Understanding Delta T and Its Importance in Spraying

What is Delta T?

Delta T is one of the most important—yet often misunderstood—weather parameters for spray applications. Simply put, Delta T is the difference between the dry bulb temperature (actual air temperature) and the wet bulb temperature (the temperature a wet surface reaches through evaporative cooling). This single value provides crucial information about atmospheric conditions that directly affect droplet evaporation, drift potential, and overall spray efficacy.

Understanding Delta T is essential for professional spray operators because it combines the effects of both temperature and humidity into one practical measure. While you could monitor temperature and humidity separately, Delta T gives you an immediate, actionable reading that tells you whether conditions are suitable for spraying.

The Science Behind Delta T

Dry Bulb vs. Wet Bulb Temperature

The dry bulb temperature is simply the ambient air temperature measured by a standard thermometer. The wet bulb temperature is measured by a thermometer with a wet cloth wrapped around its bulb. As water evaporates from the cloth, it cools the thermometer, resulting in a lower reading than the dry bulb temperature.

The rate of evaporation (and therefore the difference between the two readings) depends on the humidity of the air. In very humid conditions, evaporation is slow, so the wet bulb temperature is close to the dry bulb temperature, resulting in a low Delta T. In dry conditions, evaporation is rapid, creating a larger temperature difference and a high Delta T.

What Delta T Tells Us

Delta T indicates the evaporative capacity of the air. A low Delta T means the air is near saturation and has limited capacity to evaporate water. A high Delta T means the air is dry and will rapidly evaporate water droplets. For spray applications, both extremes present challenges:

• Low Delta T (below 2): Indicates high humidity and potential for temperature inversions, which trap spray droplets and increase drift risk
• High Delta T (above 8-10): Indicates very dry air where spray droplets may evaporate before reaching the target, reducing efficacy and increasing drift of small droplets

How to Calculate Delta T

Manual Calculation Method

To calculate Delta T manually, you need both a dry bulb and wet bulb thermometer (together called a psychrometer or sling psychrometer):

1. Measure the dry bulb temperature (standard air temperature)
2. Measure the wet bulb temperature (thermometer with moistened wick)
3. Subtract the wet bulb temperature from the dry bulb temperature

Example: If the dry bulb temperature is 28°C and the wet bulb temperature is 22°C, then Delta T = 28 - 22 = 6°C (which is within the ideal range).

Using Weather Stations and Apps

Modern weather stations and spray record apps like Spraybook automatically calculate Delta T from temperature and humidity readings. This is more convenient and often more accurate than manual calculation. Most digital tools will also alert you when Delta T moves outside the safe range for spraying.

Conversion from Humidity

If you only have temperature and relative humidity data, you can estimate Delta T using psychrometric charts or online calculators. However, for critical spray decisions, direct measurement using a wet bulb thermometer or a quality weather station is recommended.

Ideal Delta T Ranges for Spraying

The 2-8 Rule

The most commonly cited safe range for Delta T is 2-8°C. Within this range, conditions are generally favorable for spray applications because:

• Droplets won't evaporate too quickly (avoiding drift of tiny droplets)
• Temperature inversions are unlikely (reducing off-target movement)
• Spray coverage and penetration are optimized
• Active ingredients reach the target before significant evaporation

Regional Variations

Some regions and regulatory bodies recommend a 2-10°C range, particularly in areas with naturally drier climates. Always check local guidelines and product labels, as some chemicals have specific Delta T requirements. For example:

• Australia: Generally recommends 2-8°C
• North America: Often uses 2-10°C, with some regions accepting up to 12°C
• Europe: Recommendations vary by country, typically 2-8°C

Product-Specific Requirements

Always check chemical labels for specific Delta T requirements. Some volatile products or those requiring extended coverage time may have narrower acceptable ranges. Herbicides applied to dry soil conditions may tolerate higher Delta T values than foliar fungicides requiring optimal droplet retention.

Why Delta T Matters for Spray Applications

Evaporation and Droplet Size

When Delta T is too high (above 8-10°C), spray droplets evaporate rapidly. As droplets shrink, they become more susceptible to drift because smaller droplets fall more slowly and are more easily carried by air currents. A medium droplet that starts at 300 microns might evaporate down to 100 microns or less, changing from a relatively drift-resistant droplet to one that can travel hundreds of meters off-target.

Temperature Inversions

When Delta T drops below 2°C, conditions often favor temperature inversions—atmospheric conditions where warm air sits above cooler air near the ground. Inversions trap spray droplets in a stable air layer that can drift for kilometers before settling. Even gentle breezes during inversions can carry spray clouds significant distances from the application site.

Spray Efficacy

Optimal Delta T ensures droplets reach their target with sufficient size and moisture to spread, adhere, and penetrate. Too much evaporation (high Delta T) means less active ingredient reaches the target. Inversion conditions (low Delta T) may result in poor coverage patterns and unpredictable deposition.

Drift Management

Spray drift—the movement of spray droplets away from the intended target—is influenced heavily by Delta T. The ideal 2-8°C range minimizes drift risk by maintaining appropriate droplet sizes and avoiding inversion conditions. This protects neighboring properties, sensitive crops, waterways, and the environment.

