Bangkok hot season: coolest spot around

Almost everyone is amazed at the coolness of the sky, shown here as 16 degC (61F) on a hot humid Bangkok day (dew point 25C) when the air temperature is in the nineties (34 C). If we go up country a bit we will find it's even hotter but the water vapor will be lower and the corresponding radiant temperature of the day and night sky as low as freezing point,  0 deg C! Our sense of how hot it is where exposed to the sky (but preferably in shade) will be a compromise between radiant temperature of all the objects around us (sky above at 16 degrees and land below the horizon which here is about 34 degrees) and the air temperature. In other words clear sky exposure, out of the direct sun, helps comfort.

But the main benefit of a cool sky is to cool the ground surface at night time. Locally the moisture in the air and cloudiness are the main factors that affect this night ground cooling, with dry clear sky promoting cooling by roughly 100 watts per square meter. By comparison, the effect of raised CO2 levels in the atmosphere is less than 2 watts per square meter, though this effect is near-global and not easily reversed. The great uncertainty about human-caused global warming is the interaction between CO2 and water vapor. If CO2 promotes higher atmospheric humidity then its greenhouse effect is much amplified. This is difficult to model, hence the uncertainty in estimated impacts.