the Tair is the air temperature of the zone, i.e. the average temperature of the air in the zone (based on single node approach). The operative temperature in the zone is a parameter introduced due to ist correlation to indoor thermal comfort: as a human being we experience temperature as a combination of the indoor air temperature (convective heat exchange) and the temperature of the zonal surfaces (radiant heat exchange). This combined temperature is what the operative temperature stands for. Based on engineering judgement we commonly use a 50% air and 50% mean radiant temperature to calculate the operative temperature.