oSIST prEN 1991-1-9:2023
(Main)Eurocode 1 - Actions on structures - Part 1-9: General actions - Atmospheric icing
Eurocode 1 - Actions on structures - Part 1-9: General actions - Atmospheric icing
1.1 Scope of EN 1991-1-9
(1) EN 1991-1-9 gives principles and rules to determine the values of loads due to atmospheric icing to be used for following types of structures:
- masts,
- towers,
- antennas and antenna structures,
- cables, stays, guy ropes, etc.,
- rope ways (cable railways),
- structures for ski-lifts,
- buildings or parts of them exposed to potential icing,
- towers for special types of construction such as for example transmission lines and wind turbines.
NOTE Atmospheric icing on electrical overhead lines is covered by EN 50341-1.
(2) EN 1991-1-9 specifies values for:
- dimensions and weight of accreted ice,
- shapes of accreted ice.
(3) EN 1991-1-9 cover types of icing, ice loads acting on structures, and falling ice considerations.
NOTE Wind actions on iced structures are covered by EN 1991-1-4.
1.2 Assumptions
The assumptions given in FprEN 1990:2022, 1.2 apply to EN 1991-1-9.
Eurocode 1 - Einwirkungen auf Tragwerke - Teil 1-9: Allgemeine Einwirkungen - Atmosphärische Eisbildung
1.1 Anwendungsbereich von EN 1991-1-9
(1) EN 1991-1-9 enthält Grundsätze und Regeln für die Bestimmung der Werte von Lasten infolge atmosphärischer Eisbildung, die für die folgenden Tragwerksarten zu verwenden sind:
- Maste;
- Türme;
- Antennen und Antennentragwerke;
- Abspannungen, Stützen, Kabel usw.;
- Seilwerke (Seilbahnen);
- Tragwerke für Skilifte;
- Gebäude oder Teile davon, die potenzieller Eisbildung ausgesetzt sind;
- Türme für Sonderkonstruktionen wie z. B. Übertragungsleitungen und Windkraftanlagen.
ANMERKUNG Atmosphärische Eisbildung an Freileitungen wird in EN 50341-1 behandelt.
(2) EN 1991-1-9 legt Werte fest für:
- die Maße und die Massen der Eisablagerungen;
- die Formen der Eisablagerungen.
(3) EN 1991-1-9 behandelt die Arten der Eisablagerung, die auf Tragwerke einwirkenden Eislasten und die Grundlagen zur Berücksichtigung des Eisabfalls.
ANMERKUNG Windlasten an vereisten Tragwerken werden in EN 1991-1-4 behandelt.
1.2 Voraussetzungen
Die in FprEN 1990:2022, 1.2, angegebenen Voraussetzungen gelten für EN 1991-1-9.
Eurocode 1 - Actions sur les structures - Partie 1-9 : Givrage atmosphérique
1.1 Domaine d'application de l'EN 1991-1-9
(1) L'EN 1991-1-9 donne les principes et règles pour déterminer les valeurs des charges dues au givrage atmosphérique, à utiliser pour les types de structures suivants :
- les mâts ;
- les tours ;
- les antennes et structures associées ;
- les câbles, haubans, câbles de haubanage, etc. ;
- les câbles de téléphérique (câbles porteurs) et de funiculaires ;
- les structures pour remontées mécaniques ;
- les bâtiments ou leurs parties exposées au givrage ;
- les pylônes pour des types d'ouvrages particuliers, comme par exemple les lignes de transmission et les éoliennes.
NOTE Le givrage atmosphérique sur les lignes aériennes électriques est couvert par l'EN 50341-1.
(2) L'EN 1991-1-9 spécifie des valeurs pour :
- les dimensions et le poids des accrétions de glace ;
- les formes des accrétions de glace.
(3) L'EN 1991-1-9 traite les types de givrage, les charges de glace agissant sur les structures et les considérations relatives aux chutes de glace.
NOTE Les actions du vent sur les structures recouvertes de glace sont couvertes par l'EN 1991-1-4.
