The best time to spray is usually during daylight hours, when the wind speed is consistently above 4 to 5 km/h, predictable in direction, and is less than the permissible wind speed on the product label. Delta T value should be above 2 and the target plants or weeds not stressed.
Spray during favorable weather conditions.
Wind speed is critical. Air movement is needed to ensure that mixing occurs in the air. This helps to deposit airborne droplets. Wind speeds should be below 15 km/h as measured at the site of application, depending on the label instructions. If the wind speed is etween 15-25 km/h, spray with caution. Wind speed above 25 km/h means that the weather is unsuitable for spraying.
Rain — Ideally, you should schedule spraying for periods with no precipitation. If you experience precipitation under 0.1 mm, spray carefully. Do not spray when precipitation intensity is more than 0.1 mm.
Temperature should be under 18 °C in optimal conditions. When temperature is between 18 and 25 °C, spray with caution. The weather is unsuitable for spraying when the temperature is over 25 °C.
Humidity between 60-85% is optimal for spraying. Spray with caution when humidity is between 45-60% or 85-95%. Humidity below 45% or over 95% indicates that the weather is not suitable for spraying.
Delta T is an atmospheric moisture parameter, defined as the dry bulb temperature minus the wet bulb temperature, and provides a better indication of water evaporation rate than relative humidity alone. Avoid spraying when the Delta T is either too high or too low, it should be within a range of 2-8. If Delta T is between 0-2 or 8-10, spraying conditions are marginal, spray with caution. Do not spray if Delta T is under 0 or over 10.
Delta T is the difference between dry bulb temperature (when the air temperature sensor exposed directly to the air) and wet bulb temperature (air temperature sensor enclosed in wetted material so that water is constantly evaporating from it and cooling the bulb).
Delta T indicates evaporation potential. It is a key indicator for farmers to schedule spraying crops and decide whether conditions are right for spraying. Higher values mean drier atmosphere, which can reduce droplet survival in the air and at the target. Airborne droplets will rapidly decrease in size when the delta T value of the air exceeds 8 to 10. A Delta T less than 2 indicates a very moist atmosphere. This can extend the life of small airborne droplets and increase the potential for fine droplets to drift long distances.
Source: Graeme Tepper (2012), Grains Research and Development Corporation, Australian Government
Always consult the product label before considering spraying. Follow all label instructions.
DO NOT use an unapproved or non-labelled product.
DO NOT spray under surface temperature inversion conditions.
An inversion layer is a band of air which underlies a warmer air mass. In flat areas, this is usually caused by radiation or radiational cooling of the ground during cloud-free nights, and in mountainous areas by drainage of cool air into lower elevations. If small drops are sprayed above this layer, they will hang suspended and tend to move horizontally with the airmass. This can lead to airborne pesticides being transported to unintended locations.
One of the best indications of an inversion is smoke rising a short distance vertically, then hanging, forming a horizontal layer.
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