Thanks for your reply PKPF68-77.
My thermometer was situated in the shade but was certainly picking up the effect of the warm air rising from the solar heated patio and heat coming off the walls of my house. It was, I'm sure, an accurate measurement of the air temperature in the localised conditions on my patio but would not qualify as a true measure of air temperature in met terms as it wasn't measured within a Stevenson Screen and wasn't measured in a location that was well away from buildings, trees, etc. Having said that, all I was interested in finding out was how much warmer than the reported met conditions the air was in the shade close to where I was sitting and the answer was that it was significantly warmer.
Similar microclimates must be able to exist above a runway in certain circumstances. If the tarmac reaches say 50C and the air temperature at the airfield's weather station is 23C then on a calm sunny day the air temperature above the runway is likely to be higher than 23C.
Looking at the chart posted by PKPF68-77, it would be interesting to know what the very localised air temperature was at, say, 2ft, 3ft, 4ft 5ft and 6ft above a hot runway, just to see how the runway temperature affected the air temperature.
I suppose that, if I was a manufacturer and was calculating T/O performance figures, I would assume a worst case scenario (i.e. strong sunshine heating the runway and the surrounding air), then on cloudy days with a cool runway the performance would be better than expected and on sunny days performance would still meet the specifications. That would then explain why, as Atreyu says, it's not something that pilots need to take into account when calculating T/O performance.
Regarding the Trident, was the wing in ground effect phenomenon worse, better or the same on hot days? Are the landing characteristics of any other aircraft affected by hot runways?