Valid range of inlet air WB for cooling towers

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I got the following warning in my model.

   ** Warning ** CoolingTower:VariableSpeed "CT-T-L3-04" - Inlet air wet-bulb temperature is outside model boundaries at 29.81.
   **   ~~~   **  ...Valid range = -34.40 to 29.44. Occurrence info = CUSTOMRUN, 07/21 11:00 - 11:09
   **   ~~~   **  ...Inlet air wet-bulb temperature passed to the model = 29.444400
   **   ~~~   **  ...Wet-bulb temperatures outside model boundaries may not adversely affect tower performance.

The valid range of inlet air WB for cooling towers seems to be defined by the code, not specified by users. Why is there such a limit? The warning appeared only 3 times during annual simulation with an epw file from a location in China. The limit does not have a significant impact on the simulation result in my case, but the limit would affect the cooling tower leaving water temperature as it is usually controlled by the fixed approach setpoint.