The Gulf of St. Lawrence Is closed to commercial navigation for nearly five months each year due to ice conditions. In order to lengthen the shipping season, greater knowledge of the behaviour of the ice is necessary. A step in this direction was the inauguration in 1940 of aerial ice surveys in the gulf during the breakup season. The surveys have continued annually for the past thirteen years. Based primarily on the data provided by these surveys, maps were drawn showing the limits of the main ice areas in each breakup season. Although the maps enabled the isolation of several distinct patterns and rates of breakup, they revealed that the behaviour of the ice was extremely variable. The factors influencing ice conditions, including tides, ocean currents, temperature, and wind, were examined with the aim of discovering the causes of the breakup patterns. A number of factors were found to be important in determining the fundamental behaviour of the ice, bat the meteorological factors of temperature and wind appeared to be the chief agents in causing the variable behaviour from year to year. In spite of these variations, it was possible to trace average conditions throughout the Ice season. The chief characteristics of the ice season may be stated briefly. The Gulf of St. Lawrence is never completely covered with ice, but rather, it is partly covered with fields of shifting pack ice between which lie broad stretches of open water. The southern part of the gulf is an area of accumulation where ice conditions are most serious. The clearing of ice from the gulf begins slowly in January and February and becomes accelerated in March and April. The bulk of the ice moves through Cabot Strait to the open Atlantic rather than remaining inside the gulf until it melts. Generally, the ice either withdraws from west to east, passing through Cabot Strait directly, or it stagnates in the southern part of the gulf toward the end of the season. By the first of May the gulf is usually clear of ice which constitutes a hinderance to navigation. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate