If its really hot outside, the gas in your magazine will warm up and increase pressure. This may put your gun's FPS over the field limit, even if it was field-legal when you chrono'ed it.
With gas guns you also have the cooldown effect. After multiple shots, your magazine (and the gas inside) will start to become cold. This will decrease pressure. With rapid firing, the pressure will drop much faster, to a point where the gas won't be able to cycle your gun properly anymore.
When its cold outside, this effect is compounded. How fast the magazine cools down varies between guns.
H&K G36c (KWA) / Glock 18c (TM) / Kimber LAPD SWAT Custom (TM) / Mossberg M500 (ACM)