IEC 60721-2-4:2018

IEC 60721-2-4 ®
Edition 2.0 2018-06
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
Classification of environmental conditions –
Part 2-4: Environmental conditions appearing in nature – Solar radiation and
temperature
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IEC 60721-2-4 ®
Edition 2.0 2018-06
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
Classification of environmental conditions –

Part 2-4: Environmental conditions appearing in nature – Solar radiation and

temperature
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 19.040 ISBN 978-2-8322-5847-7

– 2 – IEC 60721-2-4:2018 RLV © IEC 2018
CONTENTS
FOREWORD . 3
1 Scope . 5
Object .
2 Normative references. 5
3 Terms and definitions . 5
4 General . 5
5 Solar radiation physics . 6
6 Levels of global radiation . 7
6.1 Maximum levels . 7
6.2 Mean monthly and annual global solar radiation . 7
6.3 Simultaneous values of maximum air temperatures and solar radiation . 8
6.4 World distribution of daily global irradiation . 8
7 Minimum levels of atmospheric radiation at night . 8
Annex A (informative) World distribution of daily global irradiation . 11
Bibliography . 16

Figure – Atmospheric radiation from a clear night sky .
Figure 1 – Spectra of electromagnetic radiation from the Sun and the surface
of the Earth . 10
Figure A.1 – Mean relative global irradiation for the month of June (in %) . 13
Figure A.2 – Mean relative global irradiation for the month of December (in %) . 14
Figure A.3 – Mean relative global irradiation for the year (in %) . 15

Table 1 – Typical peak values of global irradiance (in W/m from a cloudless sky) . 7
Table A.1 – Mean daily extra-terrestrial global irradiation (kWh/m ) . 12

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CLASSIFICATION OF ENVIRONMENTAL CONDITIONS –

Part 2-4: Environmental conditions appearing in nature –
Solar radiation and temperature

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
This redline version of the official IEC Standard allows the user to identify the changes
made to the previous edition. A vertical bar appears in the margin wherever a change
has been made. Additions are in green text, deletions are in strikethrough red text.

– 4 – IEC 60721-2-4:2018 RLV © IEC 2018
International Standard IEC 60721-2-4 has been prepared by IEC technical committee 104:
Environmental conditions, classification and methods of test.
This second edition cancels and replaces the first edition published in 1987 and
Amendment 1:1988. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) Figures updated including the addition of global irradiation information,
b) Format updated.
The text of this International Standard is based on the following documents:
FDIS Report on voting
104/800/FDIS 104/803/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 60721 series, published under the general title Classification of
environmental conditions, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The “colour inside” logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this publication using a colour printer.

CLASSIFICATION OF ENVIRONMENTAL CONDITIONS –

Part 2-4: Environmental conditions appearing in nature –
Solar radiation and temperature

1 Scope
This part of IEC 60721 presents a broad division into types of solar radiation areas. It is
intended to be used as part of the background material when selecting appropriate severities
of solar radiation for product applications.
All types of geographical areas are covered, except areas with altitudes above 5 000 m.
When selecting severities of solar radiation for product applications, the values which are
given in IEC 60721-1 should be applied.
2 Object
This document also serves to define limiting severities of solar radiation to which products are
liable to be exposed during transportation, storage and use.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
4 General
Solar radiation can affect products primarily by the heating of material and their environment
or by photochemical degradation of material.
The Solar radiation, especially its ultraviolet content of solar radiation, causes photochemical
degradation of most organic materials. Elasticity and plasticity of certain rubber compounds
and plastic materials are affected. Optical glass may become opaque.
Solar radiation bleaches out colours in paints, textiles, paper, etc. This can be of importance,
for example for the colour-coding of components.
The heating of material is the most important effect a consequence of exposure to solar
radiation. The presentation of severities of solar radiation is therefore related to the power
density radiated towards a surface, or irradiance, expressed in W/m .

