resulting in more than 20,000 individual records of Antarctic sea ice conditions since 1980. Sea-
ice thickness is reported from ranges of actual thickness from individual point locations or using
the proxy of sea-ice type. Individual point observations are made by trained ice observers who
estimate ice thickness, in centimeters, as the ice turns sideways along a ship's hull moving past a
ball of known diameter hung over the side (Worby 1999). Five cruises are chosen for this initial
analysis based on their location and time period as validation data sets to compare with weekly
ice charts. These include ship voyages from May/June 1995, August 1995, May/June 1998,
December 1998/January 1999, and December 1999/January 2000.
Figure 2. Ship-based observations as part of five separate voyages in the Ross Sea
during 1995 through 2000.