Unfortunately, you are area weighting based upon areas for which you do not have adequate information. (wattsupwiththat.com)
But remember, the existing products include a compensation to try and remove UHI, UHI only impacts our long term temperature results if the magnitude of the effect is growing, and each station's data still has to be added to the results for all other stations using Area Weighted Averaging. (skepticalscience.com)
A breakdown of contributing ACORN stations for area weighted mean minima and maxima, using a 5x5 degree cell grid since 1910, shows the percentage of the Australian continent with temperature data input by decade: (waclimate.net)