The temperature range for each drug is usually indicated on the label. In most cases, lowering the temperature means lowering the effectiveness of the drug. But be prepared for the fact that this will increase phytotoxicity and instead of reducing the result you will get the death of the treated culture.
If the ambient temperature is higher than indicated for this pesticide, it is possible either a decrease in efficiency (pyrethroids) or, on the contrary, an increase — carfentrazone (for desiccation) is in great need of hot weather and several hours of sunlight.
Let us summarize a short intermediate result: the air temperature itself can have a definite influence on the quality of treatment, but more often it should be considered as one of the factors that change the air humidity.
As mentioned in this article, high humidity carries certain risks due to stopping of the evaporation of droplets, and low humidity can cause unnecessary waste. Let's start with an approximate calculation, which shows why it is better to reduce the concentration of the working fluid to such figures that the outflow rate is not less than 200 l/ha.
Suppose a boom with nozzles is located at a height of 50 cm from the surface of the earth. To handle one hectare of a drop of liquid, an air column with a volume of 5,000 m3 (10,000 m2 * 0.5 m) must pass. When the sprayer moves at high speed, air turbulence will increase this height.
At a temperature of 20° and 100% humidity 86.5 l of water can be dissolved in it. If the humidity is 60%, then only 52 liters of water will be dissolved in 5000 m3. That is, this volume of air is “ready” to dissolve in itself 86.5l-52l = 34.5l of water. If you use highly concentrated plant protection products, with a rate of 50l/ha, then theoretically, the result of the introduction could be a loss of about 65% of the substance. About 15 liters of plant protection «may reach» the plants, and the rest will evaporate.
If the rate is 100 l/ha, then the loss in the form of evaporation of 34.5 l of water (and percent) is no longer so significant. At a rate of 200 l/ha, the dissolution of 34.5 liquids in the air means a loss of 17.5% PPP. That is, the lower the concentration and the greater the rate of outflow, the lower the loss due to evaporation of the working fluid.
It is not a good idea, to expect high humidity to reduce evaporation losses - this often happens before rain. It is much easier to reduce the concentration of the working fluid. Returning to high humidity - especially in such a situation, it is not a good idea to use small drops (which occur at a low rate of application). At best, it will rain and just wash them off to the ground. At worst - such long-lived drops will not settle on the leaves and fly away with the first breeze to the next field. The consequences will be unpredictable. The relationship between droplet size and volatility was discussed in more detail in the article «Is it better to lower the solution concentration or the outflow rate?».
Usually for a lower application rate, spray nozzles with a lower serial number are used and the sprayer can move faster over the field because the pump load is lower. The optimal size (which is well resistant to wind, evaporation and not very volatile) is 200-400 microns. Drops smaller than 200 microns behave more like an aerosol, therefore, even after «reaching» to the level of the ground or leaves will not always settle on them.
Increased volatility makes them «easy prey» for the wind, thermals (air flows rising from the heated soil) and evaporation. Add here the vortex flows arising behind the boom and the sprayer tank. If the speed of movement across the field exceeds 18 km/h, even a modern computer will not be able to simulate them, so you can simply write them to the section of useless wastes of working fluid.
Thus, the high temperature can do a disservice if accompanied by low humidity - in this case, you can expect significant costs of PPP to evaporate the working fluid.