Interpreting ice charts: chapter 1

Table of Contents

The Egg Code

The basic data concerning concentrations, stages of development (age) and form (floe size) of ice are contained in a simple oval form. A maximum of three ice types is described within the oval. This oval and the coding associated with it, are referred to as the "Egg Code". To indicate ice observations interpreted from radar imagery, the oval shall be omitted.

In the following figures and tables where ranges are shown for thickness, floe sizes or other dimensions, a report coinciding with the end point of a range shall be coded as the higher value.

The following is a summary diagram of the Egg Code. This code conforms to international convention and shall be used in coding all visual sea ice and lake ice observations without exception.

Diagram of the components of the egg code: total and partial concentration, stage of development and forms of ice. See description below for details.

The symbols Ca CbCc and Fa FbFc correspond to SaSb Sc respectively.

There are some minor additions to the egg code symbology that are Canadian practice. In Canada, to enable the reporting of additional ice classes, especially during freeze-up and break-up, Cd Se and Fe can be used. This should not be a common occurrence.

The following pages describe the specific details and rules for completing each level of information within the egg.

Concentration (C)

Diagram of the egg code indicating the location of concentration of ice.  See description below for details.

Total concentration (Ct ) of ice in the area reported in tenths and partial concentrations of thickest (Ca ), second thickest (Cb ), third thickest (Cc ) and fourth thickest (Cd ) ice in tenths.


  1. Less than 1/10 (i.e. traces) shall not be reported within the oval except to describe open water (see Example 1, section 3.8).
  2. Cd shall only be included when Sd and Se are reported (see Example 2, section 3.8).
  3. When Sd is used and Cd is omitted, Cd equals Ct-(Ca+Cb+Cc) (see Example 3, section 3.8).
  4. When only one ice type is present, the partial concentration shall not be indicated (see Example 4, and Example 5, section 3.8).
  5. When one ice type is present with a trace of a thinner type, only total concentration of the major ice type shall be indicated (see Example 5, section 3.8 ).

Stage of Development (S)

Diagram of the egg code indicating the location of stage of development. See description below for details.

Stage of development of thickest (So), second thickest (Sa), third thickest (Sb) and fourth thickest (Sc) ice and the thinner ice types Sd and Se, of which the concentrations are reported by Ca Cb Cc Cd respectively.


  1. Reference to thicker ice should be understood to mean older ice and conversely, thinner ice to mean younger ice types.
  2. Ice is designated as Sea, Lake or River Ice depending on where it forms. In Canada, the practice is to use lake-ice coding to report ice in the Great Lakes and the St. Lawrence Seaway. Elsewhere, including the St. Lawrence River east of Montreal, sea-ice coding is used for stages of development.
  3. Sa, Sb and Sc shall have concentrations of at least 1/10, except when Ct is zero (see Example 1, section 3.8).
  4. Reporting of Sa, Sb and Sc should generally be restricted to a maximum of three significant classes. In exceptional cases further classes may be reported as follows:
    • So- Stage of ice development thicker than Sa, but having a concentration less than 1/10 (see Example 6, section 3.8).
    • Sd- Stage of development of the thickest remaining ice types (if more than one type remains). It is the fourth stage present after Sa, Sb and Sc.
    • Se- Shall only be reported when a thinner ice type remains after Sd. Partial concentration of Se is obtained by subtracting partial concentrations (Ca Cb Cc Cd) from total concentration (Ct) (see Example 2, section 3.8).
  5. When Se is not present, Sd may be a trace of ice (see Example 6, section 3.8).
  6. Concentration shall not be indicated for So and Se(see Example 2, section 3.8, and Example 6, section 3.8).
  7. Concentration shall not be indicated for Sd when Se is not present (see Example 3, section 3.8, and Example 5, section 3.8).

Coding for Sea-Ice Stages of Development (So Sa Sb Sc Sd Se)

Description Thickness Code
New ice < 10 centimetres 1
Nilas, Ice rind < 10 centimetres 2
Young Ice 10 - 30 centimetres 3
Grey Ice 10 - 15 centimetres 4
Grey-white ice 15 - 30 centimetres 5
First-year ice >= 30 centimetres 6
Thin first-year ice 30 - 70 centimetres 7
First stage thin first-year 30 - 50 centimetres 8
Second stage thin first-year 50 - 70 centimetres 9
Medium first-year ice 70 - 120 centimetres 1·
Thick first-year ice > 120 centimetres 4·
Old ice - 7·
Second-year ice - 8·
Multi-year ice - 9·
Ice of land origin -  Ice of land origin symbol
Undetermined or unknown - X·


Coding for Lake-Ice Stages of Development (So Sa Sb Sc Sd Se)

Description Thickness Code
New lake ice < 5 centimetres 1
Thin lake ice 5 -15 centimetres 4
Medium lake ice 15 - 30 centimetres 5
Thick lake ice 30 -70 centimetres 7
Very thick lake ice > 70 centimetres 1·


  1. On the horizontal line giving So Sa Sb Sc Sd, only one dot (·) shall be placed to indicate the distinction between classes of ice. Every coded figure to the left of the (·) is understood to have the (·) as part of its code (see Examples 2, 3 and 6, section 3.8).
  2. Codes 3 and 6 shall only appear on Canadian charts if the Ice Service Specialist cannot confidently determine the stages of the ice in the area observed.
  3. Codes 8 and 9 shall only appear when measurements have been taken.
  4. Codes 8·and 9·shall normally appear on Canadian charts only from 01 October to 31 December, but if the Ice Service Specialist is confident of the report, it may be used throughout the year, otherwise 7·is used.
  5. The symbol Ice of land origin symbol shall only be used within the egg and when the concentration of ice of land origin is 1/10 or more.
  6. The symbol X (meaning "undetermined") shall be used to designate stages of development or forms of ice only if it is impossible to specify otherwise.

