Blog · Agriculture
Spray windows explained: wind, humidity and inversions
A spray window is a period when the weather allows a spray to reach its target and stay there. It depends mainly on wind speed and direction, temperature and humidity, whether a temperature inversion is present, and whether rain is on the way.
This guide explains the weather side of spraying in general terms. It gives no product or rate advice. The product label sets the legal conditions of use, and the label and your agronomist or local extension service always take precedence over anything written here.
What spray drift is
Spray drift is the movement of spray droplets or vapour away from the target area. It wastes product, reduces the effect on the target, and can damage neighbouring crops, gardens, water and wildlife.
Droplet size sits at the centre of the problem. Small droplets fall slowly, so they stay in the air longer and travel further. They also evaporate faster, which makes them smaller still. Coarser droplets fall quickly and drift less, though they may cover the target less evenly. Nozzle choice, pressure, boom height and travel speed all affect droplet size and drift. Weather decides what happens to the droplets once they leave the nozzle.
Wind
Drift risk rises with wind speed. The stronger the wind, the further droplets are carried before they land, and the more of the spray ends up outside the target area. Gusty wind is harder to manage than steady wind, because speed and direction keep changing.
Wind direction matters as much as speed. What lies downwind is the question: a sensitive crop, a watercourse, a house, a road or beehives. A steady breeze blowing away from anything sensitive is a very different situation from the same breeze blowing towards it.
It is tempting to think that no wind at all is the safest condition. It is not, and the reason is inversions. In still air you cannot tell where fine droplets will go, and dead calm in the early morning or evening often signals an inversion. Many labels and guidance documents therefore ask for a light, steady breeze in a known direction, and set both a lower and an upper wind limit. Follow the limits on your label.
Wind should be measured at the site, at about the height of the boom, and checked again through the job. A forecast or a distant station gives a general picture. Our guide to microclimate and weather stations explains why local readings differ.
Temperature inversions
In normal daytime conditions the air is warmest near the ground and cools with height. Warm air rises, the air mixes, and anything suspended in it is diluted and dispersed.
In an inversion the pattern is reversed. A layer of cool air sits at the surface with warmer air above it. The cool air is dense and stable, so there is almost no vertical mixing. Surface inversions usually form on clear, calm evenings as the ground cools, last through the night, and break down after sunrise once the sun warms the ground or the wind picks up.
An inversion is a no-spray condition. Fine droplets released into that stable layer do not settle as expected and do not disperse. They stay suspended as a concentrated cloud, which can then move off target with a slight drift of air or flow downhill with draining cold air. It may travel a long way and come down somewhere unpredictable, still concentrated enough to cause damage.
Signs that an inversion may be present include:
- clear sky and little or no wind in the evening, overnight or early morning
- mist or fog, or dew or frost forming
- smoke or dust that hangs in the air or spreads out in a flat layer instead of rising
- sounds and smells carrying unusually far
- a lower temperature near the ground than a few metres above it
These are the same conditions that lead to radiative frost, which our guide to frost risk describes.
Temperature, humidity and Delta T
Hot, dry air evaporates droplets quickly. A droplet that shrinks in flight becomes more drift-prone, and a water-based spray that dries too fast on the leaf may not perform as intended. Some products can also turn to vapour after application in hot conditions and move as vapour. Labels for those products usually include temperature limits.
Delta T is a single figure that combines temperature and humidity. It is the difference between the dry-bulb temperature, which is the ordinary air temperature, and the wet-bulb temperature, which is the temperature shown by a thermometer whose bulb is kept wet and ventilated. The drier the air, the more evaporation cools the wet bulb, and the bigger the gap.
- A high Delta T means dry air and fast evaporation.
- A low Delta T means air close to saturation. Droplets last a long time, which can extend drift of fine droplets, and very low values often coincide with dew, fog and inversions.
Delta T is widely used in Australia, where guidance commonly describes a middle band, often quoted as roughly 2 to 8, as preferred. Treat any such figure as a general indicator and check what your own label and local guidance say.
Rain and leaf wetness
Many products need a dry period after application to be absorbed or to dry onto the leaf. This is often called the rainfast period and is given on the label. Heavy rain soon after spraying can also wash product into drains and watercourses. Heavy dew can cause run-off from the leaf for some applications.
Putting a window together
A workable window is a run of hours in which every condition is met at once:
| Factor | What helps | What rules spraying out |
|---|---|---|
| Wind speed | Light, steady breeze within label limits | Strong or gusty wind, or dead calm |
| Wind direction | Away from sensitive areas | Towards sensitive areas |
| Inversion | None, air is mixing | Inversion present or likely |
| Temperature and humidity | Moderate evaporation | Hot, dry air, or limits on the label exceeded |
| Rain | Dry for the period the label requires | Rain due too soon |
Forecast windows are for planning. The decision is made in the field, with measurements taken on site at the time, and recorded. Many jurisdictions require spray records that include the weather conditions, so a note of wind speed, direction, temperature and humidity at the start and end of the job is good practice.
Where Prism fits
Prism Microclimate, built with FarmGrid, covers 961k fields across one whole state. It provides field-level weather, soil moisture, frost, spray and field-work windows, growing degree days and a morning brief per field. There is more on our agriculture page.