EN IEC 60721-2-2:2024

SLOVENSKI STANDARD
01-februar-2025
Klasifikacija okoljskih pogojev - 2-2. del: Okoljski pogoji v naravi - Padavine in
veter (IEC 60721-2-2:2024)
Classification of environmental conditions - Part 2-2: Environmental conditions appearing
in nature - Precipitation and wind (IEC 60721-2-2:2024)
Klassifizierung von Umgebungsbedingungen - Teil 2-2: Natürliche
Umgebungsbedingungen - Niederschlag und Wind (IEC 60721-2-2:2024)
Classification des conditions d'environnement - Partie 2-2: Conditions d'environnement
présentes dans la nature - Précipitations et vent (IEC 60721-2-2:2024)
Ta slovenski standard je istoveten z: EN IEC 60721-2-2:2024
ICS:
19.040 Preskušanje v zvezi z Environmental testing
okoljem
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 60721-2-2

NORME EUROPÉENNE
EUROPÄISCHE NORM December 2024
ICS 19.040 Supersedes EN 60721-2-2:2013
English Version
Classification of environmental conditions - Part 2-2:
Environmental conditions appearing in nature - Precipitation and
wind
(IEC 60721-2-2:2024)
Classification des conditions d'environnement - Partie 2-2: Klassifizierung von Umgebungsbedingungen - Teil 2-2:
Conditions d'environnement présentes dans la nature - Natürliche Umgebungsbedingungen - Niederschlag und
Précipitations et vent Wind
(IEC 60721-2-2:2024) (IEC 60721-2-2:2024)
This European Standard was approved by CENELEC on 2024-11-29. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2024 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 60721-2-2:2024 E

European foreword
The text of document 104/1066/FDIS, future edition 3 of IEC 60721-2-2, prepared by TC 104
"Environmental conditions, classification and methods of test" was submitted to the IEC-CENELEC
parallel vote and approved by CENELEC as EN IEC 60721-2-2:2024.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2025-12-31
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2027-12-31
document have to be withdrawn
This document supersedes EN 60721-2-2:2013 and all of its amendments and corrigenda (if any).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 60721-2-2:2024 was approved by CENELEC as a
European Standard without any modification.
In the official version, for Bibliography, the following note has to be added for the standard indicated:
IEC 60721-2-1 NOTE Approved as EN 60721-2-1

IEC 60721-2-2 ®
Edition 3.0 2024-10
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Classification of environmental conditions –

Part 2-2: Environmental conditions appearing in nature – Precipitation and wind

Classification des conditions d'environnement –

Partie 2-2: Conditions d'environnement présentes dans la nature –

Précipitations et vent
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 19.040  ISBN 978-2-8322-9873-2

– 2 – IEC 60721-2-2:2024 © IEC 2024
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 8
4 Rain . 8
4.1 General . 8
4.2 Global distribution of rainfall . 9
4.3 Characteristics of rain . 11
4.3.1 Formation . 11
4.3.2 Types of rain . 11
4.3.3 Distribution of raindrop sizes . 12
4.3.4 Terminal velocity . 12
4.4 Rates of rainfall . 13
4.4.1 Instantaneous rates and clock-hour totals . 13
4.4.2 Frequency of instantaneous rates . 14
4.5 Heavy rates of rainfall . 15
4.5.1 Thunderstorm rain . 15
4.5.2 Prolonged heavy rain . 15
4.5.3 Worldwide extremes . 15
4.6 Spatial variations of rainfall rate . 16
4.6.1 General . 16
4.6.2 Rainfall rates below 2 mm/h . 17
4.6.3 Rainfall rates between 2 mm/h and 10 mm/h . 17
4.6.4 Rainfall rates between 10 mm/h and 25 mm/h . 17
4.6.5 Rainfall rates exceeding 25 mm/h . 18
4.7 Rainfall probabilities along a line . 18
5 Snow . 19
5.1 General . 19
5.2 Global distribution of the incidence of snowy weather . 19
5.3 Global distribution of the incidence and depth of lying snow . 22
5.4 Mass, size, and fall-speed of snow . 22
6 Hail. 26
6.1 General . 26
6.2 Global distribution of hailstones . 26
6.3 Seasonal variations in global distribution . 27
6.4 Diurnal variation . 28
6.5 Size of hail . 28
6.6 Terminal velocity . 30
6.7 Duration and diameter of hail cell . 31
7 Ice . 32
7.1 General . 32
7.2 Hoar frost . 32
7.3 Rime ice (including in-cloud icing and freezing fog) . 32
7.4 Glazed frost (including freezing rain and freezing drizzle) . 33
7.5 Wet snow accretion . 34

