Of all the activities scored here, crop spraying has the most precise and consequential weather requirements, and the score is built around them. When you apply a pesticide, herbicide or fungicide, the goal is for the product to land on the target and stay there, and wind is the main thing working against you. The counterintuitive truth is that both too much wind and too little are problems. The professional ideal is a light, steady breeze of roughly five to sixteen kilometres per hour, or about three to ten miles per hour, and the score peaks in exactly this window. Above roughly fifteen miles per hour, drift becomes unacceptable and the labels on many products legally prohibit application, so the score falls away sharply.
The most important and least intuitive part of the spraying score is that it penalises dead-calm conditions, because still air often signals a temperature inversion. On a clear, still night or early morning, the ground cools and a layer of cold air becomes trapped beneath warmer air above, and this layer is extremely stable, refusing to mix. Spray released into an inversion does not disperse and settle; the fine droplets hang suspended and can drift sideways for long distances, damaging neighbouring crops or sensitive areas hours later. The score watches for the tell-tale combination of very low wind, clear skies and low humidity, and flags a likely inversion, dropping the score even though the calm conditions might otherwise seem ideal.
Beyond wind, the score accounts for the other conditions that affect a spray application. Rain too soon after spraying can wash the product off before it has become rainfast, wasting it and risking runoff, so the score checks that conditions are dry. Temperature and humidity matter too: very hot, dry conditions cause fine droplets to evaporate before they reach the plant, reducing effectiveness, while the product's own label often specifies a temperature range for best results. The score weighs these alongside the wind and inversion factors to give an overall sense of whether the conditions favour an effective, low-drift application.
This score is a planning aid, not a substitute for the product label, professional judgement or the law. Always read and follow the specific label instructions for the product you are applying, which set legal requirements for wind speed, buffer zones and conditions that override any general score. Keep records, be mindful of sensitive neighbouring areas, watercourses and bystanders, and when in doubt, wait for a better window. The score is designed to help you quickly identify the days and hours when conditions fall within the safe, effective range, but the responsibility for a safe, legal and effective application always rests with the operator and the label.