– 6 – IEC 60721-2-4:2018 RLV © IEC 2018
An object subjected to solar radiation will attain a temperature that depends primarily on the
surrounding air temperature, the energy radiated from the Sun, and the incidence angle of the
radiation on the object. Other factors, for example wind and heat conduction to mountings,
can be of importance. In addition, the absorptance α of the surface for the solar spectrum
s
is of importance.
An artificial air temperature t may be defined, which, under steady-state conditions, results in
s
the same surface temperature of an object as the combination of the actual air temperature t
u
and the solar radiation of the irradiance E.
An approximate value can be obtained from the following equation:
α ⋅E
s
t =t +
s u
h
y
The coefficient h is the heat transfer coefficient for the surface, in W/(m · °C). It includes
y
thermal radiation to the surroundings, heat conduction of the surface material and convection
due to wind.
The absorptance α , depends on the thermal colour, the reflectance and the transmittance of
s
the surface.
Typical clear sky values for common materials are:
α = 0,7
s
h = 20 W/(m · °C)
y
E = 900 W/m
resulting in an "over temperature" due to solar radiation of about 30 °C. It can then be seen
that an error of 10 % in the estimation of the intensity of the solar radiation will influence the
temperature involved by less than 5 °C. Therefore, there is no need in this classification for
extremely accurate severities of solar radiation and minor factors affecting the heat radiated
have therefore been disregarded here.
The heating effect is caused mainly by short-term radiation of high intensity, i.e. the solar
radiation around noon on cloudless days. Such values are presented in Table 1.
It may also be of interest to identify the lowest possible value of atmospheric radiation during
clear nights in order to determine the “under temperature” of products exposed to the night
sky.
Such values are given in figure 1.
5 Solar radiation physics
The electromagnetic radiation from the Sun to the Earth covers a rather broad the spectrum
from the ultraviolet to the near infra-red. Most of the energy reaching the surface of the Earth
is in the wavelength range of 0,3 µm to 4 µm with a maximum in the visible range around
0,5 µm. Typical spectra are shown in Figure 1.
The amount of radiant energy from the Sun which falls upon the unit area of a plane normal to
the Sun's rays just outside the atmosphere at the mean distance from the Earth to the Sun is
2 2
called the solar constant. Its value is approximately 1,37 kW/m 1 367 W/m .

The distance from the Earth to the Sun varies during the year, and consequently the radiation
2 2
varies from approximately 1,41 kW/m in January to approximately 1,32 kW/m in July.
Approximately 99 % of the energy of the Sun is emitted at wavelengths below 4 µm. Most
of the energy below 0,3 µm is absorbed by the atmosphere and does not reach the surface of
the Earth. Further absorption and scattering of the radiation takes place, due to particles and
gases, during passage through the atmosphere. The scattering of the direct solar radiation in
the atmosphere results in diffuse radiation from the sky. Thus, the energy received at a
certain place on Earth is the sum of the direct solar radiation and the diffuse solar radiation,
which is referred to as "global radiation". From the point of view of heating effects, this sum is
of interest and the levels given in this document are therefore related to global radiation.
6 Levels of global radiation
6.1 Maximum levels
The maximum level of global radiation on a clear day occurs at noon. The highest value of
the power achieved on a cloudless day at noon at a surface perpendicular to the direction
of the Sun depends on the content of aerosol particles, ozone and water vapour in the air. It
varies considerably with geographical latitude and type of climate.
The global radiation on a surface perpendicular to the direction of the Sun may can reach a
value of 1 120 W/m in a range of 280 nm to 3 000 nm at noon on a cloudless day with
approximately 1 cm of water vapour content, 2 mm of ozone and aerosols of β = 0,05, where β
is the Ångström turbidity coefficient. The value 1 120 W/m is typical for flat land far away
from industrial areas and from large cities at solar elevations exceeding 60°.
NOTE 1 The water vapour content of a vertical column of the atmosphere is measured as the height, in
centimetres, of the corresponding precipitated water. Analogously, the ozone content of a vertical column of the
atmosphere is measured as the height of the corresponding ozone column at normal temperature and pressure.
The scattering and absorption by aerosol particles is expressed by the Ångström turbidity coefficient, which is the
optical depth of the atmosphere with respect to extinction of monochromatic radiation of wavelength λ = 1 µm.
NOTE 2 During partly clouded days the global solar irradiation can increase up to 1 300 W/m for a few minutes.
This short-term phenomenon occurs when the Sun comes out behind the clouds and the radiation is reflected from
the edges of the clouds.
The direct solar radiation decreases with increasing turbidity. Turbidity is high in subtropical
climates and in deserts where the concentration of particles in the air is high. It is also high in
large cities and low in mountainous areas.
The levels in Table 1 are recommended for application as peak values of global irradiance at
noon, experienced by a surface perpendicular to the direction of the Sun in a cloudless sky.
The level varies only by a few per cent within the hours nearest to noon and can therefore be
assumed to be representative for a few hours at a time.
Table 1 – Typical peak values of global irradiance
(in W/m from a cloudless sky)
Area Large cities Flat land Mountainous areas
Subtropical climates and deserts 700 750 1 180
Other areas 1 050 1 120 1 180
6.2 Mean monthly and annual global solar radiation
Whilst the maximum heating effect of solar radiation on a surface is normally dependent on
short-term irradiance around noon, the photochemical effects are related to radiation,
integrated over time, i.e. irradiation. For the purpose of comparison, daily global irradiation is
the most convenient and commonly used value.

