy
than the same size volume of air just above the Earth’s surface
A strong wind possesses more kinetic energy than a light breeze
Energy exists in different forms as it can be transformed and it can be
conserved (stored)
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Temperature and heat
Temperature is the measure of the average
speed of the atoms and molecules, where
higher temperatures correspond to faster
average speeds
Due to their motion, atoms and molecules
acquire kinetic energy, often referred to as
heat energy
Heat is also the energy transferred from one
object to another due to temperature
difference
Transfer of heat in the atmosphere is by
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Radiant energy
Figure 3 : Radiant energy from the sun
The energy received from the
sun is called radiant energy
(radiation)
Weather and climate on Earth
are determined by the amount
and distribution of radiation
It travels in the form of waves
that release energy when it is
absorbed by an object
The speed, c = 2.9979 ∗ 108
ms−1
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Electromagnetic waves
Wavelength (λ) and frequency (ν) are related as
c = λ ν
Radiation can be described as a flux of particles (photons) or as an
electromagnetic wave, i.e. exhibits a dual nature
The electromagnetic waves do not need molecules for their transfer
It can transverse even in a vacuum
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Laws of radiation
Stefan-Boltzmann relation:
E = σT4
All objects with temperatures above absolute zero (0 K) emit
radiation
Wien’s displacement law
λmax T = constant; Theconstant ≈ 2, 897 µmK
Determines the wavelength at which any object radiates most of its
energy
Kirchhoff’s law:
Good absorbers are good emitters at a particular wavelength, and
poor absorbers are poor emitters at the same wavelength
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Sun’s and Earth’s radiation
Figure 4 : Electromagnetic spectrum
Solar radiation
(shortwave) is mostly
emitted at wavelengths
less than 2 µm;
⇒ Maximum near 0.5 µm
Terrestrial radiation
(longwave) is mostly
emitted at about
5 − 25 µm
⇒ Maximum near 10 µm
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Sun’s and Earth’s radiation
Earth’s radiation means the sum of radiation which is emitted from
the Earth’s surface and its atmosphere
Earths observed average surface temperature is 288 Kand the sun’s
surface temperature is 6000 K
All objects that radiate energy, absorb it as well. The rate at which
an object radiates and absorbs energy depends on its surface
characteristics, such as colour, texture, moisture, and temperature.
A black object is a good absorber especially for visible light,
converting it into internal energy, and so raise its temperature
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Sun’s and Earth’s radiation
A blackbody is any object that perfectly absorbs and perfectly emits
radiation
At any time the Earth is partly in sunlight and partly in darkness
As a blackbody, it absorbs solar radiation and emits infrared radiation
at equal rates at a radiative equilibrium temperature, which is about
255 K
The rate at which an object radiates and absorbs energy depends on
its surface characteristics, such as colour, texture, moisture, and
temperature
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Sun’s and Earth’s radiation
The temperature is however lower than its surface temperature due to
absorption and emission of Earth’s atmosphere
The atmosphere absorbs some wavelengths of radiation and is
transparent to others
It is a selective absorber as it selectively absorbs and emit radiation
Most gases in the atmosphere are selective absorbers and emitters
Both water vapour (H2O) and carbon dioxide (CO2) are strong
absorbers of infrared radiation and poor absorbers of visible solar
radiation
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Sun’s and Earth’s radiation
Other selective absorbers include nitrous oxide (N2O), methane
(CH4), and ozone (O3); while oxygen (O2) and nitrogen (N2) are poor
absorbers of infrared energy
So most of the infrared energy emitted from Earth’s surface keeps the
lower atmosphere warm
(H2O) and (CO2) also selectively emit radiation at infrared
wavelengths in all directions
In turn, the Earth constantly radiates infrared energy upward, where
it is absorbed and warms the lower atmosphere
ndettoel@2016 ENV 111: Introduction to Meteorology
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Sun’s and Earth’s radiation
. . . .. . . .
The spectrum
region between
8 − 11 µm is
known as an
atmospheric
window
At this region
neither water
vapour nor CO2
readily absorb
infrared radiation
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Sun’s and Earth’s radiation
The Earth constantly radiates infrared energy upward, where it is
absorbed and warms the lower atmosphere
Clouds are also very effective emitters of infrared radiation both from
their top and base, emitting respectively radiation to space and to the
Earth’s surface
Thus nights with the presence of clouds are warmer in comparison
with nights with clear sky
The absorption of infrared radiation from Earth by water vapour and
CO2 is popularly called the greenhouse effect.
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Sun’s and Earth’s radiation
Table 1 : Strong and weak absorption of solar radiation by most important greenhouse
gases at different wavelengths
Spectrum region (λ) Ability of radiation absorption
Bellow 5 µm Insignificant absorption
5 − 8 µm Strong absorption from H2O
8 − 12 µm Weak absorption from H2O and O3
12 − 17 µm Strong absorption from CO2
17 − 19 µm Weak absorption from H2O
Above 19 µm Strong absorption fromH2O
However, the sun’s rays appears to remain fairly constant at about
1361 W/m2
. This value is called the solar constant
ndettoel@2016 ENV 111: Introduction to Meteorology
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Earth’s annual energy balance
For an equilibrium climate, the outgoing longwave radiation (OLR)
should necessarily balance the incoming absorbed solar radiation
(ASR),
Incoming radiant energy may be scattered and reflected by clouds and
aerosols or absorbed in the atmosphere
The energy budget is determined between the combined energy of the
Earth and the atmosphere against the Sun’s energy. The balance
must also happen between Earth’s surface and the atmosphere
If no energy balance, the Earth’s average surface temperature would
change. The energy balance is determined for a year
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Earth’s annual energy balance
Given 100 units of solar energy reach the top of Earth’s atmosphere
(TOA)
On the average, clouds, Earth, and the atmosphere reflect and scatter
30 units back to space, and that the atmosphere and clouds together
absorb 19 units, which leaves 51 units of direct and indirect solar
radiation to be absorbed at Earth’s surface
The 51 units reaching the surface areused as:
⇒ 23 units is used to evaporate water
⇒ 7 units lost through conduction and convection
⇒ 21 units are radiated away as infrared energy
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Earth’s annual energy balance
Figure 6 : Simple annual energy budget
ndettoel@2016 ENV 111: Introduction to Meteorology
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Factors affecting the sun’s radiation flux
Figure 7 : Sunlight at a location
Certain factors can affect the
amount of radiation flux
received at a location on
Earth
Astronomical factors:
⇒ Earth-Sun distance,
⇒ Sun’s height above the
horizon
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Factors affecting the sun’s radiation flux
Geographical factors
⇒ Latitude (φ)
⇒ The height of the site from mean sea level
⇒ The topography of the area around the site
⇒ Reflection (albedo) of the surface
Geometrical factors
This is about the location of the surface in relation to the incoming
radiation
Radiation may also decrease due to absorption and scattering in the
atmosphere
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Introduction Radiant energy Energy spectrum Annual energy Factors affecting radiation
Factors affecting the sun’s radiation flux
Table 2 : Typical albido of various surfaces
Surface Reflection (%)
Fresh snow 75 − 95
Old snow, dirty 40 − 60
Ice 70
Water (average value) 10
Surface without vegetation 15
Sand 18 − 28
Forest 5 − 10
Grass 15 − 25
Inhabited areas (towns) 14 − 18
Cloud (thick layer) 50 − 85
Cloud (thin layer) 5 − 50
Albedo is the percent
of radiation returning
from a given surface
compared to the
amount of radiation
initially striking that
surface
Represents the
reflectivity of the
surface
ndettoel@2016 ENV 111: Introduction to Meteorology