IEC 60721-2-2:2024 © IEC 2024 – 3 –
7.6 Water content of fog and cloud at temperatures below 0 °C . 34
7.7 Altitude variations . 35
8 Wind . 35
8.1 General . 35
8.2 Mean wind speed . 36
8.2.1 General . 36
8.2.2 Variation of mean wind speed with height . 37
8.2.3 Frequency of winds. 39
8.3 Gustiness . 46
8.3.1 General . 46
8.3.2 Gust factor . 46
8.3.3 Gust ratios . 46
8.3.4 Variation of gust speed with height . 47
8.4 Effects of topography . 48
8.5 Wind types and extreme winds . 48
8.6 Effects of wind . 50
8.7 Wind in conjunction with other damaging agents . 52
Bibliography . 53

Figure 1 – Average annual rainfall (AAR) for global land areas, based on 1961 to 1990
data [1] . 9
Figure 2 – Estimated conversion factor for converting to hours at instantaneous rate [1] . 14
Figure 3 – Average decay of correlation with distance, of 1 min rainfall rate and total
storm rainfall rate [1] . 17
Figure 4 – Estimated percentage of days on which an amount of snow equivalent to at
least 1 mm of rainfall falls in northern and southern hemispheres [1] . 21
Figure 5 – Satellite derived average rainfall equivalent of lying snow and ice [1] . 25
Figure 6 – Estimated average annual number of days per year with hail of diameter
≥ 15 mm [1]. 27
Figure 7 – Estimated average seasonal number of days per year with hail of diameter
≥ 15 mm, based on post-processed global model data (oceans excluded) [1] . 29
Figure 8 – Relationship between diameter and terminal velocity of spherical
hailstones [1] . 31
Figure 9 – Air temperature and wind speed criteria for the formation of different types
of ice [1] [15] . 33
Figure 10 – Power spectrum of wind speed fluctuations [1] . 36
Figure 11 – Annual mean 10 m wind speed (m/s) for global land areas, averaged over
the period 1961 to 1990 [1] . 38
Figure 12 – Measurement stations with station numbers . 40
Figure 13 – Vortex formation produced when wind strikes the corner of a structure . 51
Figure 14 – Contours of pressure coefficients produced by vortices . 51

Table 1 – Estimated number of raindrops per cubic metre for various rates of
rainfall [1] . 12
Table 2 – Terminal velocity of raindrops in still air [1] . 13
Table 3 – Duration in the average year of instantaneous point rainfall equalling or
exceeding specified rates [1] . 14
Table 4 – Predicted worldwide extremes of rainfall [1]. 15

– 4 – IEC 60721-2-2:2024 © IEC 2024
Table 5 – Observed world maximum rates of rainfall [1] . 16
Table 6 – Estimated duration (h) in the average year when stated distances along
given tracks simultaneously have rainfall at or exceeding specific rates [1] . 18
Table 7 – Snow crystals: relation between mass (mg) and diameter (mm) of the sphere
which just contains the crystal [1] . 23
Table 8 – Values of α and β for different crystal types [11] . 24
Table 9 – Number of hailstones per cubic metre (at about 4 000 m) for specified
maximum hailstone size and specific ranges of stone size [1] . 30
Table 10 – Meteorological parameters controlling atmospheric ice accretion [15] . 33
Table 11 – Details of stations, their location, elevation, observation rate and total
number of observations [19] . 41
Table 12 – Percentile mean wind speed and percentage frequencies of measured wind
speeds for each station [19] [20] [21] . 43
Table 13 – Terrain type and gust factor [1] . 46
Table 14 – Ratio of the probable maximum gust speed, averaged over time, to the
mean hourly wind speed for level sites in open country [1] . 47
Table 15 – Suggested ratios for estimating maximum gust speed over short periods
from a known mean hourly wind speed . 47
Table 16 – Factors for calculating maximum mean wind speed for various intervals

using the mean speed measured over the hour [1] . 47
Table 17 – Spatial and temporal scales of meteorological wind systems plus
characteristic wind speed ranges . 49

