An extreme‐value analysis of wind speeds at five Canadian locations

What strikes one most about the data set of this case study is its sheer size; there are approximately 1.7 x 106 observations each of wind speed and wind direction, and 0.6 x 106 observations of surface pressure. The last two were included on the speculation that they might be useful in understanding the behaviour of the observed winds. Indeed, strong spatial gradients of surface pressure are strongly linked to high wind speeds through the geostrophic equation (see Holton 1972), and strong spatial gradients at a particular time are also generally associated with strong temporal gradients of surface pressure. Thus, on the face of it, the surface pressure record should contain useful information about "extreme" winds. However, because of the size of the data set and the limited amount of time available, this connection has not been pursued. In order to make the data-handling task a manageable one, only the daily maxima of the hourly observations were studied. This decision reduces the total number of wind-speed observations to approximately 69,000. The second thing which impresses one is the fact that this is a very "dirty" data set, as are almost all long geophysical time series. The observations, one-minute "mean" wind speeds recorded hourly, are taken subjectively either from a chart recorder or by observing the movements of a dial over a one-minute period. Judging from the instructions which meteorological technicians are given regarding the method for taking these observations, the observations might be better described as subjectively determined one-minute median wind speeds. Observers are instructed to ignore short gusts and lulls. The observations were taken with a variety of instruments over the years. The height and/or location of the measuring instruments has been changed at least once at every station, and the information which is available about the history of these changes is rather scant. The local topography, which has a profound effect on the characteristics of air flow past a given point, has changed over time with encroaching urbanization and the gradual development of the airports at which the instruments are located. Additional complications which are present in most geophysical time series include the effect of the seasonal cycle on the characteristics of the wind speed and the fact that the atmosphere exhibits variability on long time scales which is not seen in "textbook" weakly stationary stochastic processes.