You can "take this to the bank." Hot and humid weather makes an engine richen. The air is thinner and you have to lean the engine to compensate if you want optimum mixture setting.

Cool, dry air is more dense, and will lean an engine out. With large temperature changes to cooler air you need to richen your engine to regain optimum mixture.

Barometric pressure and field altitude also make differences. Especially field altitude. An engine loses approximately 3% power for each 1000' increase in density altitude. Density altitude is a function of true altitude, barometric pressure, and temperature. An engine at a density altitude of 7000' will have about 21% less power than the same engine at sea level, 59 degrees, and 29.92 barometric pressure. (ISA) This sounds like a high altitude, but it is commonly encountered in the western United States due to high field elevations and high temperatures. I have seen many 7000' foot days in Tucson, Arizona.

At high density altitudes; over 5000' due to high temperatures and true altitudes, Continental and Lycoming recommend leaning their engines for take off power! In these conditions, you can add full power, and start leaning the mixture on a full size airplane, and just watch the tach start climbing; gaining power because the mixture was too rich. Baseline full rich mixture is set for standard conditions, (ISA) of sea level, 29.92 barometric pressure, and 59 degrees. Any thing above that in temperature, altitude, or lower in barometric pressure, or any combination thereof, causes a richer than optimum setting. Humidity has the effect of lowering air density as well, but is less of an effect than temperature or altitude.

AV8TOR