IEC 60721-2-2:2024 © IEC 2024 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CLASSIFICATION OF ENVIRONMENTAL CONDITIONS –

Part 2-2: Environmental conditions appearing in nature –
Precipitation and wind
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) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC 60721-2-2 has been prepared by IEC technical committee 104: Environmental conditions,
classification and methods of test. It is an International Standard.
This third edition cancels and replaces the second edition published in 2012. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) the layout of the information provided has been re-organized;
b) the information provided has been extensively enhanced and revised;
c) new information on wind severities has been included.

– 6 – IEC 60721-2-2:2024 © IEC 2024
The text of this International Standard is based on the following documents:
Draft Report on voting
104/1066/FDIS 104/1074/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
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 webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

IEC 60721-2-2:2024 © IEC 2024 – 7 –
INTRODUCTION
This part of IEC 60721 presents fundamental properties, quantities for characterization, and a
classification of environmental conditions dependent on precipitation and wind relevant to
electrotechnical products. The information presented is intended to be used as background
material when selecting appropriate severities of parameters related to precipitation and wind
for product applications.
Precipitation encompasses all forms of hydrometeors, both liquid and solid, which are free in
the atmosphere, and which reach the Earth's surface. At altitudes below the freezing level,
precipitation can occur as liquid or solid particles but above this level snow or hail will
predominate. For this document, the different forms of hydrometeors are addressed separately
and under the more commonly referred to meteorological conditions of rain, snow and hail. Also
encompassed are icing conditions but only that occurring at ground level.
This document additionally and separately addresses wind.
The majority of the information presented in this document has been assembled by the UK Met
Office from published sources as well as historical and forecasting weather records. The
information has been assembled and maintained for the UK Ministry of Defence for equipment
design and testing purposes [1] . The historical meteorological data employed for this work
meets World Meteorological Organization criteria for validity. However, such data are only
available from a limited number of worldwide locations (typically a few hundred). Forecasting
weather records, which were extensively utilized for this work, are available from a significant
number of locations (typically tens of thousands) but are not necessarily verified. Whenever the
latter information has been used, an appropriate strategy was adopted to remove spurious data.

___________
Numbers in square brackets refer to the Bibliography.

– 8 – IEC 60721-2-2:2024 © IEC 2024
CLASSIFICATION OF ENVIRONMENTAL CONDITIONS –

Part 2-2: Environmental conditions appearing in nature –
Precipitation and wind
1 Scope
This part of IEC 60721 presents fundamental properties, quantities for characterization, and a
classification of environmental conditions dependent on precipitation and wind relevant to
electrotechnical products.
The information presented within this document is intended to be used as background material
when selecting appropriate severities of parameters related to precipitation and wind for product
applications.
For the purpose of this document, precipitation is considered to encompass all forms of
hydrometeors, both liquid and solid, which are free in the atmosphere, and which reach the
Earth's surface. The different forms of hydrometeors are addressed separately and under the
more commonly referred to meteorological conditions of rain, snow and hail. Whilst icing
conditions are additionally considered, only that occurring at ground level, is addressed.
This document separately addresses the climatic condition of wind and provides methodologies
and quantitative information to enable wind severities and frequencies to be estimated
worldwide.
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 terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
4 Rain
4.1 General
Rain is the primary focus of Clause 4 as it is the dominant meteorological condition associated
with the wetting of electrotechnical products.

