• Full size image -
  • Full size image -
  • Thumbnail -
  • Thumbnail -

Monitor weather conditions to avoid spray drift

Spray drift can become a huge issue between rural neighbours. Not only can those affected by drift damage take a financial hit, but the potential conflict can ruin even the most solid friendships.

It’s wise to remember that if drift damage occurs, the operator is 100 per cent responsible. So, it only makes sense that farmers and operators equip themselves with the tools and knowledge to avoid spray drift.

While weather plays a major role in spray drift it’s something that’s out of the operator’s control. Having said that, observing certain weather conditions including wind, temperature and relative humidity, and how they all interact, goes a long way towards preventing spray drift.


Wind is the most common cause of drift. It’s also the easiest to avoid. The windier the conditions, the more likely it is that the chemical will be carried away from its intended target. Product labels specify the maximum wind speed for application, which is usually between five and 15 km/hr. If the wind exceeds the recommended velocity, do not spray.

Droplet size influences how wind carries particles through the air. Finer particles are lighter, making them more susceptible to movement. A particle the size of 100 microns, about the diameter of a human hair, will move more by air turbulence than by gravity. Therefore, nozzle selection, spray pressure and spray mix viscosity are all important to ensure that particles are adequately coarse, while still ensuring acceptable coverage.

Boom height determines the distance that a particle must fall. And since wind velocity increases with height, a higher boom not only increases the travel distance of the particles, it may also expose them to higher wind speeds than if spraying closer to the ground.

Wind direction is also important and the operator should always consider downwind hazards. Even when conditions are ideal for spraying, extra caution should be taken near neighbouring residences, public areas, waterways and sensitive crops. Always increase buffer distances and if possible, spray when downwind of such areas.

Temperature and humidity

Delta-T is a tool farmers can use to help them determine when conditions are acceptable for spraying. Delta-T indicates acceptable spraying conditions using air temperature and relative humidity and is calculated by subtracting the wet bulb temperature from the dry bulb temperature. Ideally, Delta-T should be between two and eight.

Values below two occur when the air is moist. This can result in small droplets surviving for extended periods, increasing the risk of travel to off-target areas. Such conditions normally occur during the night or any time humidity is high.

Higher Delta-T values, those greater than eight or 10, occur when the air is hot and dry. Under these conditions, droplets evaporate faster, becoming smaller, more concentrated, and airborne for longer. The longer a particle remains suspended, the more likely it is to float away. Generally, coarser nozzles will allow spraying with slightly higher Delta-T values.

Some products may also be more prone to vaporization when Delta-T is high. Depending on the product’s volatility, temperatures greater than 25 to 30°C will increase the risk of vaporization. Product in vapour form is extremely prone to longdistance travel with air currents. Unlike particle drift, which is only a concern while spraying, vaporization can occur hours to days after application and is not affected by the equipment used.


Normally, the air closest to the ground is warmer than the air above. This causes a natural circulation — more buoyant air near the surface rises, gradually cools and is replaced by the cooler, denser air from above.

During an inversion, cool, dense air remains close to the ground, resulting in a stable stratification that prevents any vertical movement of air — or the fine particles or vapour that may be suspended. These particles are much more likely to travel horizontally, often pushed by the slightest air movement.

Despite being an important factor for spraying, inversions are difficult to detect. There are certain factors that are conducive to inversions: calm winds, clear skies, dry air and high pressure. Most often, inversions start to form several hours before sunset and persist through the night.

In general, the time of minimum air temperature and maximum inversion intensity is just before sunrise. Inversions normally persist one to two hours past sunrise, and are broken up as winds pick up, or as the ground surface heats up.


Clearly, an important part of avoiding spray drift is being familiar with the weather conditions that contribute to the risk. Direct measurement, by means of a weather station located near the field is the most effective means of monitoring conditions during field activities. Tracking wind, temperature and relative humidity in real time allows the operator to spray at optimal times and shut down when conditions change.

In the event of a spray drift issue, historical data from the weather station and detailed machinery records become important. Being able to prove, using hard data, that an application was made under appropriate conditions can avoid unnecessary headaches.

A reliable, detailed forecast is a must for any applicator. While there are several good forecasts, finding one that you are can trust for your local area is important. You can also consult several forecast models to see how closely they agree with each other. If the various forecasts disagree, the outcome is less certain.

Above all, being extremely aware of one’s surroundings, monitoring weather conditions, and adhering to labels will greatly reduce the risk of spray drift. This will ultimately save money by preventing drift damage and will help maintain good neighbourly relations.