Applications made when cool cloudy days follow cool nights will result in lower herbicide efficacy than applications made in warm sunny days. Cloudy days don’t provide the photosynthetic activity required for many herbicides, including Group 10 glufosinate. And nights near freezing followed by days with highs that barely reach 10 C will not provide high metabolic activity required for best results from Group 9 glyphosate.
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.