Picking the right time to spray can be tricky. There are a number of factors that can increase the risk of spray drift and ineffective treatment.
In this article we will take a look at the conditions necessary for the perfect spray day…

When is the perfect time to spray?

Picking the right time to go spraying isn’t really a case of looking at the clock. There are a number of factors that we need to consider to find the right conditions to minimise spray drift and evaporation.

There are four key factors that we need to consider:

  • Significant Weather
  • Wind
  • Temperature & Humidity (Delta-T)
  • Inversion Layers

Significant Weather

Just like on a TAF, significant weather refers to precipitation – rain, hail, sleet, snow.

Spraying in rain is definitely not a good idea. Excess rainwater can cause chemical to be washed away from the target plants and into stormwater drainage and waterways, causing an ecological hazard to marine life.

Wind

Wind speed and direction needs to be constantly monitored for successful spraying. More often than not, a day that starts out with perfect conditions may become unsuitable by late morning as thermal winds begin sweeping across the landscape.

High wind speeds cause droplets to be blown away from the target area, particularly when the aircraft is flown too high or too fast.
When combined with stronger gusts, erratic and shifting wind direction can make spray fall in different patterns. This can make it hard for an operator to plan buffer zones and flight paths.

Zero wind is also bad for spraying! A light breeze agitates the air, aiding the droplets to fall. Droplets in flat calm conditions will remain suspended for longer, leading to an increased risk of drift and evaporation.

The Department of Primary Industries (DPI) in NSW recommends that the ideal safe wind speed is 3-10 km/h, a light breeze. (Leaves and twigs are in constant motion.) 11-14 km/h (a moderate breeze) is suitable for spraying if using low drift nozzles or higher volume application, say 80-120 L/ha.

Application techniques to reduce wind drift in higher wind speed

Droplet Size & Flow Rate – Utilise a coarse droplet size or higher (>300 microns) and high flow rate (>80L/ha).

Aircraft Speed & Height – The recommendation is to fly no faster than 5m/s at a height not more than 3 meters above crop height. This allows the droplets to fall vertically rather than fanning out behind the aircraft and drifting in the wind.

Flight Path – Try to fly a path predominantly with the wind (headwind/tailwind). This helps to predict how the droplets will fall and maintain even coverage. Also, planning buffer zones at the downwind edge of the field will help to prevent off-target application.

Temperature & Humidity (Delta-T)

Developed in Australia in the 1990s, Delta-T is now accepted around the world as one of the standard indicators for acceptable spray conditions.

Delta-T is calculated as the temperature difference between a dry bulb (air temperature sensor exposed directly to the air) and a wet bulb (air temperature sensor enclosed in wetted material so that water is constantly evaporating from it and cooling the bulb). The higher the Delta-T value, the drier the atmosphere is with greater potential to evaporate spray droplets.

The graph shows how the Delta-T value changes relative to humidity and temperature. A Delta-T value of 2-8 is considered ideal for spraying.

Inversion Layers

An inversion layer occurs when nighttime temperatures are low, with clear skies and calm or very light winds. A layer of cold air becomes trapped at ground level with a boundary layer of warmer air acting as a “lid” to keep it in place. When combined with high humidity these inversion layers are visibly present as radiation fog.

Why is it bad for spraying?

The colder air, trapped beneath the inversion layer has nowhere to go. This means suspended particulates and droplets are forced sideways rather than upwards. As the cooler air has the potential to hold more evaporated liquid, spray droplets are prone to remaining suspended in the air and will drift on the lateral air currents.

Inversion layer conditions are best avoided for spray operations. Make note of the nighttime temperature, wind and humidity to give you an idea of what to expect, however sometimes the only way to know for sure is by onsite inspection. Check your Delta-T and wind speed, and NEVER spray in fog.

Conclusion

It can be frustrating as an operator to find conditions are not suitable when you have a lot of area to cover and a client to keep happy. The temptation to push ahead in less than ideal conditions can be strong but you must always consider the consequences of an operation that at best fails to achieve efficacy or at worst goes off-target and causes significant damage.

Prepare yourself for successful spraying by studying weather conditions on site. There are a number of different tools and instruments available that you should make part of your spray gear for every job:

  • Anemometer – for measuring wind speed
  • Thermometer – capable of recording air temperature and humidity
  • Delta-T Chart – to check for acceptable conditions

There are also instruments available that will do all the hard work for you. The Kestrel 3550AG weather meter is a handheld device, developed for spray operators, that takes all relevant measurements and indicates Delta-T levels on screen.

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