All thermodynamic processes are more efficient at higher absolute temperatures. This was discovered by Sadi Carnot in the 1800's. Carnot applies equally to heat engines as well as cooling systems. This means that the thermal efficiency of a thermal process increases as the temperature rises, so the ACH actually is more efficient as ambient temperature rises. At 80º F, the dT, or temperature difference in cooling mode is apx. 15º F below ambient, but at 110º F the dT will rise to apx. 20+º F below ambient in cooling mode. This is very fortunate, because, obviously, the hotter it gets, the more cooling is desired. If ambient is 110º F and cooling is -20º F, the net air temp. will be 90º F, which is still 5-6º F below skin temperature, so cooling will occur. Without cooling, a skin temp. of 95-96º F will actually absorb heat from ambient air at temperatures above 95-96º F, leading to heat exhaustion if exposed long enough to high enough temps.
If the ACH is used in a comfortable environment, scalp cooling will feel more intense and could even produce goosebumps on the arms, etc.,, since it could get into the area of excessive body cooling. This is why ventilation mode will be useful for those times when the environment is marginal.
Relative humidity effects cooling because the specific heat, or power, required to change the temperature of a given amount of moist air, is higher than that of the same amount of dry air, requiring more power to cool. However, the relative humidity, density, and specific heat of the cooled moist air has more cooling power on the user's head, can absorb more heat for a given temperature rise, so that almost balances out.