IEC 60721-2-2:2024 © IEC 2024 – 9 –
4.2 Global distribution of rainfall
Compared with meteorological air temperature which, at any particular time is often
substantially the same (±5 °C) over relatively large regions, rainfall is a much more spatially
variable climatic condition. In particular, the precipitation intensity that constitutes a near
extreme value is peculiar to the highly localized area. Even a relatively short distance away,
the intensity can differ by a factor of two or more. Thus, it is impracticable to relate precipitation
intensity to specific geographical areas of the world, as is the case with temperature.
Precipitation intensity is defined as the rate at which precipitation falls. Although values of
precipitation intensity may be considered as instantaneous rates, in practice they are averages
taken over periods of one minute or longer. When using any rainfall data, it should be
remembered that as the pattern of rainfall is infinitely variable both in time and space, only
general information can be given by means of maps or diagrams.
For most places, readily available rainfall data are limited to observations of the rainfall catch
made once daily. Where precipitation is of snow, the observations record the rainfall equivalent
of that snow. The daily observations may be summarized to provide average monthly, seasonal
or annual amounts. Figure 1 represents such a summary for annual amounts, based on
observations from 27 075 locations worldwide over the period from 1961 to 1990.

NOTE At higher latitudes, an increasing proportion of this "rainfall" will fall as snow.
Figure 1 – Average annual rainfall (AAR) for global land areas,
based on 1961 to 1990 data [1]

– 10 – IEC 60721-2-2:2024 © IEC 2024
The fundamental requirements for precipitation to fall in significant amounts are high
atmospheric moisture content and a mechanism for the uplift of air. Ascending air cools by
expansion due to the decreasing atmospheric pressure with height. Given that the lower the
temperature the less moisture the air can retain in vapour state, then if cooling and moisture
content are sufficient, precipitation is the result. As a generalization, the wetter land regions of
the world belong to one of the following three broad geographical categories:
a) Along the equator ±15° of latitude, e.g. Indonesia, equatorial Africa, and the Amazon rain
forest. The high rainfall of these regions is primarily due to convection, triggered by solar
heating and accentuated by the convergence of the northern hemisphere tropic's north-
easterly winds and the southern hemisphere tropic's south-easterly winds along the "inter-
tropical convergence zone". Here, copious moisture is provided by either rain forest or
warm, tropical ocean.
b) The western side of continents in mid latitudes, e.g. UK, Western Europe, the north-western
coastal fringe of North America and the south-western coastal fringe of South America
(southern Chile). At these latitudes, winds blow predominantly from the west and therefore
reach the western side of continents having picked up moisture over a long ocean track.
The lifting mechanisms are varied and include convection and orographic uplift, but the
dominant lifting mechanism is cyclonic or frontal uplift in weather disturbances that develop
on, and move eastwards along, the boundary between polar and tropical air masses. In
North and South America, the inland penetration of high rainfall is severely limited by the
high, north-south aligned mountain chains of the American Cordillera. By contrast, the
western fringe of Europe has no north-south aligned barrier of such proportion, so that
moderate rainfall is able to penetrate well inland across the European plain. Iceland, the
Falkland Islands, Tasmania, and the exposed west of South Island New Zealand also belong
to this regime.
c) Extending poleward from the equatorial regions along the eastern seaboard of continents,
e.g. Eastern Asia from India to Kamchatka, North America from the Gulf of Mexico to Quebec
(including the southern extremity of Greenland), South America from southern Brazil to
north-eastern Argentina, the east of South Africa and the eastern fringe of Australia. The
reasons for high rainfall in these areas are complex but include the predominantly easterly
moist onshore winds of tropical latitudes and, at sub-tropical latitudes, the drawing of moist
summer monsoon winds of tropical ocean origin inland towards a heat-generated continental
low-pressure area, as in the Indian and south-east Asian summer monsoons. Further
poleward, at mid latitudes, the prevailing wind is from the west, blowing offshore; however,
the eastern seaboards of both North America and Asia are favoured regions for the
development of precipitation-bearing cyclonic weather systems which then move north-
eastwards close to the mid-latitude coastline.
The important influence of topography on rainfall is demonstrated by the heavier rainfall in
mountainous regions, particularly where a mountain range runs parallel to the coast and
intercepts moisture laden winds as they blow onshore. Mountains also usually reduce rainfall
downwind – the "rain-shadow" effect.
Many of the great deserts of the world lie within or close to latitudes 20° to 30° of latitude, where
relatively high atmospheric pressure dominates e.g. the Sahara Desert and Saudi Arabia, the
deserts of California and Arizona, the Atacama Desert in Chile, the Namibian and Kalahari
Deserts of southern Africa and much of interior and western Australia. The dryness of some
deserts is accentuated by the rain-shadow effect of adjacent mountain barriers (e.g. the inland
deserts of California and Arizona). An additional factor in some coastal deserts is a cold ocean
current offshore that suppresses convection, for example the narrow Atacama Desert of Chile
is trapped between high mountains to the east and a cool ocean current offshore.
In Asia the circum-global belt of high pressure at sub-tropical latitudes is displaced by the Asian
monsoon, which blows outwards from intense high pressure over Siberia in winter and blows
into low pressure over southern interior Asia in summer. This effectively transfers the latitudinal
desert belt by approximately 15° of latitude poleward to lie north and inland of the areas reached
by the Indo-Asian monsoon e.g. the Gobi Desert of Mongolia and China.

