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Gas turbine output is quoted at ISO conditions of 15 C at sea level. Real output falls roughly 0.7% per degree above 15 C because hot air is less dense and the compressor moves less mass, and further with altitude as ambient pressure drops. Inlet filters and exhaust ducting take another 1-3%.
Temperature derate
Factor = 1 - 0.007 x (ambient T - 15)
Altitude derate
Factor = (1 - 2.25577e-5 x altitude)^5.2559
They are constant-volume machines, so mass flow falls directly with air density. A 40 C day can cost 15-20% of ISO output — exactly when electricity demand peaks.
Inlet air cooling — evaporative coolers, fogging or mechanical chillers — restores much of the temperature derate and is standard in hot climates.