High humidity, high temperatures, higher altitude,and low pressure systems all make the air less dense. So you need to run leaner to keep the fuel/air ratio right. Low humidity, low temperatures, lower altitudes and high pressure systems make the air denser so you need to run richer to get the proper fuel/air ratio. Air density also affects prop performance, airplane performance, and engine cooling, so treat the previous statements as incomplete, but useful to think about.

Jim