Delta T and spray drift

Delta T (ΔT) is a meteorological parameter representing the difference between the dry bulb temperature (T_dry) and the wet bulb temperature (T_wet). It is calculated as:

ΔT = T_dry - T_wet

The dry bulb temperature is the standard air temperature measured by a thermometer freely exposed to the air but shielded from radiation and moisture. The wet bulb temperature is measured by a thermometer with its bulb wrapped in a wet cloth, which is then exposed to a flow of air. Evaporation of water from the cloth cools the bulb, and the amount of cooling depends on the humidity of the air. The drier the air, the more evaporation occurs, and the lower the wet bulb temperature.

Therefore, Delta T is an indicator of atmospheric conditions that affect the evaporation rate of water droplets. A large Delta T indicates dry air with significant evaporative potential, while a small Delta T suggests high humidity and less evaporation. It is not a direct measure of humidity or temperature alone, but a combined index that is particularly useful in applications where droplet evaporation is critical, such as agricultural spraying.

The primary concern for agricultural spraying is ensuring that the applied product reaches its target. Spray droplets are typically small, often in the range of 100 to 400 micrometres, making them highly susceptible to evaporation. When droplets evaporate too quickly, they shrink, becoming lighter and more prone to drift by wind. This reduces the amount of active ingredient reaching the target, leading to inefficient application and potential environmental contamination.

Delta T provides a practical guide to the rate of evaporation:

• High Delta T (e.g., above 10°C): Indicates dry air and rapid evaporation. Droplets shrink quickly, increasing drift risk and reducing coverage.
• Low Delta T (e.g., below 2°C): Indicates high humidity or even saturation. Evaporation is slow, but this can lead to droplets remaining airborne longer, also increasing drift risk, or coalescing and falling as larger drops, potentially causing run-off or uneven coverage.

Optimal droplet life balances the need for sufficient time to reach the target with minimal evaporation, ensuring effective deposition. This balance is crucial for both efficacy and environmental stewardship.

Agricultural industry guidance, particularly in Australia and North America where large-scale spraying is common, often cites an optimal Delta T range for spraying. This range typically falls between 2°C and 8°C.

Within this band:

• 2°C to 8°C: Evaporation rates are moderate. Droplets retain their size sufficiently to reach the target with minimal drift, yet evaporate quickly enough post-deposition to avoid excessive run-off or prolonged surface wetness. This range is commonly cited by agricultural chemical manufacturers and industry bodies as providing the best compromise between efficacy and drift management.

Outside this band, risks increase:

• Below 2°C: High humidity. Droplets may remain airborne longer, increasing drift potential, or coalesce on leaf surfaces, leading to run-off. This can also indicate conditions conducive to temperature inversions, further increasing drift risk.
• Above 8°C: Low humidity and high evaporation. Droplets shrink rapidly, making them highly susceptible to wind drift and reducing deposition efficiency. For example, if the dry bulb temperature is 25°C and the wet bulb is 15°C, the Delta T is 10°C, which is outside the optimal band, indicating rapid evaporation and increased drift risk.

It is important to note that these are commonly cited guidelines, and the user's own product labels and local regulations always govern application parameters.

Temperature inversions are a critical factor in spray drift. An inversion occurs when a layer of warmer air sits above cooler air, trapping air near the ground. This stable atmospheric layer prevents vertical mixing, meaning any spray released into it will not disperse upwards but will instead move horizontally with the prevailing low-level wind, often for considerable distances.

Inversions are most common on clear, calm nights and early mornings when the ground cools rapidly, radiating heat away. As the sun rises, the ground heats up, breaking the inversion. Spraying during an inversion can lead to significant off-target movement of pesticides, even with low wind speeds. Indicators of an inversion include:

• Calm or very light winds (below 3 m/s).
• Clear skies.
• Dew or frost formation.
• Fog or mist.
• Odours or smoke hanging in the air or moving horizontally.

The Wind Agent does not directly forecast inversions, but conditions such as very low wind speeds and specific diurnal temperature patterns can suggest their presence. The instrument's Shear Glass can also show very low wind speeds at 10m but higher speeds at 80m, indicating a stable layer.

While Delta T addresses droplet evaporation, wind speed and direction are paramount for managing physical drift. Even under optimal Delta T conditions, excessive wind speeds will cause spray droplets to be carried off-target. Conversely, very low wind speeds, especially during inversions, can also lead to unpredictable drift.

Commonly cited wind speed limits for spraying vary by crop, product, and local regulations, but a typical range is 3 to 20 km/h (approximately 0.8 to 5.5 m/s). Below 3 km/h, the risk of inversion-related drift increases. Above 20 km/h, physical drift becomes difficult to control, regardless of droplet size or Delta T.

Wind direction is equally critical. Before spraying, it is essential to identify all sensitive areas downwind, such as residential properties, water bodies, organic farms, or non-target crops. The Wind Agent's direction persistence chart can help identify the consistency of the wind direction over the planned spraying period. If the wind is forecast to shift towards a sensitive area, operations should be delayed.

For example, if a field borders a sensitive watercourse to the east, and the forecast shows winds FROM the west at 15 km/h, spraying might proceed. However, if the wind is FROM the north-west and forecast to shift FROM the north, the risk to the watercourse increases, and a delay may be warranted. The Wind Agent's exceedance fan can be configured to show the probability of wind exceeding a specific speed from a particular direction.

Delta T, like temperature and humidity, follows a diurnal cycle. Understanding this cycle is crucial for planning spray operations.

• Early Morning: Often characterised by low Delta T due to high humidity and potentially cool temperatures, especially if an inversion is present. This period carries a higher risk of inversion-related drift.
• Mid-Morning to Afternoon: As the sun heats the ground, temperatures rise, and humidity often falls, leading to an increase in Delta T. This can bring conditions into the optimal 2–8°C range. However, wind speeds can also increase during this period due to thermal mixing.
• Late Afternoon to Evening: As temperatures begin to drop and humidity rises, Delta T typically decreases. Inversions can start to re-form, leading to conditions similar to early morning.

Worked Example: Consider a forecast for a summer day in Ireland. At 06:00, T_dry is 10°C, and relative humidity is 95%. The calculated T_wet is approximately 9.5°C, giving a Delta T of 0.5°C. This is below the optimal range, indicating high humidity and potential inversion. By 14:00, T_dry is 22°C, and relative humidity is 50%. The calculated T_wet is approximately 15.5°C, resulting in a Delta T of 6.5°C. This falls within the commonly cited optimal range of 2–8°C, suggesting good evaporative conditions. However, wind speed must also be checked.

The Wind Agent's Delta T chart provides a continuous forecast, allowing operators to identify windows within the day that are most suitable for spraying.

Accurate record-keeping of meteorological conditions during spraying operations is a critical aspect of compliance and due diligence. Many agricultural regulations and product labels require detailed records to be maintained, demonstrating that applications were conducted within specified parameters.

These records typically include:

• Date and time of application.
• Location of application.
• Product used and application rate.
• Wind speed and direction at the time of spraying.
• Air temperature (dry bulb).
• Relative humidity or Delta T.
• Presence of temperature inversions (observed or inferred).
• Nozzle type and pressure.
• Name of applicator.

The Wind Agent provides an evidence record for every forecast and observation, which can be used to support compliance documentation. By logging the conditions at the time of decision-making and application, users can demonstrate adherence to best practices and regulatory requirements, mitigating potential liabilities associated with off-target spray drift. This digital record offers a precise, timestamped account of the atmospheric conditions, complementing manual logs.