– 8 – IEC 60721-2-4:2018 RLV © IEC 2018
In December, the monthly mean average of daily irradiation reaches approximately
10,8 kWh/m close to the South Pole, because of the duration of daylight. Outside the
Antarctic area, daily levels reach approximately 8,4 kWh/m .
The highest annual mean averages of daily global irradiation, up to 6,6 kWh/m , are found
mainly in desert areas.
6.3 Simultaneous values of maximum air temperatures and solar radiation
The lowest values of the turbidity coefficient β are found in cold air masses. Therefore, the
levels in Table 1 do not occur at the highest values of air temperature.
It may be assumed that global irradiance does not reach more than 80 % of the values given
in table 1 at the maximum air temperatures given in IEC 60721-2-1.
6.4 World distribution of daily global irradiation
For the distribution of daily global irradiation, see Annex A.
7 Minimum levels of atmospheric radiation at night
In cloudless nights when the atmospheric radiation is very low, objects exposed to the night
sky will attain surface temperatures below the surrounding air temperature.
The theoretical temperature T , in kelvins, of an object in equilibrium with the atmospheric
radiation is given by Boltzmann's law:
1/4
A
 
T =
 
σ
 
where
−8 2 4
σ is Stefan-Boltzmann's constant, 5,67 ⋅ 10 W/(m ⋅ K );
A is the atmospheric radiation in W/m (see figure 1).
In practice, temperatures will be higher due to heat conduction, convection and water
condensation.
As an example, it has been found that the surface of a horizontal disk thermally isolated from
the ground and exposed to the night sky during a clear night can attain a temperature of
−14 °C when the air temperature is 0 °C and the relative humidity is close to 100 %.
Figure 1 shows the atmospheric radiation from the night sky in clear air as a function of air
temperature at a height of 2 m above the ground level. The relative humidity is normally very
high on clear nights.
IEC  2345/02
Figure 1 – Atmospheric radiation from a clear night sky

– 10 – IEC 60721-2-4:2018 RLV © IEC 2018

Ultraviolet Infra-red
Visible light
2 000
A
B
1 000
C
D
G
H
F
E
0,1 0,2 0,5 1 2 5 10 20 50 100
Wavelength  (µm)
A Radiation outside the atmosphere from the Sun E Absorption bands of water vapour
represented as a black body at temperature and carbon dioxide
6 000 K (1,60 kW/m )
B Solar radiation outside the atmosphere F Absorption by oxygen and ozone
)
(1,37 kW/m
C Direct solar radiation at the surface of the Earth G Radiation of a black body at 300 K
perpendicular to the direction of radiation (0,47 kW/m )
(e.g. 0,9 kW/m )
D Diffuse solar radiation at the surface of the Earth H Thermal radiation from the Earth
2 2
(e.g. 0,10 kW/m ) (e.g. 0,07 kW/m )
Figure 21 – Spectra of electromagnetic radiation from the Sun
and the surface of the Earth
Spectral irradiance (power density per unit of wavelength)  W/(m •µm)

