THEME 3: CLIMATIC ELEMENTS
3.1 INTRODUCTION
According to the International Meteorological Vocabulary (IMV) a climatic
element is “any one of the properties or conditions of the atmosphere which
together specify the physical state of the weather or climate at a given place, for
any particular moment or period of time” (Lowry, 1972). Examples of climatic
elements thus include dry-bulb temperature, wet-bulb temperature, wind speed
and direction, cloud cover, visibility, sunshine duration, soil moisture content,
human discomfort index, etc.
Clearly, the values of some climatic elements are obtainable from suitable
instruments such as wind speed from an anemometer or dry-bulb temperature
from a dry-bulb thermometer or a thermograph. Others such as the cloud cover
or cloud type are not. There is a great variety of climatic elements and the means
of obtaining values for them are equally various. Several references can be
consulted for a more detailed discussion of the climatic elements; however, only
one source is listed here as the information contained therein is sufficient for our
needs.
3.2 RADIATION
Refer to pages 17 – 38 of Robinson & Henderson-Sellers (1999). Additional
notes are provided below:
Energy may be transferred from one place to another by:
• Conduction
• Convection and advection
• Radiation
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,Radiation is the process whereby energy is transferred from one body to another
in the form of electromagnetic waves, without the help of a medium (solid, liquid,
gas, etc.). All matter, at any temperature above absolute zero (0 K) emits such
radiation.
Radiation is the only method of energy transfer that can bridge the vacuum of
outer space, thus energy coming to and leaving planet Earth must be in the form
of radiation. We can actually think of radiation as streams of photons that are
discrete packets of energy, moving along a sinusoidal wave trajectory (packets of
photons make up waves, and groups of waves make up a beam of radiation).
3.2.1 ELECTROMAGNETIC SPECTRUM
A fundamental characteristic of radiation is the wavelength (λ) of propagation. All
bodies radiate at a large number of wavelengths. Radiation is therefore
conceived as containing a continuous range of wavelengths, the totality of which
is called the electromagnetic spectrum (Figure 3.1). The unit of wavelength
commonly used is the micrometer, µm, which is equal in length to 0.001 mm.
Climatologically important wavelengths range from 0.1 to 100 µm. Within this
range the human eye responds only to visible light with wavelengths between
0.4 µm (violet) and 0.7 µm (red). The spectral range between 0.4 µm and 0.7 µm
is also referred to as photosynthetic active radiation (PAR) and is used by
photosynthetic organisms (e.g. plants) in the process of photosynthesis.
Radiation with wavelengths shorter than 0.4 µm is termed ultraviolet (UV), while
radiation with wavelengths longer than 0.7 µm is termed infrared radiation (IR).
Of the sun’s radiation, 9% is in the ultraviolet, 45% in the visible and 46% in the
infrared portion of the electromagnetic spectrum.
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, Figure 3.1 The electromagnetic spectrum.
Shortwave radiation: that portion of the solar spectrum having wavelength
less than 4 µm
Long-wave radiation: the terrestrial and atmospheric radiation which are in
the wavelengths between 4 and 120 µm
Since the spectral distributions of solar and terrestrial radiation overlap very little,
they can very often be treated separately in measurements and computations. In
meteorology, the sum of both types is called total radiation.
3.2.2. RADIATION LAWS
Radiation laws have been developed for theoretically perfect radiators, termed
black bodies. A black body is a body comprising a sufficient number of
molecules absorbing and emitting electromagnetic radiation in all parts of the
electromagnetic spectrum so that:
a) all incident radiation from every wavelength and from all directions are
absorbed
b) all radiation from every wavelength and in all directions are emitted
c) no radiation is thus reflected.
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, The degree to which a real body approaches a black body is given by the
emissivity of the body:
Eλ (realbody)
ελ =
Eλ (blackbody)
where: ελ = emissivity of a radiating object (0 ≤ ε ≤ 1)
Eλ = irradiance - the amount of energy emitted (W m-2 µm-1)
The emissivity of a black body is unity at all wavelengths. In contrast, the
emissivity of natural materials varies with wavelength of emission between
values of 0 and 1.
In general, the radiation laws…
• Refers to objects said to be in thermal equilibrium.
• Does not refer to objects undergoing chemical reactions such as burning
• Provide the maximum amount of radiation that a body can give off as a
function of its temperature.
PLANCK’S LAW:
Determines the basic shape of the emission curve by calculating the radiation
emitted at a particular wavelength, λ, by a black body at temperature T (Figures
3.2 and 3.3):
c1
Eλ =
λ [exp(c 2 / λT ) − 1]
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where: Eλ = irradiance - the amount of energy emitted (W m-2 µm-1)
λ = wavelength (µm)
T = temperature (K)
c1 = 3.74 × 108 W m-2 µm4 (a constant)
c2 = 1.44 × 104 µm K (a constant)
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