Summer.
Aircraft just work better in lower temperatures. Colder air is denser, and so colder air means that the wings produce more lift, the engine produces more power, the propeller produces more thrust and so on. This results in much lower fuel consumption during winter months.
Humidity is generally higher in summer than in winter.
It is higher in the summer
It is higher in the summer
It is higher in the summer
It is higher in the summer
It is higher in the summer
It is higher in the summer
In the summer
In the summer
In the summer
Summer because photochemical smog is created by the actions of sunlight on pollutants, and sunlight is stronger in the summer than winter.
In the summer Phoenix is very hot. In the winter it is warm and comfortable.