· · · Repository Hydraulic Engineering Reports

In this report information is given on the probability of (extreme)dry or wet periods during the growing season. It is rather difficult to define an objective parameter for the indication of what is called "dry"or "wet". In this report we have chosen precipitation RR, open water evaporation Eo and the potential precipitation surplus RR-Epot, where Epot is the potential evapotranspiration. Of these three quantities frequency distributions for different periods in the growing season are given for six selected stations using the time series 1911-1975.

The numbers of easterly gales that occurred in the North Atlantic Ocean north of 25°N been estimated by using a selection criterion involving minimum values for the meridional pressure gradient, for each day of the period 1881-1970. To do this use has been made of magnetic tapes from the "Deutscher Wetterdienst" with daily pressure values (for 12 hours GMT) in grid points. This method is not without shortcomings, because the wind field depends on more factors than the pressure gradient only. When the pressure difference indicates an easterly gale there are two possibilities: the statement is correct or not correct. By applying a statistical method an estimate could be made of the percentages of (in)correct statements. The percentage of correct statements appeared to be around 80% in the area between 70°N and 35°N. South of 35°N this percentage decreases considerably. On the North Atlantic Ocean January is the month with the most frequent and July the month with the least frequent easterly gales. In general the maximum number of easterly gales is found in the area between Iceland and Greenland, while a secondary maximum is found south of the American coast between New York and Newfoundland. From the maxima southwards the numbers of easterly gales gradually decrease. In autumn a secondary maximum occurs in the area between the Azores and the English Channel, while in spring to the contrary a secondary minimum is found in this area. More interesting is the following. Looking at the number of days per year on which an easterly gale did occur it appears that especially in the area between 70°N and 50°N there are considerable differences between the successive periods 1881-1900, 1901-1920, 1921-1940, 1941-1960 with a minimum in the second period and a maximum in the last period. The differences between these periods are often highly significant. The fluctuations in the numbers of days per year with an easterly gale can be explained by the climatic fluctuation connected with the cycle of solar activity with a period of 80-90 years.

Verslag van de stormvloed van 2 januari 1979.

Verslag van de Stormvloed van 6 november 1979.