Annex A
(informative)
World distribution of daily global irradiation
Figures A.1, A.2 and A.3 are world maps showing isohels of relative global irradiation
(June, December and annual mean values), derived from satellite measurements (see [1] ).
Relative global irradiation is defined as the ratio of global irradiation measured at the Earth's
surface, divided by the extra-terrestrial global irradiation, which is the solar radiation on a
plane perpendicular to the direction of the Sun just outside the atmosphere.
In order to obtain the mean daily value of global irradiation at the Earth's surface, the
percentage value shown on the maps should be multiplied by the appropriate mean daily
value of extra-terrestrial global irradiation, which is given as a function of geographical
latitude in Table A.1.
NOTE The basis for determining the daily irradiation values in kWh/m is the values of monthly and annual
irradiation in MJ/m divided by the number of days in June (30), in December (31), and in the year (365).
EXAMPLE:
Determination of the mean daily global irradiation to be expected in June at the southern point of the Californian
peninsula.
From Figure A.1, the point (at an approximate geographical latitude of 23° N) is surrounded by an isohel of 60 %,
and the percentage value for the point is estimated to be 62 %.
In Table A.1, interpolation for 23° N in the June column gives 11,16 kWh/m , which is multiplied by the percentage
value above.
The mean daily global irradiation will thus be approximately 6,9 kWh/m .
___________
Numbers in square brackets refer to the Bibliography.

– 12 – IEC 60721-2-4:2018 RLV © IEC 2018
Table A.1 – Mean daily extra-terrestrial global irradiation (kWh/m )
Latitude June December Annual
90 N 12,47 0,0 4,17
85 N 12,42 0,0 4,20
80 N 12,28 0,0 4,30
75 N 12,05 0,0 4,49
70 N 11,72 0,0 4,76
65 N 11,40 0,11 5,16
60 N 11,40 0,65 5,71
55 N 11,48 1,36 6,29
50 N 11,56 2,16 6,87
45 N 11,61 3,00 7,42
40 N 11,61 3,85 7,93
35 N 11,56 4,72 8,40
30 N 11,44 5,57 8,82
25 N 11,26 6,40 9,19
20 N 11,00 7,20 9,49
15 N 10,68 7,96 9,73
10 N 10,30 8,68 9,90
5 N 9,84 9,34 10,01
0 9,33 9,95 10,04
5 S 8,76 10,50 10,01
10 S 8,13 10,98 9,90
15 S 7,46 11,39 9,73
20 S 6,74 11,73 9,49
25 S 5,99 12,00 9,19
30 S 5,21 12,19 8,82
35 S 4,41 12,32 8,40
40 S 3,60 12,37 7,93
45 S 2,79 12,37 7,41
50 S 2,01 12,31 6,86
55 S 1,27 12,22 6,29
60 S 0,60 12,13 5,71
65 S 0,10 12,12 5,16
70 S 0,0 12,45 4,75
75 S 0,0 12,80 4,48
80 S 0,0 13,05 4,30
85 S 0,0 13,20 4,20
90 S 0,0 13,25 4,16
IEC
Figure A.1 – Mean relative global irradiation for the month of June (in %)

– 14 – IEC 60721-2-4:2018 RLV © IEC 2018

IEC
Figure A.2 – Mean relative global irradiation for the month of December (in %)

IEC
Figure A.3 – Mean relative global irradiation for the year (in %)

– 16 – IEC 60721-2-4 RLV © IEC 2018
Bibliography
[1] G. Major et al., World maps of relative global radiation
[2] World Meteorological Organization, Technical Note No. 172, Annex. WMO-No. 557,
Geneva (1981)
[3] Haarto, Antti, 2001, Estimation methods for sky radiance distribution from
multipyranometer observations, Annales Universitatis Turkuensis Ser A1, vol 265,
Turku, University of Turku, 121 p. ISBN 951-29-1889-7

___________
IEC 60721-2-4 ®
Edition 2.0 2018-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Classification of environmental conditions –
Part 2-4: Environmental conditions appearing in nature – Solar radiation and
temperature
Classification des conditions d’environnement –
Partie 2-4: Conditions d'environnement présentes dans la nature – Rayonnement
solaire et température
– 2 – IEC 60721-2-4:2018 © IEC 2018
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 General . 5
5 Solar radiation physics . 6
6 Levels of global radiation . 7
6.1 Maximum levels . 7
6.2 Mean monthly and annual global solar radiation . 7
6.3 Simultaneous values of maximum air temperatures and solar radiation . 8
6.4 World distribution of daily global irradiation . 8
7 Minimum levels of atmospheric radiation at night . 8
Annex A (informative) World distribution of daily global irradiation . 10
Bibliography . 15

