Anything that effects the weight of air that goes into the cylinder every cycle will effect the output.

If the air is cold, then it's more dense and therefore more weight of air enters the cylinder per induction stroke. More air to expand when it combusts, therefore more power. That's what the intercooler in a turbocharged engine does. There's a heat exchanger (radiator) between the turbocharger compressor and the inlet manifold so that the compressed (and therefore hot) air is cooled to increase its density.

Humidity is a bit more difficult as water dissolved in the air does not expand when it gets hot in the same way as the air itself and the burn temperature is different (hence the problems you had in setting the mixtures).

So, low pressure from altitude and high temperature will reduce the output, but a variation in humidity is more difficult to predict!

-David C.