Weather Consideration
Just about every herbicide label carries the recommendation to apply products "when weeds are actively growing." While there may be some variations, in general, if it is nice warm day and the sun is shining, the weeds are actively growing.
Cool overcast conditions aren't conducive for plant growth, including weeds. So at cool or even very hot temperatures weed control can be compromised. An ideal temperature range on a sunny day sits between 16 and 24°C . Weeds may be killed slowly below 15°C. Some herbicides may injure crops if applied above 29°C. Avoid applying volatile herbicides such as 2,4-D ester, MCPA ester and dicamba during hot weather, especially near susceptible broadleaf crops, shelterbelts, or farmsteads.
For more specific recommendations consult product labels.