Form of Ice (F)

Diagram of the egg code indicating the location of forms of ice. See description below for details.

Floe Size corresponding to Sa Sb Sc Sd and Se (when Sd and Se are greater than a trace).


  1. World Meteorological Organization International procedures also permit reporting of Fpand Fs as the primary and secondary forms of all the ice without reference to stage of development.
  2. It is Canadian practice to report FaFbFc as predominant floe sizes of SaSbScrespectively. This makes it necessary, when only Sa and Sb are present, that Fa and Fb shall be followed by a dash (-) where Fc would normally appear (see Example 7, section 3.8)

Coding for Forms of Ice (Fa Fb Fc Fd Fe)

Description >Width Code
Pancake ice - 0
Small ice cake, brash ice, agglomerated brash < 2 metres 1
Ice cake 2 - 20 metres 2
Small floe 20 - 100 metres 3
Medium floe 100 - 500 metres 4
Big floe 500 - 2,000 metres 5
Vast floe 2 - 10 kilometres 6
Giant floe > 10 kilometres 7
Fast ice - 8
Icebergs, growlers or floebergs - 9
Undetermined, unknown or no form - X


  1. Width refers to the maximum horizontal extent.
  2. At least one code 8 must be used for fast or consolidated ice. Other ice types embedded may retain their floe size (see Example 9, section 3.8).
  3. Occasionally the stage of development of fast ice cannot be determined. The area shall be blackened-in to denote fast ice (see Table 3.9).
  4. New sea ice does not have a definite form; therefore, when this stage of development occurs as Sa, Sb or Sc, the symbol X shall be used to designate floe size (see Example 4, section 3.8).
  5. Floe size is not included for So, Sd and Se if the concentration of these ice types is less than 1/10. Otherwise floe sizes for Sd and Se are optional.
  6. If there is a significant variation in floe sizes in an area containing only one particular ice type, the Ice Service Specialist may enter the applicable floe-size Categories in the lowest part of the oval reserved for floe size. The largest floe-size category shall be put on the left side within the oval, followed by the other applicable floe sizes. In this case, the partial concentrations listed (Ca Cb Cc Cd) would match the partial concentration of floe sizes, instead of different ice types.


Coding and Symbology for Strips and Patches

The symbol, placed at the bottom of the oval in the section reserved for Form of Ice, indicates that the ice is in strips and patches; the concentration within the strips and patches is represented by C.(see Example 11, section 3.8).

Strips and patches symbol

When strips and patches are observed in open-water areas, the symbol shall be placed to denote the position of the strips and patches. If the ice in the strips and patches is of the same composition as that inside an adjacent ice edge, no oval is required. If the ice in the strips and patches is of a different composition, an oval shall be used with an arrow or arrow(s) to the strips-and-patches symbol(s). To avoid confusion, the strip symbol must be included with the total concentration (see Example 10, section 3.8).

In an area where the ice is arranged in strips and patches and the ice floes are medium or greater, the floe size shall be indicated by using two ovals. The floe sizes are indicated as normal in the first oval, with the symbol placed between the first and second ovals. Thesymbol is repeated in the second oval beside the total concentration of the strips and patches (see Example 12a, section 3.8).

Strips and patches symbol
Strips and patches symbol

An alternate way of reporting the same situation as above:

In an area where the ice is arranged in strips and patches and the ice floes are medium or greater, the floe sizes shall be indicated as normal. Both the total concentration and the concentration within the strips will be placed in the space reserved for Ct, with the symbol between them. When this option is used, Ca Cb Cc and possibly Cd refer to the total concentration and not the concentration within the strips. For example, Ct can be reported as 29 meaning the total concentration is 2 tenths with strips of 9 tenths and the partial concentration(s) shall equal 2 tenths (see Example 12b, section 3.8).

Strips and patches symbol
Strips and patches symbol

In an area of ice where some thicker ice type(s) is (are) embedded as strips and patches, these shall be indicated by the use of two ovals. The overall partial concentrations of the ice types are indicated in the first oval and the concentrations within the strips and patches are indicated in the second oval. The symbol shall be placed between the two ovals and along with the total concentration in the second oval (see Example 13, section 3.8).

Strips and patches symbol


Coding for Brash

Diagram of the egg code indicating the location of brash coding. See description below for details.

If 1 tenth or more of brash is present, it will always be Ca.

If brash is present, Sa will always be a dash (-), otherwise the normal table is to be used.

Brash is already indicated in the table as 1, therefore Fa= 1 confirms the dash (-) for Sa.

Four digits (VKMT) shall be added below the oval to indicate the thickness concentration breakdown of the brash that is present. Table 3.4 (below) shows the thickness Categories for agglomerated brash. The breakdown shall be entered going from right (T) to left (V). In the case where there is no thickness for thin but there are entries for medium, thick and very thick a zero (0) shall be placed in the thin column. This also holds true for medium (M) and thick (K) regardless of the combination (see Example 14, to Example 17, section 3.8).

Thickness Categories for Brash (VKMT)

Description Thickness
Very Thick (V) > 4 metres
Thick (K) > 2 - 4 metres
Medium (M) 1 - 2 metres
Thin (T) < 1 metre

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