Establishment of Climatologies

Climatologies discussed in this study are based on measurements near local solar noon and daily doses. The latter were derived by integrating measurements over 24-hour periods. Spectra were measured hourly until 1997 and quarter-hourly thereafter. The following times (provided in Universal Time) were associated with noon: McMurdo: 01:00; Palmer: 16:00; South Pole: 00:00; Ushuaia: 17:00; San Diego: 20:00; Barrow: 22:00; and Summit: 15:00. To set up a climatology for noontime irradiance, we calculated for every site and every day of year the average, median, and maximum, as well as the 10th and 90th percentiles, using data from the periods indicated in the last column of Table 3.1. As the maximum may not always occur at noon due to changing clouds and total ozone, we also calculated the maximum within ± 2 hours (± 12 hours for South Pole) of the times indicated above. This value is denoted "daily maximum."

Figure 3.2(a) shows the resulting climatology for the UV Index (i.e., erythemal irradiance multiplied with 0.4 cm /p,W (WHO, 2002)) at the South Pole. The time-axis of the plot starts at winter solstice (22 December). Individual measurements are indicated by small dots. Average and median are plotted as red and green lines, respectively. Ten percent of the measurements are below (above) the lower (upper) blue line. The daily maximum is indicated by a thin grey line. All lines exhibit a large day-to-day variability. To facilitate interpretation, an 11-day running-average filter was applied to the average, median, and 10th and 90th percentiles. The resulting graph is plotted in Fig. 3.2(b) and will be discussed further below.

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