1.2 Hypothèses
Les hypothèses formulées dans le FprEN 1990:2022, 1.2 s'appliquent à l'EN 1991-1-9.
Evrokod 1 - Vplivi na konstrukcije - 1-9. del: Splošni vplivi - Atmosferska zaledenitev
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
oSIST prEN 1991-1-9:2023
01-maj-2023
Evrokod 1 - Vplivi na konstrukcije - 1-9. del: Splošni vplivi - Atmosferska
zaledenitev
Eurocode 1 - Actions on structures - Part 1-9: General actions - Atmospheric icing
Eurocode 1 - Einwirkungen auf Tragwerke - Teil 1-9: Allgemeine Einwirkungen -
Atmosphärische Eisbildung
Eurocode 1 - Actions sur les structures - Partie 1-9 : Givrage atmosphérique
Ta slovenski standard je istoveten z: prEN 1991-1-9
ICS:
91.010.30 Tehnični vidiki Technical aspects
oSIST prEN 1991-1-9:2023 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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DRAFT
EUROPEAN STANDARD
prEN 1991-1-9
NORME EUROPÉENNE
EUROPÄISCHE NORM
March 2023
ICS 91.010.30
English Version
Eurocode 1 - Actions on structures - Part 1-9: General
actions - Atmospheric icing
Eurocode 1 - Einwirkungen auf Tragwerke - Teil 1-9:
Allgemeine Einwirkungen - Atmosphärische Eisbildung
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee
CEN/TC 250.
If this draft becomes a European Standard, CEN 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.
This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC
Management Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.
Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2023 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 1991-1-9:2023 E
worldwide for CEN national Members.
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Contents Page
European foreword . 4
Introduction . 5
1 Scope . 8
1.1 Scope of EN 1991-1-9 . 8
1.2 Assumptions . 8
2 Normative references . 8
3 Terms, definitions and symbols . 9
3.1 Terms and definitions . 9
3.2 Symbols and abbreviations . 10
3.2.1 Latin upper case letters . 10
3.2.2 Latin lower case letters . 11
3.2.3 Greek lower-case letters . 11
4 Design situations . 11
5 Classification of actions . 11
6 Ice load on structures . 12
6.1 Basic values . 12
6.2 Ice classes and characteristic values of glaze and rime . 13
6.3 Glaze . 13
6.4 Rime . 14
6.4.1 General. 14
6.4.2 Rime on single members . 14
6.4.3 Direction of ice vanes on the structure . 16
6.4.4 Rime on lattice structures . 17
6.5 Variation with height above the ground . 17
7 Combination of ice loads with other actions . 18
7.1 General. 18
7.2 Supplementary provisions for combination of ice and wind actions . 19
Annex A (informative) Falling ice considerations . 20
A.1 Use of this Informative Annex . 20
A.2 Scope and field of application . 20
A.3 Impact of falling ice . 20
Annex B (informative) Information on how ice loads acts on structures . 21
B.1 Use of this Informative Annex . 21
B.2 Scope and field of application . 21
B.3 General considerations . 21
B.4 Glaze on lattice structures . 21
B.5 Rime icing on members inclined to the wind direction . 22
B.6 Wet snow . 22
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B.7 Unbalanced ice load . 23
Annex C (informative) Types of icing and data collection . 24
C.1 Use of this Informative Annex . 24
C.2 Scope and field of application . 24
C.3 Icing types . 24
C.4 Data Collection . 26
Annex D (informative) Guidance how to use EN 1991-1-9 . 28
Bibliography . 29
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European foreword
This document (prEN 1991-1-9:2023) has been prepared by Technical Committee CEN/TC 250
“Structural Eurocodes”, the secretariat of which is held by BSI.
CEN/TC 250 is responsible for all Structural Eurocodes and has been assigned responsibility for
structural and geotechnical design matters by CEN.
This document is currently submitted to the CEN Enquiry.
This document is a new part of EN 1991-1.
