The exact moment the Landsat 8 satellite is passing above Bangkok you can find it in the metadata file of the Landsat image.
If it passes in the morning, then maybe you should find another satellite platform that might fly closer to the afternoon period, like ASTER or MODIS to estimate the Land Surface Temperature (LST).
You should also have meteorological data from a meteorological station.
Remember that thermal satellite images estimate the LST (the Surface Urban Heat Island - SUHI), not the air temperature ( classic Urban Heat Island - air temperature). So, there are 2 distinctions between SUHI and UHI.