Author(s): C. Watte; C. Marche
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Keywords: No Keywords
Abstract: The most remarkable feature of the ice season, is its extreme variability. A given location, such as Port-Alfred, could be still ice-covered at the end of March on some years while being completely ice-free on others. Hence the importance of the construction of average and extreme ice conditions charts. Two statistical procedures that could be used to analyse ice observations are developed. The climatological approach consists of classifying winters according to the concept of freezing degree-days by means of which a specific ice distribution is deduced. On the other hand, the ice navigability approach is a statistical study of ice observations based on an "operating conditions" format. Bradford scale converts ice type, concentration and thickness into impediment factor to navigation (minimum horse-power required per foot length of vessel) the statistical nature of this method leads to a higher confidence in the determination of average and extreme values. A comparison between the two approaches reveals the importance of wind as a factor in the break-up process. Climatological and Hydrological factors affect in a complex manner the formation and growth of ice cover. So, an evaluation of the principal parameters involved led to the development of several empirical formulae for short-turn forecasting of cover formation and break-up dates and the prediction of ice thickness. These formulae, being based on the concept of thermal balance, need systematic recording of meteorological and hydrological data. However, atmospheric general circulation forecasts could be used for long-range prediction of ice-cover parameters. An estimate of climatologic winter parameters leads to probable ice distribution by analogy with a similar previous year.
Year: 1978