It is (likely) showing the 0.1% percentile of measured wet-bulb temperatures. i.e. in a given year, we expect 0.1% of the wet-bulbs to exceed this temperature o

It is (likely) showing the 0.1% percentile of measured wet-bulb temperatures. i.e. in a given year, we expect 0.1% of the wet-bulbs to exceed this temperature or approximately 8 to 9 hours in a year.