Interpreting ice charts: chapter 4

Canadian Ice Charts (1967-1981)

Ratio Code

In 1967, a new ice observing and reporting system was developed in Canada which conveyed information simultaneously concerning sea ice concentration and stage of development (thickness). This digital code was in use until 1981. The basis of the system was that the amount of ice in each of six stages of development categories was reported in an invariable order:

• Multi Year Ice
  • Second Year Ice
    • First Year Ice
      • Grey-White Ice
        • Grey Ice
          • Nilas and New Ice

The concentration of the thinnest ice type was required with a zero used to maintain the order if ice types were absent.   The older (thicker) forms were omitted if they were not present. This meant that nilas and new ice were always in the units column of the digital report, grey ice was in the tens column, first year ice was the thousands column, etc.

Examples:

• 42: Indicated 4 tenths grey ice and 2 tenths of nilas or new ice.
• 402: Indicated 4 tenths of grey-white ice, no grey ice and 2 tenths of nilas or new ice.
• 4200: Indicated 4 tenths of first year ice and 2 tenths of greywhite ice. There were no grey, nilas or new ice.

This system was useful in southern waters during the winter but in the Arctic, when the older forms of ice were present, the number of digits was found to be unwieldy, particularly when only zeros denoting the absence of an ice type were used. For those situations, a decimal point was used - always between second and first year ice. It was considered as part of the second year ice report.

Thus 3.4000 would indicate 3 tenths of second year ice with 4 tenths of first year ice; there were no grey-white, grey, nilas or new ice. With the decimal reference point, there was no need to report the zeros, for their function was merely to put the 4 into the proper column. The report 3.40 conveyed the same information, the single zero being added by convention as a reminder.

Examples:

• 21.60: Indicated 2 tenths multi year ice, 1 tenth second year ice, 6 tenths first year ice and no younger forms of ice.
• 1.420: Indicated 1 tenth second year ice, 4 tenths first year ice, 2 tenths grey-white ice and no younger forms of ice.

Other Features:

Floe Size

In addition to the amount of ice in each class which was present, the floe size of the ice also had an effect on the progress a ship could make through the ice. For this reason the tenths of medium floe or greater was also reported. This appeared on the second line of the digital report. A medium floe was over 100 meters long (about 300 feet).

Example:

: Indicates 4 tenths of first year ice of which 3 tenths were in floes medium size or greater; 2 tenths of grey-white ice were present, all small floe or less in size; 1 tenth of grey ice were present, all medium floe or larger. No new ice was present.

Other Features:

1. A slash (/) was used to indicate less than 1 tenth of medium or larger ice floes.

Representation on a chart: