Weather Conditions for Safe and Effective Spraying

Why Weather Matters

Weather conditions during spray application determine success or failure. The right conditions ensure chemicals reach their targets, remain active, and minimize off-target movement. Poor conditions can result in drift, reduced efficacy, crop damage, environmental contamination, and legal liability.

Professional applicators understand that monitoring weather isn't just good practice—it's a legal requirement in most jurisdictions. Chemical labels specify weather restrictions, and violating these requirements can void insurance, attract regulatory penalties, and compromise application effectiveness.

Critical Weather Parameters

Temperature

Optimal Range

Most agricultural chemicals perform best between 10°C and 25°C (50°F to 77°F). Within this range, plants are actively growing, chemical uptake is efficient, and evaporation is manageable.

Temperature Effects

Too Cold (below 10°C/50°F):

• Reduced plant metabolism and chemical absorption
• Some chemicals become less effective or inactive
• Increased risk of crop stress and damage
• Slower breakdown of chemicals on surfaces
• Potential for frost damage to treated plants

Too Hot (above 30°C/86°F):

• Rapid evaporation reduces droplet size and increases drift
• Volatile chemicals may vaporize before reaching target
• Plant stress reduces chemical absorption
• Increased risk of phytotoxicity (chemical damage to plants)
• Operator heat stress and safety concerns

Temperature Monitoring

Measure temperature at the spray site, not at distant weather stations. Temperature varies significantly across landscapes, especially in hilly terrain. Record temperature at the start, middle, and end of application to document changing conditions.

Wind Speed and Direction

Optimal Wind Speed

The ideal wind speed for spraying is between 3 and 15 km/h (2 to 10 mph). This "Goldilocks zone" provides enough air movement to prevent temperature inversions while not being strong enough to cause significant drift.

Wind Speed Guidelines

Below 3 km/h (2 mph) – Too Calm:

• Likely indicates a temperature inversion
• Spray droplets can drift long distances in stable air
• Chemicals may accumulate in temperature layers
• High drift risk despite apparent calm

3-15 km/h (2-10 mph) – Ideal:

• Good atmospheric mixing
• Acceptable drift potential with proper nozzle selection
• Droplets reach target before evaporating or drifting
• No temperature inversion risk

15-25 km/h (10-15 mph) – Marginal:

• Increased drift risk, especially with fine droplets
• Use only with low-drift nozzles and coarse droplets
• Pay close attention to spray direction and sensitive areas
• Some chemical labels prohibit application

Above 25 km/h (15 mph) – Too Windy:

• Unacceptable drift risk
• Most chemical labels prohibit application
• Cannot achieve uniform coverage
• Stop application immediately

Wind Direction

Wind direction is as important as speed. Always consider what lies downwind:

• Neighboring crops (especially sensitive species like grapes, tomatoes, cotton)
• Organic farms (zero tolerance for synthetic chemicals)
• Water bodies (environmental contamination)
• Residential areas (human exposure concerns)
• Schools, hospitals, and public spaces
• Apiaries (bee protection)

Plan spray blocks to work away from sensitive areas when possible. If wind direction is toward sensitive areas, postpone application until conditions change.

Wind Measurement

Use handheld anemometers or weather stations positioned at spray height (approximately 2 meters above ground). Wind speed varies significantly with height—ground-level measurements may not reflect conditions at boom or drone height.

Monitor wind continuously during application, not just at the start. Wind conditions can change rapidly, especially during morning and evening transitions.

Humidity

Optimal Humidity Range

Relative humidity between 40% and 80% is generally ideal for most spray applications. This range allows adequate droplet lifespan while avoiding excessive moisture on plant surfaces.

Humidity Effects

Low Humidity (below 40%):

• Rapid evaporation reduces droplet size
• Increased drift potential as droplets shrink
• Reduced coverage as droplets may evaporate before reaching target
• Higher Delta T values (see Delta T section)
• Common in hot, dry conditions

High Humidity (above 90%):

• Slow drying reduces chemical absorption
• Increased risk of runoff
• Extended period for rain washoff
• Potential for disease issues on wet foliage
• May indicate impending rain

Humidity and Temperature Interaction

Humidity and temperature work together to determine evaporation rate. This relationship is captured in the Delta T calculation, which provides a single number representing evaporative conditions.

Delta T: The Critical Measurement

What is Delta T?

Delta T (ΔT) represents the evaporative capacity of the atmosphere by measuring the difference between the actual air temperature and the wet bulb temperature (the temperature of evaporation). It's the single most useful number for assessing spray conditions.

Delta T Guidelines

• Below 2: High risk of temperature inversion. Do not spray.
• 2-8: Ideal spraying conditions. Good atmospheric mixing, acceptable evaporation rate.
• 8-10: Marginal. Evaporation becoming rapid. Use larger droplets, monitor closely.
• Above 10: Poor conditions. Excessive evaporation, very high drift risk. Avoid spraying.

How to Measure Delta T

Delta T is calculated from dry bulb temperature and wet bulb temperature, typically measured with a sling psychrometer or electronic weather station. Many modern spray record apps, including Spraybook, calculate Delta T automatically from temperature and humidity readings.

For more detailed information about Delta T, see our dedicated article on Understanding Delta T.

Cloud Cover

Effects on Spraying

Clear Skies:

• Higher temperatures and lower humidity
• Greater risk of inversions at night and early morning
• Rapid heating during the day
• UV degradation of some chemicals

Overcast Conditions:

• More stable temperatures
• Higher humidity
• Better mixing (no inversion risk during day)
• Often indicates approaching weather systems

Light cloud cover (30-70%) often provides excellent spray conditions with stable temperatures and good atmospheric mixing.