The first generation of EN Eurocodes was published between 2002 and 2007. This document forms part
of the second generation of the Eurocodes, which have been prepared under Mandate M/515 issued to
CEN by the European Commission and the European Free Trade Association.
The Eurocodes have been drafted to be used in conjunction with relevant execution, material, product,
and test standards, and to identify requirements for execution, materials, products and testing that are
relied upon by the Eurocodes.
The Eurocodes recognize the responsibility of each Member State and have safeguarded their right to
determine values related to regulatory safety matters at national level through the use of National
Annexes.
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Introduction
0.1 Introduction to the Eurocodes
The structural Eurocodes comprise the following standards generally consisting of a number of Parts:
— EN 1990, Eurocode: Basis of structural and geotechnical design
— EN 1991, Eurocode 1: Actions on structures
— EN 1992, Eurocode 2: Design of concrete structures
— EN 1993, Eurocode 3: Design of steel structures
— EN 1994, Eurocode 4: Design of composite steel and concrete structure
— EN 1995, Eurocode 5: Design of timber structures
— EN 1996, Eurocode 6: Design of masonry structures
— EN 1997, Eurocode 7: Geotechnical design
— EN 1998, Eurocode 8: Design of structures for earthquake resistance
— EN 1999, Eurocode 9: Design of aluminium structures
— < New parts >
The Eurocodes are intended for use by designers, clients, manufacturers, constructors, relevant
authorities (in exercising their duties in accordance with national or international regulations),
educators, software developers, and committees drafting standards for related product, testing and
execution standards.
NOTE Some aspects of design are most appropriately specified by relevant authorities or, where not
specified, can be agreed on a project-specific basis between relevant parties such as designers and clients. The
Eurocodes identify such aspects making explicit reference to relevant authorities and relevant parties.
0.2 Introduction to EN 1991
(1) EN 1991 provides the actions to be considered for the structural design of buildings, bridges and
other civil engineering works, or parts thereof, including temporary structures, in conjunction with
EN 1990 and the other Eurocodes.
(2) The actions on structures, including in some cases geotechnical structures in conjunction with
EN 1997 as appropriate, provided in EN 1991 are intended to be applied in conjunction with the other
Eurocodes for the verification of safety, serviceability and durability, as well as robustness of structures,
including the execution phase.
(3) The application of this document for the verifications mentioned in (2) follows the limit state
principle and is based on the partial factor method, unless explicitly prescribed differently.
(4) EN 1991 does not cover actions for structures in seismic regions, unless explicitly prescribed by
EN 1998. Provisions related to such requirements are given in EN 1998, which complements and is
consistent with EN 1991.
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(5) EN 1991 is also applicable in the case of existing structures for their:
— structural assessment,
— design of repairs, improvements and alterations,
— assessment for changes of use.
NOTE In this case additional or amended provisions can be necessary.
(6) EN 1991 is also applicable to the design of structures where materials or actions outside the scope
of the other Eurocodes are involved.
NOTE In this case additional or amended provisions can be necessary.
0.3 Introduction to EN 1991-1-9
EN 1991-1-9 gives design guidance for actions due to atmospheric icing on structures and civil
engineering works.
EN 1991-1-9 is intended to be used with EN 1990, the other Parts of EN 1991 and EN 1992 to1999 for
the design of structures.
Atmospheric icing on electrical overhead lines is covered by the CENELEC (European Committee for
Electrotechnical Standardization) standard EN 50341-1.
0.4 Verbal forms used in the Eurocodes
The verb “shall” expresses a requirement strictly to be followed and from which no deviation is
permitted in order to comply with the Eurocodes.
The verb “should” expresses a highly recommended choice or course of action. Subject to national
regulation and/or any relevant contractual provisions, alternative approaches could be used/adopted
where technically justified.
The verb “may” expresses a course of action permissible within the limits of the Eurocodes.
The verb “can” expresses possibility and capability; it is used for statements of fact and clarification of
concepts.