Figure 1 – Spectra of electromagnetic radiation from the Sun and the surface
of the Earth . 9
Figure A.1 – Mean relative global irradiation for the month of June (in %) . 12
Figure A.2 – Mean relative global irradiation for the month of December (in %) . 13
Figure A.3 – Mean relative global irradiation for the year (in %) . 14

Table 1 – Typical peak values of global irradiance (in W/m from a cloudless sky) . 7
Table A.1 – Mean daily extra-terrestrial global irradiation (kWh/m ) . 11

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CLASSIFICATION OF ENVIRONMENTAL CONDITIONS –

Part 2-4: Environmental conditions appearing in nature –
Solar radiation and temperature

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60721-2-4 has been prepared by IEC technical committee 104:
Environmental conditions, classification and methods of test.
This second edition cancels and replaces the first edition published in 1987 and
Amendment 1:1988. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) Figures updated including the addition of global irradiation information,
b) Format updated.
– 4 – IEC 60721-2-4:2018 © IEC 2018
The text of this International Standard is based on the following documents:
FDIS Report on voting
104/800/FDIS 104/803/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 60721 series, published under the general title Classification of
environmental conditions, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
CLASSIFICATION OF ENVIRONMENTAL CONDITIONS –

Part 2-4: Environmental conditions appearing in nature –
Solar radiation and temperature

1 Scope
This part of IEC 60721 presents a broad division into types of solar radiation areas. It is
intended to be used as part of the background material when selecting appropriate severities
of solar radiation for product applications.
All types of geographical areas are covered, except areas with altitudes above 5 000 m.
This document also serves to define limiting severities of solar radiation to which products are
liable to be exposed during transportation, storage and use.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
4 General
Solar radiation can affect products primarily by the heating of material and their environment
or by photochemical degradation of material.
Solar radiation, especially its ultraviolet content, causes photochemical degradation of most
organic materials. Elasticity and plasticity of certain rubber compounds and plastic materials
are affected. Optical glass may become opaque.
Solar radiation bleaches out colours in paints, textiles, paper, etc. This can be of importance,
for example for the colour-coding of components.
The heating of material is a consequence of exposure to solar radiation. The presentation of
severities of solar radiation is therefore related to the power density radiated towards a
surface, or irradiance, expressed in W/m .
An object subjected to solar radiation will attain a temperature that depends primarily on the
surrounding air temperature, the energy radiated from the Sun, and the incidence angle of the
radiation on the object. Other factors, for example wind and heat conduction to mountings,
can be of importance. In addition, the absorptance α of the surface for the solar spectrum
s
is of importance.
– 6 – IEC 60721-2-4:2018 © IEC 2018
An artificial air temperature t may be defined, which, under steady-state conditions, results in
s
the same surface temperature of an object as the combination of the actual air temperature t
u
and the solar radiation of the irradiance E.
An approximate value can be obtained from the following equation:
α ⋅E
s
t =t +
s u
h
y
The coefficient h is the heat transfer coefficient for the surface, in W/(m · °C). It includes
y
thermal radiation to the surroundings, heat conduction of the surface material and convection
due to wind.
The absorptance α , depends on the thermal colour, the reflectance and the transmittance of
s
the surface.
Typical clear sky values for common materials are:
α = 0,7
s
h = 20 W/(m · °C)
y
E = 900 W/m
resulting in an "over temperature" due to solar radiation of about 30 °C. It can then be seen
that an error of 10 % in the estimation of the intensity of the solar radiation will influence the
temperature involved by less than 5 °C. Therefore, there is no need in this classification for
extremely accurate severities of solar radiation and minor factors affecting the heat radiated
have therefore been disregarded here.
The heating effect is caused mainly by short-term radiation of high intensity, i.e. the solar
radiation around noon on cloudless days. Such values are presented in Table 1.
It may also be of interest to identify the lowest possible value of atmospheric radiation during
clear nights in order to determine the “under temperature” of products exposed to the night
sky.
5 Solar radiation physics
The electromagnetic radiation from the Sun to the Earth covers the spectrum from the
ultraviolet to the near infra-red. Most of the energy reaching the surface of the Earth is in the
wavelength range of 0,3 µm to 4 µm with a maximum in the visible range around 0,5 µm.
Typical spectra are shown in Figure 1.
The amount of radiant energy from the Sun which falls upon the unit area of a plane normal to
the Sun's rays just outside the atmosphere at the mean distance from the Earth to the Sun is
called the solar constant. Its value is 1 367 W/m .
The distance from the Earth to the Sun varies during the year, and consequently the radiation
varies.
Approximately 99 % of the energy of the Sun is emitted at wavelengths below 4 µm. Most
of the energy below 0,3 µm is absorbed by the atmosphere and does not reach the surface of
the Earth. Further absorption and scattering of the radiation takes place, due to particles and
gases, during passage through the atmosphere. The scattering of the direct solar radiation in
the atmosphere results in diffuse radiation from the sky. Thus, the energy received at a
certain place on Earth is the sum of the direct solar radiation and the diffuse solar radiation,

