Persistence of wind direction

Persistence of wind direction is a neglected subject. However, it is one of the significant parameters in dosage evaluation from short term releases of pollutants to the atmosphere. In this paper this topic is studied in detail using the continuous wind data from three nuclear power station sites in India. It is found from the analysis that longer persistence periods are less frequent. The frequency of occurrence of different persistence periods obeyed a log-normal distribution at two sites whereas at the third site, though the behaviour was qualitatively similar, no known analytic function could be fitted. The extrema and the averages of the wind speeds averaged over individual persistence periods, showed a dependence upon the persistence period as follows: 1. (1) the average maximum wind speed decreased with persistence; 2. (2) the average minimum wind speed increased with persistence; 3. (3) the average of the average wind speed first increased with increasing persistence and then tended to a stable value. Such analysis is useful in defining the worst meteorological conditions at nuclear station sites for applications in hazard evaluation. Some of the applications of the persistence analysis are discussed, and it is shown that dosage at any site from short term (h) continuous release of fission products has a maximum and that this maximum, which is different for different sites, can be used to grade the sites. An expression for return periods of persistences has been derived. This expression gives the return period in actual time units.