0.5 National Annex for EN 1991-1-9
National choice is allowed in this standard where explicitly stated within notes. National choice includes
the selection of values for Nationally Determined Parameters (NDPs).
The national standard implementing EN 1991-1-9 can have a National Annex containing all national
choices to be used for the design of buildings and civil engineering works to be constructed in the
relevant country.
When no national choice is given, the default choice given in this standard is to be used.
When no national choice is made and no default is given in this standard, the choice can be specified by
a relevant authority or, where not specified, agreed for a specific project by appropriate parties.
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National choice is allowed in EN 1991-1-9 through notes to the following clauses:
6.1 (2) NOTE
6.1 (3) NOTE 1
6.1 (4) NOTE
6.1 (5) NOTE 1
6.1 (5) NOTE 2
6.1 (5) NOTE 3
6.1 (5) NOTE 4
6.4.1 (3) NOTE
6.4.2.1 (1) NOTE 1
6.5 (5) NOTE 2
7.2 (1) NOTE
National choice is allowed in EN 1991-1-9 on the application of the following informative annexes:
— Annex A Falling ice considerations (A.1 (1));
— Annex B Information on how ice loads act on structures (B.1 (1));
— Annex C Types of icing and data collection (C.1 (1));
The National Annex can contain, directly or by reference, non-contradictory complementary
information for ease of implementation, provided it does not alter any provisions of the Eurocodes.
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1 Scope
1.1 Scope of EN 1991-1-9
(1) EN 1991-1-9 gives principles and rules to determine the values of loads due to atmospheric icing to
be used for following types of structures:
— masts,
— towers,
— antennas and antenna structures,
— cables, stays, guy ropes, etc.,
— rope ways (cable railways),
— structures for ski-lifts,
— buildings or parts of them exposed to potential icing,
— towers for special types of construction such as for example transmission lines and wind turbines.
NOTE Atmospheric icing on electrical overhead lines is covered by EN 50341-1.
(2) EN 1991-1-9 specifies values for:
— dimensions and weight of accreted ice,
— shapes of accreted ice.
(3) EN 1991-1-9 cover types of icing, ice loads acting on structures, and falling ice considerations.
NOTE Wind actions on iced structures are covered by EN 1991-1-4.
1.2 Assumptions
The assumptions given in FprEN 1990:2022, 1.2 apply to EN 1991-1-9.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
NOTE See the Bibliography for a list of other documents cited that are not normative references, including
those referenced as recommendations (i.e. in “should” clauses), permissions (“may” clauses), possibilities (“can”
clauses), and in notes.
FprEN 1990:2022, Eurocode — Basis of structural and geotechnical design
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3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in FprEN 1990:2022 and the
following apply.
3.1.1
in-cloud icing
icing due to super cooled water droplets in a cloud or fog
3.1.2
precipitation icing
icing due to either
a) freezing rain or drizzle, or
b) accumulation of wet snow
3.1.3
accretion
process of building up ice on the surface of an object, resulting in different types of icing on structures
3.1.4
fundamental basic ice load
theoretical maximum characteristic value of rime ice mass or characteristic value of glaze ice thickness
obtained on a reference collector, irrespective of wind direction, orientation of the object of icing and of
the time of year, at 10 m above the ground level in open country, with an annual probability of
exceedance of 0,02
3.1.5
glaze
clear, high-density ice
3.1.6
rime
white ice that forms when water droplets freeze to the outer surface of an object, trapping air
3.1.7
reference collector
30 mm diameter cylinder not less than 0,5 m in length, which slowly rotates around its own axis
3.1.8
ice class
IC
classification of the characteristic values of ice load that is expected to occur with an annual probability
of exceedance of 0,02 on a reference ice collector situated in this particular location at 10 m height
above the ground
3.1.9
characteristic rime ice mass
rime ice mass on the reference collector with an annual probability of exceedance of 0,02
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3.1.10
characteristic glaze ice thickness
glaze ice thickness on the reference collector with an annual probability of exceedance of 0,02
3.1.11
basic ice load
fundamental basic ice load modified to account for the direction of icing wind, the characteristics of the