IEC 60721-2-2:2024 © IEC 2024 – 11 –
Precipitation in polar regions is generally not particularly high on account of the reduced amount
of water vapour in the air at low temperatures.
4.3 Characteristics of rain
4.3.1 Formation
Clouds are formed when air is cooled below its dewpoint, usually as a result of lifting and
consequent expansion. At first the cloud droplets grow by the condensation of water on to them,
but it can be shown that this process alone cannot produce drops of the size found in rain. Two
mechanisms are thought to be important in the formation of raindrops.
Firstly, droplets which are slightly larger than the average will fall, relative to the air, and towards
neighbouring smaller droplets, and so can collide and coalesce with some of them to become
larger still. This process can continue until a droplet eventually falls out of the base of the cloud.
This mechanism is confined mainly, but not exclusively, to the tropics, where clouds can remain
devoid of solid precipitation throughout their depth. Theoretical studies have shown that a
significant amount of rain can be produced in this way, provided the cloud is several kilometres
deep.
Secondly, when a cloud top becomes appreciably colder than 0 °C, it contains a mixture of ice
crystals and supercooled water drops. At first the crystals grow by direct sublimation of water
vapour on to them, but as they become larger, they can collide with the supercooled droplets
and other ice crystals to form snowflakes, and when these snowflakes have fallen below the
level at which the temperature is 0 °C they will melt to form raindrops. This is the dominant
mechanism in middle and high latitudes, but it also occurs within the tropics and applies to
clouds with a top colder than about −10 °C. In convective (cumuliform) cloud, graupel or small
hail, rather than snowflakes, can be produced.
4.3.2 Types of rain
Rainfall is often classified according to the process causing the uplift of air initiating the rain
formation; there are three main types of rain which are not mutually exclusive, and these are
known as orographic, cyclonic, and convective.
Orographic rain is caused by one, or sometimes both, of two primary mechanisms. The most
commonly known mechanism is the forced ascent of a moist airstream over the physical barrier
of the high ground. The ascending airstream cools by expansion, often to the temperature at
which saturation occurs, above which altitude cloud forms. This can result in drizzle or rain over
the high ground when there is none on the adjacent low ground, but more often it enhances
cyclonic cloud and rain that are also affecting adjacent low ground. This enhancement is often
due primarily to raindrops scavenging additional water as they fall through the perpetually
reforming layer of low cloud formed by the ascent. This "seeder/feeder" mechanism can
massively augment precipitation on, particularly, windward slopes of mountains exposed to
moist airstreams, and this type of rainfall often continues for many hours or even days. The
effect often extends for some distance upwind, and for a lesser distance downwind (spill over),
of the foot of the mountain barrier. The other type of orographic rainfall does not require a
flowing airstream and is caused by the convective (i.e. non-forced) ascent of moist air from sun-
heated mountain slopes and summits. This type of orographic rainfall is a daytime, and
particularly summer, phenomenon and is typically more intense but of shorter duration, an
example being the afternoon thunderstorms to which the Alps are prone in summer. Such
convection occurs more readily over mountains than over adjacent lowlands because
convective air bubbles, formed over sun-heated mountain surfaces, more readily attain a
temperature higher than that of the surrounding atmosphere at that altitude. Sometimes
orographic convection is triggered, not by sun-heated surfaces, but by the initial forced
orographic ascent of an unstable airstream, such triggering not necessarily confined to daytime.

