"Foliar absorption of herbicides occurs in a liquid phase only; once a water droplet has dried on the leaf surface and herbicides have crystallized little to no additional absorption occurs. Therefore, any environmental condition speeding the drying of spray droplets on a leaf surface will reduce absorption. Low humidity and high winds can greatly reduce drying time, thereby allowing little time for absorption to occur. Conversely, high humidity with little wind slows the rate of drying and lengthens absorption time. Rainfall shortly after (< ½ hour) glyphosate application can wash spray droplets from the leaf surface. A foliar application should be "rain fast" once droplets have dried on the leaf surface.
Temperature, soil moisture, and solar radiation that optimize plant growth facilitate absorption and translocation of glyphosate. When photosynthetic rates are high photoassimilate produced in leaf epidermal cells is rapidly loaded into the phloem, other organic molecules like glyphosate are similarly loaded, and both are quickly translocated to sink organs. The rapid removal of glyphosate molecules from epidermal cells maintains a high concentration gradient that increases absorption rate. The time of day glyphosate is applied can also impact its efficacy. Applications made between 9 a.m. and 6 p.m. tend to maximize glyphosate activity. Short-lived temperature spikes (> 90 ºF) can also enhance absorption by reducing cuticle viscosity and allowing easier passage of foliar-applied herbicides."
From Purdue University:
"Glyphosate must penetrate the leaf surface to provide effective weed control. While absorption occurs relatively quickly, rain after an application can wash glyphosate off before it has a chance to enter the leaf. The rain-free period required to prevent reduced activity is in uenced by the susceptibility of the target weed and the glyphosate rate. Small weeds of a sensitive species will require a shorter rain-free period than large or dif cult to control weeds. A 30-minute rain-free period may be adequate under ideal conditions. When spraying larger weeds, however, several hours between application and rain may be required to avoid reduced activity. Differences in rainfastness among glyphosate products are generally small. Adding more surfactant appears to have marginal benefits on the rain-free requirement."