Temperature Inversions Explained

What is a Temperature Inversion?

Normally, air temperature decreases with altitude. During an inversion, this pattern reverses, with warmer air sitting above cooler air near the ground. This creates a stable atmospheric "lid" that prevents vertical air movement and traps anything suspended in the lower air layer, including spray droplets.

Identifying Inversions

Signs of a temperature inversion include:

• Delta T below 2°C
• Smoke or dust hanging in distinct horizontal layers
• Fog or dew formation
• Calm conditions with light, variable winds
• Clear skies in evening or early morning
• Strong odors that persist or travel long distances

When Inversions Occur

Temperature inversions typically develop:

• On clear, calm evenings as the ground radiates heat and cools
• During early morning hours before the sun warms the ground
• Under high-pressure weather systems with light winds
• In valleys where cool air settles and warm air remains above

Why You Must Not Spray During Inversions

Spraying during an inversion is dangerous because spray droplets can drift for kilometers—sometimes tens of kilometers—in unpredictable directions. Many spray drift incidents resulting in crop damage, environmental harm, and legal action have occurred during inversion conditions. No matter how careful you are with equipment and technique, you cannot control atmospheric drift during an inversion.

Practical Tips for Monitoring Delta T

Invest in Quality Monitoring Equipment

Accurate Delta T monitoring requires reliable equipment. Options include:

• Sling Psychrometer: Simple, reliable, no batteries required; manual reading of dry and wet bulb temperatures
• Digital Weather Station: Automatic Delta T calculation, data logging, often includes wind speed; requires calibration
• Portable Weather Meters: Handheld devices measuring temperature, humidity, wind; calculate Delta T automatically
• Smartphone Apps: Mobile weather apps can provide local weather data and Delta T calculations

Monitor Continuously, Not Just at Start

Delta T can change significantly during a spray operation, especially in the early morning or late afternoon. Check Delta T:

• Before starting any application
• Every 30-60 minutes during extended operations
• When you notice weather changes (wind shifts, cloud cover changes)
• If you're operating near the margins of the acceptable range

Measure at Application Height

For ground-based spraying, measure weather conditions at the height where you're spraying (typically boom height). For aerial or drone applications, conditions at application height may differ significantly from ground level. Use multiple measurement points if possible.

Keep Records of Delta T

Recording Delta T is increasingly important for compliance and due diligence. Document Delta T at the start, during, and end of applications. This provides evidence that you operated within safe parameters and can be crucial if questions arise about drift or efficacy.

Understand Local Patterns

Delta T patterns vary by location and season. Learn when your area typically experiences ideal conditions. In many regions, the best spray windows are mid-morning after inversions lift and mid-afternoon before evening inversions form. However, local geography, season, and weather patterns all influence timing.

Use Forecast Tools

Some agricultural weather services provide Delta T forecasts, helping you plan spray operations in advance. While on-site measurement is essential for final go/no-go decisions, forecasts help with scheduling and resource planning.

Common Delta T Mistakes

Relying on Humidity Alone

Many operators monitor humidity but ignore Delta T. This is a mistake because the same humidity percentage represents very different evaporation conditions at different temperatures. At 20°C, 60% humidity might be fine, but at 35°C, 60% humidity could produce a dangerously high Delta T.

Ignoring Low Delta T Risks

Operators often focus on avoiding high Delta T (to prevent evaporation) but ignore the serious drift risks of low Delta T inversions. Low Delta T conditions are actually more dangerous for off-target movement than moderately high Delta T.

Using Uncalibrated Equipment

Weather instruments need regular calibration. An inaccurate thermometer or humidity sensor can give false confidence in unsafe conditions. Check calibration regularly, especially at the beginning of each spray season.

Spraying at Range Margins

If Delta T is at 1.8°C or 8.5°C, technically outside the 2-8 range, don't spray. Conditions at the margin can quickly move into dangerous territory. Build in a safety buffer—aim for 3-7°C rather than accepting 2-8°C.

Delta T and Other Weather Factors

Delta T is crucial, but it doesn't work in isolation. Always consider the complete weather picture:

• Wind Speed: Even with ideal Delta T, excessive wind causes physical drift
• Wind Direction: Know where your spray will travel if drift occurs
• Temperature: Affects chemical volatility and plant physiology
• Rainfall Timing: Impacts product wash-off and efficacy
• Cloud Cover: Influences temperature inversions and product breakdown

Modern spray record apps like Spraybook integrate all these parameters, providing comprehensive weather assessment for each application.

Conclusion

Delta T is a fundamental parameter for safe, effective spray applications. By understanding what Delta T represents, monitoring it accurately, and staying within the ideal 2-8°C range, you minimize drift risks, optimize product efficacy, and demonstrate professional spray operation.

The investment in quality weather monitoring equipment and the discipline to check Delta T regularly are small prices to pay for protecting your crops, your neighbors, the environment, and your professional reputation. Make Delta T monitoring a non-negotiable part of every spray operation—your results will prove its value.

Published 13/12/2025