The relationship between meteorological and oceanographic parameters to the onset of ice formation and winter severity in Cook Inlet is investigated using data from 1969 to 1980. Air temperature, in the form of adjusted frost degree-days, is the parameter most highly correlated with onset of significant ice (r = 0.960) in the upper inlet. From this relationship, a curve is developed to provide a simple operational method for predicting ice formation. In the lower inlet, a more complex relationship exists between heat flux from the Gulf of Alaska waters, river runoff, and air temperature to the onset of ice. The heat exchange between Gulf of Alaska and Cook Inlet waters is of primary importance to onset of ice. Attempts at correlating winter severity to the meteorological and oceanographic parameters show that the southern extent of the ice in the lower inlet is due to climatic differences affecting the rate of inflow and temperature of the Gulf of Alaska coastal water into Cook Inlet and modification by the local wind-driven drift of ice.