4 EXPECTED ICE CONDITIONS
The following Map Set 1 shows the mean ice con-
would be encountered near this boundary. Similarly,
ditions for the period of record (January 1986 to April
the regions with the highest likelihood of any ice are
1999) for each biweekly interval between 1 Decem-
indicated by the >75100% occurrence zones. These
ber and 31 March. These were calculated simply from
zones are highly probable of having ice, even during
the arithmetic mean of ice concentration and stage of
the least severe ice years.
development. Map Set 2 shows the probability of oc-
Map Set 3 shows the same probability zones of oc-
currence of any ice, regardless of concentration or stage
currence but for a more severe ice condition, that of at
of development, and shows the 0%, >2550%, >50
least 5/10ths concentration. Certain applications of in-
75%, and >75100% zones of ice occurrence prob-
terest may depend not simply on the mere presence of
ability. Map Set 3 shows the same zones of probabil-
any ice, but may require the additional knowledge of
ity of ice occurrence but for ice of at least 5/10ths
its concentration. For example, certain vessel naviga-
concentration. The process by which these maps were
tions may be affected only when ice concentrations
derived is explained in Appendix C.
are 5/10ths or greater, and certain behaviors of marine
The use of the different map sets depends on the
animals, fish stocks, and waterfowl may be similarly
particular application of interest. The most compre-
dependent. As with Map Set 2, during a very severe
hensive information is contained in Map Set 1, which
ice year, 5/10ths ice concentration may be encoun-
shows two different ice variables: mean ice concen-
tered at the boundary between 0% and >025% oc-
tration (indicated by color) and mean stage of devel-
currence. Likewise, during minimum ice years, 5/10ths
opment (indicated by hatching pattern). Concentration
or greater ice concentrations are very likely to occur
is a relative measure of the water surface that is actu-
within the >75100% occurrence zones.
ally covered with ice. Stage of development provides
It is important to remember that these maps were
a general indication of the ice thickness and its integ-
developed from ice charts that the NWS constructed
rity, or strength. Both conditions are important and
using a combination of remote sensing observations
necessary for mariners who need to know whether a
and visual reports. While the NWS uses a wide range
particular region might be navigable at a particular
of available tools to construct their charts as accurately
time during an average ice year.
as possible, that accuracy is limited by the amount and
Map Sets 2 and 3 show only the probabilities of ice
resolution of the available data. For example, small
occurrence, without respect to its stage of develop-
amounts of ice could exist outside the 0% probability
ment. By extension, they indicate the extreme mini-
boundary, because these amounts may have been too
mum and maximum regions of ice extent for two ice
small to be observed visually or detected by remote
concentration levels. For example, an interest in know-
sensing during the initial NWS analysis. The follow-
ing where in Cook Inlet any ice has occurred, regard-
ing maps are therefore not meant for actual naviga-
less of concentration, would require the use of Map
tion or for any other purpose upon which their accu-
Set 2. The extreme limit of occurrence of any ice is
racy may be relied for the safety of life or property.
denoted by the boundary between the 0% occurrence
They are simply statistical representations of the ice
and the >025% occurrence. In other words, during
conditions as reported by the NWS for the period of
the most severe ice year, the maximum extent of ice
record.
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