– 12 – IEC 60721-2-2:2024 © IEC 2024
In the case of cyclonic rainfall, large scale forced uplift is associated with features of the general
weather situation, such as fronts and depressions. As with rainfall due to orographic forced
ascent, cyclonic rainfall is often relatively steady but of long duration. However, as in the case
of orographic forced ascent, cyclonic forced ascent can also trigger more intense pulses of
convective rainfall given an unstable atmosphere.
Convective rain falls from cumulonimbus clouds forming within an unstable air mass, this being
one in which temperature falls relatively rapidly with altitude. The formation of a cumulonimbus
cloud requires a trigger to initiate the convective ascent of a large bubble of moist air which
then, despite cooling by expansion as it rises, cools less quickly than the temperature of the
surrounding atmosphere, so remaining buoyant and continuing to rise through a considerable
depth of atmosphere. This forms clouds of relatively small lateral extent but great vertical depth.
The trigger is often the daytime heating of the ground surface by the sun or the passage of a
cold air mass over a relatively warm sea surface. However, convection can also be initiated by
cyclonic uplift, particularly near some cold weather fronts, where the approaching cold air mass
sometimes overrides the warmer air in advance of the front, causing extreme instability. Another
trigger can be the initially forced ascent of an unstable air mass over high ground. Convective
rainfall is typically more intense, but of shorter duration, than cyclonic or orographic rain,
individual areas of rain being relatively small (20 km diameter or less), with dry areas close by.
4.3.3 Distribution of raindrop sizes
It is known that drops of a diameter greater than about 6 mm break up before reaching their
terminal velocity, and this fixes the upper limit for raindrop diameter. Table 1 presents estimates
of the number and size of raindrops for various rates of rainfall. It should be noted that the
values quoted represent averages based on a large number of observations of size
distributions, and that individual measurements of drop sizes can show wide variations about
these average values. Additionally, the table was derived using rain from predominantly
stratiform clouds at mid-latitudes and therefore it is possible that it does not represent the
predominantly convective rainfall of tropical regions as accurately.
Table 1 – Estimated number of raindrops per cubic metre for various rates of rainfall [1]
Estimated number of raindrops per cubic metre for various rates of rainfall
Rate of rainfall Drop diameter
mm/h or l/m h 1 mm to 2 mm 2 mm to 3 mm 3 mm to 4 mm 4 mm to 5 mm > 5 mm
1 32 0,53 0,01 0,00 0,00
5 140 7,5 0,41 0,20 0,00
10 230 19 1,5 0,12 0,01
25 415 51 6,4 0,75 0,11
50 615 101 17 2,8 0,55
100 850 179 38 7,9 2,1
200 1 140 259 113 20 6,9
500 1 590 525 173 57 28
1 000 1 970 754 289 111 69
4.3.4 Terminal velocity
The rate at which a raindrop falls through still air depends both on the size of the drop and on
the resistance offered by the air. The speed of fall will increase until the air resistance is equal
to the weight of the drop, after which it will fall at a steady rate known as the terminal velocity.

IEC 60721-2-2:2024 © IEC 2024 – 13 –
Table 2 shows the variation of terminal velocity with drop size, for the following still air
conditions:
air pressure 101,3 kPa;
temperature 20 °C;
relative humidity 50 %.
Table 2 – Terminal velocity of raindrops in still air [1]
Terminal velocity of raindrops in still air
Drop diameter Terminal velocity
mm m/s
0,1 0,27
0,5 2,06
1,0 4,03
1,6 5,65
2,0 6,49
2,6 7,57
3,0 8,06
3,6 8,60
4,0 8,83
4,6 9,03
5,0 9,09
5,8 9,17
4.4 Rates of rainfall
4.4.1 Instantaneous rates and clock-hour totals
Data on the frequency and duration of rainfall rates are relatively scarce, although the use of
recording rain gauges with suitable time scales provides some information on the occurrence
of specific rates of rainfall, and radar data contributes in some areas. Indirect estimates have
been made from daily rainfall summaries, using regression equations based on a limited number
of stations; these methods can be misleading when marked orographic or seasonal effects
occur, especially at high rainfall intensities.
For many stations, especially in North America and Europe, routine tabulations of clock-hour
rainfall amounts have been made but are only available from respective national meteorological
organizations. Empirical formulae have been developed to make use of these clock-hour totals
to derive frequencies of occurrence of specified rainf
...