Spray Drift Risk and the Weather

Date Posted:20 November 2014 

Spray Drift Risk and the Weather main image Spray Drift Risk and the Weather image

The risk of off target movement of product that is associated with all spraying activities is governed by how much product we leave in the air, either as droplets or as vapour.  Careful product selection can minimize the vapour component, but what can happen to any droplets that remain in the air after they have been released is purely a function of the weather conditions during spraying, and for several hours after the spraying has taken place.  Most of the damage we have seen in recent years as a result of spray drift has been attributed to the movement of airborne droplets as a result of spraying under the wrong conditions, which nearly always occur at night.


Air movement at Night is more dangerous than air movement during the Day In terms of spray drift risk daytime spraying is always safer, particularly when the sun is heating the ground and the wind speed is consistently above 3-4 km/h. This is because air movement over a warm surface tends to be more turbulent, which helps air to mix and brings airborne droplets back to the surface.

Spraying at night when the ground has cooled, and surface temperature inversion conditions are likely to exist is dangerous because the air flow across the surface becomes less turbulent and more laminar (where the air flow becomes parallel to the ground). Typically wind speeds less than 11 km/h at night will be laminar, which can lead to droplets remaining suspended in the air for long periods of time. These droplets will continue to move with that airflow until the inversion breaks some time after sunrise.

In a single, un-replicated, drift study a couple of years back ,  which compared the amount of drift produced from daytime spraying versus night time spraying, the amount of chemical remaining in the air after spraying at night was 5-6 times greater than that which occurred from spraying after sunrise. The amount of chemical remaining in the air from less than 1 hour of spraying at night spraying was equivalent to 1 hectares worth of chemical for every 60 hectares that were sprayed at night, even when using a coarse spray quality.


The point of summer  spraying is to control the target weeds. We do this to conserve moisture, retain nutrients and to reduce the weed seed bank for future crops and fallows.  The more efficient these operations are, particularly in relation to completing spraying while the target weeds are most susceptible, the better the outcome is likely to be.

If the desire is to get over the country as quickly as possible, so we can target weeds while they are most susceptible, this is a valid strategy. However with increased speed, there is usually a need to reduce droplet size to the smaller end of the coarse spectrum to maintain efficacy. This will increase drift risk, particularly if the conditions are wrong.

In my view, this means that the only safe time to spray when travelling at higher speeds is during daylight hours, when the sun is clearly up and heating the ground, the wind speed is above 3-4 km/h, and a suitable downwind buffer exists.

Until our understanding of air movement at night improves, Night spraying should be considered Too risky.

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