object of icing (e.g. fixed or rotating object, surface property, colour), the orientation of the iced object
(e.g. vertical or horizontal), the season and the variation with height above the ground
3.1.12
directional factor
factor taking into account the reduction of the basic ice load in cases where the icing is not uniform with
respect to wind direction
3.1.13
object factor
factor taking into account the reduction of the basic ice load due to ice shedding from non-rotating
objects triggered by e.g. solar radiation, temperature variations, turbulent wind or other local
meteorological conditions
3.1.14
orientation factor
factor taking into account the reduction of the basic ice load in cases where the ice load depends on the
object orientation, e.g. a reduction for glaze ice on vertical elements in areas where icing occur under
low wind speed condition
3.1.15
seasonal factor
factor taking into account the reduction of the basic ice load in cases of temporary structures (in order
of seasons length) and for structures in the execution phase
3.1.16
ice action
accreted ice on a structure, both as gravity load due to the self-weight of the ice and additional wind
action on the iced structure
3.2 Symbols and abbreviations
For the purpose of this European standard, the symbols given in FprEN 1990:2022, 3.2 apply together
with the following additional notations which are specific to this Part.
NOTE The notation used is based on ISO 3898:2013.
3.2.1 Latin upper case letters
D Diameter of accreted ice or total width of object including ice
H Height above the ground
L Length of ice vane measured in windward direction
T Air temperature
W Width of object (excluding ice) perpendicular to wind direction
c
W Wind direction
dir
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3.2.2 Latin lower case letters
c Direction factor
dir
c Height factor
h
R
c Height factor for rime
h
G
Height factor for glaze
ch
c Object factor
object
c Orientation factor
orient
c Seasonal factor
season
d Diameter of object (excluding ice)
k Factor for reduction of velocity pressure for determination of wind action
m Mass of accreted ice per unit length
i Basic ice load
b
i Fundamental basic ice load
b,0
m Ice mass for ice on big objects
w
t Ice thickness
3.2.3 Greek lower-case letters
ρ Density of ice
4 Design situations
(1) The relevant ice loads shall be determined for each design situation identified, in accordance with
FprEN 1990:2022.
(2) The combination of wind actions with icing in the persistent/transient design situations which
include icing should be taken into account.
NOTE This leads to the increased vertical loads on the iced structure and increased wind drag caused by the
increased wind-exposed area. The latter can lead to more severe wind loads than without icing.
(3) The impact of falling ice should be considered as accidental design situation.
NOTE Guidance for considering ice falling off the structure can be found in Annex A.
5 Classification of actions
(1) Ice loads shall be classified as variable, fixed actions unless otherwise specified in this standard.
NOTE See FprEN 1990:2022, 6.1.1 for classification of actions.
(2) For the particular case of falling ice, ice load should be considered as an accidental action.
(3) Ice loads covered in this standard should be classified as static actions.
NOTE See FprEN 1990:2022, 7.1.2 for modelling of static actions.
(4) Ice loads should be classified to “glaze”, (G), and “rime”, (R), due to the different characteristics of
the icing types, see Clause 6.
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6 Ice load on structures
6.1 Basic values
(1) The basic ice load i is the basis for specification of the ice load on a structure and shall be
b
determined from the fundamental basic ice load (defining the icing climate).
(2) The fundamental values of the basic ice loads i should be defined.
b,0
NOTE The fundamental values of the basic ice loads ib,0 to be used in a country can be found in the National
Annex.
(3) The fundamental values of the basic ice loads i should be specified as a glaze (G) thickness or as a
b,0
rime (R) mass.
NOTE 1 The choice between glaze thickness and rime mass is given in the National Annex.
NOTE 2 The typical properties of the different icing types are indicated in Table C.1.
(4) The fundamental values of the basic ice loads ib,0 should be provided either as characteristic values
(i.e. characteristic glaze ice thickness or characteristic values of rime ice mass) directly or classified
using the definition of icing classes.