which is referred to as "global radiation". From the point of view of heating effects, this sum is
of interest and the levels given in this document are therefore related to global radiation.
6 Levels of global radiation
6.1 Maximum levels
The maximum level of global radiation on a clear day occurs at noon. The highest value of
the power achieved on a cloudless day at noon at a surface perpendicular to the direction
of the Sun depends on the content of aerosol particles, ozone and water vapour in the air. It
varies considerably with geographical latitude and type of climate.
The global radiation on a surface perpendicular to the direction of the Sun can reach a value
of 1 120 W/m in a range of 280 nm to 3 000 nm at noon on a cloudless day with
approximately 1 cm of water vapour content, 2 mm of ozone and aerosols of β = 0,05, where β
is the Ångström turbidity coefficient. The value 1 120 W/m is typical for flat land far away
from industrial areas and from large cities at solar elevations exceeding 60°.
NOTE 1 The water vapour content of a vertical column of the atmosphere is measured as the height, in
centimetres, of the corresponding precipitated water. Analogously, the ozone content of a vertical column of the
atmosphere is measured as the height of the corresponding ozone column at normal temperature and pressure.
The scattering and absorption by aerosol particles is expressed by the Ångström turbidity coefficient, which is the
optical depth of the atmosphere with respect to extinction of monochromatic radiation of wavelength λ = 1 µm.
NOTE 2 During partly clouded days the global solar irradiation can increase up to 1 300 W/m for a few minutes.
This short-term phenomenon occurs when the Sun comes out behind the clouds and the radiation is reflected from
the edges of the clouds.
The direct solar radiation decreases with increasing turbidity. Turbidity is high in subtropical
climates and in deserts where the concentration of particles in the air is high. It is also high in
large cities and low in mountainous areas.
The levels in Table 1 are recommended for application as peak values of global irradiance at
noon, experienced by a surface perpendicular to the direction of the Sun in a cloudless sky.
The level varies only by a few per cent within the hours nearest to noon and can therefore be
assumed to be representative for a few hours at a time.
Table 1 – Typical peak values of global irradiance
(in W/m from a cloudless sky)
Area Large cities Flat land Mountainous areas
Subtropical climates and deserts 700 750 1 180
Other areas 1 050 1 120 1 180
6.2 Mean monthly and annual global solar radiation
Whilst the maximum heating effect of solar radiation on a surface is normally dependent on
short-term irradiance around noon, the photochemical effects are related to radiation,
integrated over time, i.e. irradiation. For the purpose of comparison, daily global irradiation is
the most convenient and commonly used value.
In December, the monthly mean average of daily irradiation reaches approximately
10,8 kWh/m close to the South Pole, because of the duration of daylight. Outside the
.
Antarctic area, daily levels reach approximately 8,4 kWh/m
The highest annual mean averages of daily global irradiation, up to 6,6 kWh/m , are found
mainly in desert areas.
– 8 – IEC 60721-2-4:2018 © IEC 2018
6.3 Simultaneous values of maximum air temperatures and solar radiation
The lowest values of the turbidity coefficient β are found in cold air masses. Therefore, the
levels in Table 1 do not occur at the highest values of air temperature.
6.4 World distribution of daily global irradiation
For the distribution of daily global irradiation, see Annex A.
7 Minimum levels of atmospheric radiation at night
In cloudless nights w
...