Herbicides in general tend to work best in warm sunny conditions when weeds are actively growing and cycling nutrients into their growing points. In these conditions, weeds will take in herbicides most efficiently. In cool cloudy conditions, weed growth is slower and herbicide efficacy may be lower.
Cool humid conditions are also prime conditions for herbicide injury to the crop. The leaf cuticle (waxy layer) is thinner allowing more rapid uptake of herbicide into the plant and cool conditions reduce the speed at which the herbicide is inactivated in the crop. This can lead to a flash of injury in the crop. This injury is temporary in most cases and once good growing conditions return, the crop recovers and yield loss is rare.
Talk to your local product rep to see how they will support the use of their product when applied in cool temperatures. Set expectations according to weather conditions.