NOTE The choice of providing the fundamental basic ice loads either as characteristic values or as icing
classes is given in the National Annex.
(5) The basic ice load i shall be calculated from Formula (6.1).
b
i = c ⋅⋅c c ⋅ c ⋅⋅ci (6.1)
b dir object orient season h b,0
where
is the directional factor, see NOTE 1;
c
dir
c is the object factor (e.g. fixed or rotating objects), see NOTE 2;
object
c is the orientation factor (e.g. vertical or horizontal), see NOTE 3;
orient
c is the seasonal factor, see NOTE 4;
season
R G
c is the height factor (c = c for rime mass and c = c for glaze thickness), see 6.5;
h h h h h
is the fundamental value of the basic ice load, see 6.1 to 6.4.
i
b,0
NOTE 1 Value of the directional factor is 1,0 unless the National Annex gives a different value for use in the
country.
NOTE 2 Value of the object factor is 1,0 unless the National Annex gives a different value for use in the country.
NOTE 3 Value of the orientation factor is 1,0 unless the National Annex gives a different value for use in the
country.
NOTE 4 Value of the seasonal factor is 1,0 unless the National Annex gives a different value for use in the
country
(6) In case of tall structures, the basic ice loads should be calculated at multiple representative heights
(see 6.5).
NOTE The flow chart shown in Annex D can be used as guidance for the calculation procedure.
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6.2 Ice classes and characteristic values of glaze and rime
(1) If classes (ICs) are to represent the fundamental basic ice load (see 6.1 (3)), they should be
determined for both glaze (ICG) and rime (ICR), because the characteristics for these differ. ICG should
be determined for glaze deposits and ICR for rime deposits.
NOTE Guidance to estimate the characteristic values for rime ice mass or characteristic glaze ice thickness is
given in Annex C.
6.3 Glaze
(1) If ice accretion is glaze, ice action shall be based on the Ice class for Glaze (ICG’s) at 10 m above the
ground given in Table 6.1, or the characteristic glaze ice thickness at 10 m above the ground at the site.
Table 6.1 — Ice thicknesses for ICGs
Ice classes ICG G1 G2 G3 G4 G5 G6
Characteristic ice thickness t (mm) 10 20 30 40 50 *
*To be used for extreme ice accretions
NOTE The numbers represent the upper bound for the corresponding ICGs.
(2) The glaze ice thickness should be used for determination of the ice load on a structure according to
the icing model for glaze illustrated in Figure 6.1.
(3) The ice mass on a cylinder for a given ice thickness should be calculated from
m π⋅⋅ρ td + t (6.2)
( )
where
m is the glaze mass;
t is the glaze thickness;
d is the cylinder diameter;
ρ
3
is the glaze density; ρ= 900 kg/m .
(4) The ice mass on other section types should be calculated by adding a constant ice thickness t to
the original cross section.
Key
t thickness of ice on different profiles
Figure 6.1 — Ice accretion model for glaze
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6.4 Rime
6.4.1 General
(1) If ice accretion is rime, ice action shall be based on the Ice Class for Rime (ICRs) at 10 m above the
ground given in Table 6.2, or the characteristic rime ice mass at 10 m above the ground at the site.
Table 6.2 — Ice masses for ICRs
Ice classes for rime R1 R2 R3 R4 R5 R6 R7 R8 R9 R10
Characteristic Ice mass m (kg/m) 0,5 0,9 1,6 2,8 5,0 8,9 16,0 28,0 50,0 *
*To be used for extreme ice accretions
NOTE The numbers represent the upper bound for the corresponding ICRs.
(2) Unless otherwise specified, all rime ice should be considered vane-shaped (see Figure 6.2) on
profiles up to a width of 0,3 m.
NOTE 1 The point of reference for the rime ice vane accretion is defined as the most windward point of the
object, as seen from the icing wind direction.
NOTE 2 For asymmetric objects, the apparent cross-sectional area perpendicular to the icing wind direction
defines the object width.
NOTE 3 Guidance for calc
...
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