Rainfall

Pre-Application Considerations

Recent rainfall affects spray decisions:

• Wet foliage reduces chemical adhesion and uptake
• Soil moisture affects product activation (pre-emergent herbicides)
• Wet ground may restrict equipment access
• High humidity often follows rain

Generally, allow foliage to dry before spraying. Some products specify minimum drying periods in their labels.

Post-Application Rainfall

Most chemicals require a rain-free period after application for proper absorption and efficacy:

• Immediate (0-1 hour): High risk of washoff and reduced efficacy
• 1-4 hours: Moderate risk; depends on product formulation
• 4-6 hours: Most contact products adequately absorbed
• 6+ hours: Most systemic products adequately absorbed

Always check chemical labels for specific rain-fastness requirements. If rain occurs before the specified period, reapplication may be necessary.

Temperature Inversions: The Hidden Danger

Understanding Inversions

Temperature inversions occur when warm air sits above cooler air near the ground, creating a stable atmospheric layer. Under normal conditions, air temperature decreases with altitude. During an inversion, this pattern reverses.

Why Inversions Are Dangerous

Inversions create stable air conditions where spray droplets become trapped and can drift long distances (sometimes kilometers) before eventually settling. This creates extreme off-target movement risk.

Identifying Inversions

Visual Signs:

• Smoke or dust hanging in horizontal layers
• Ground fog or mist
• Extremely calm conditions (wind below 3 km/h)
• Condensation on grass or vehicles

Measurement:

• Temperature increase with height (measure at ground level and 2+ meters up)
• Delta T below 2
• Stable or rising temperature during night hours

High-Risk Times:

• Evening to early morning (sunset to 2 hours after sunrise)
• Clear, calm nights
• Following hot, clear days
• Valley bottoms and low-lying areas

Inversion Precautions

If you suspect an inversion:

• Stop spraying immediately
• Wait for atmospheric mixing (typically 1-2 hours after sunrise)
• Look for wind increase and Delta T above 2
• Resume only when clear signs of mixing appear

Seasonal Considerations

Spring

• Variable conditions, rapid weather changes
• Morning frosts possible
• Increasing temperatures and day length
• Higher humidity from soil moisture
• Good mixing during midday

Summer

• High temperatures limit spray windows
• Low humidity increases evaporation
• Best spraying: early morning after inversion clears, late afternoon/evening
• Avoid midday heat
• Watch for afternoon thunderstorms

Autumn

• Excellent spray conditions often available
• Moderate temperatures and humidity
• Longer inversion periods as day length decreases
• Increasing chance of rain systems
• Plant metabolic activity decreasing

Winter

• Cold temperatures limit chemical effectiveness
• Short spray windows between frosts
• Slow plant growth reduces uptake
• Extended inversions common
• Best conditions: midday with sunshine

Time of Day Guidelines

Early Morning (Sunrise to 10 AM)

Challenges: Temperature inversions, dew on foliage, rapidly changing conditions

Opportunities: Cool temperatures (summer), calm conditions (once mixing starts)

Best Practice: Wait at least 1-2 hours after sunrise for inversion to break

Midday (10 AM to 3 PM)

Challenges: High temperatures (summer), low humidity, high evaporation

Opportunities: Excellent mixing, no inversion risk, stable conditions

Best Practice: Ideal in cooler seasons, avoid in hot weather

Late Afternoon/Evening (3 PM to Sunset)

Challenges: Inversions can form near sunset, dew formation, approaching darkness

Opportunities: Cooling temperatures, increasing humidity, often calm conditions

Best Practice: Stop well before sunset to avoid inversion formation

Weather Monitoring Tools

Essential Equipment

Handheld Weather Meters: Portable devices measuring temperature, humidity, wind speed, and calculating Delta T. Essential for on-site monitoring. Price range: $100-$500.

Weather Stations: Fixed or portable stations providing continuous monitoring. Can log data and alert to changing conditions. Price range: $200-$2,000+.

Mobile Apps: Smartphone applications providing local weather data, forecasts, and spray condition assessments. Many free or low-cost options available.

Sling Psychrometer: Manual device for measuring wet and dry bulb temperature to calculate Delta T. Low-cost backup option: $50-$150.

Data Logging

Record weather conditions throughout application, not just at the start. Modern spray record apps like Spraybook automatically log weather data at regular intervals, providing complete documentation of spray conditions.

Chemical Label Requirements

Understanding Label Restrictions

Chemical labels are legal documents specifying weather restrictions. Common requirements include:

• Maximum wind speed (typically 15-25 km/h)
• Minimum buffer zones under various wind conditions
• Temperature restrictions (minimum and maximum)
• Rain-fastness periods
• Time-of-day restrictions
• Droplet size specifications

Compliance Obligations

Violating label weather restrictions:

• Voids product warranty
• May void insurance coverage
• Creates regulatory compliance issues
• Increases liability in drift incidents
• Can result in license suspension

Always read and follow label instructions. When in doubt, consult product manufacturers or regulatory authorities.

Practical Weather Monitoring Protocol

Pre-Application (Day Before)

• Check multi-day forecasts
• Identify potential spray windows
• Note forecast temperature, wind, rain, and cloud cover
• Plan application timing and order of fields

Pre-Application (Day Of)

• Check updated forecast for changes
• Verify expected conditions still favorable
• Plan start time to avoid inversions
• Prepare equipment and chemical mixes

Start of Application

• Measure and record all weather parameters
• Calculate Delta T
• Check for inversion signs
• Verify conditions meet label requirements
• Document conditions in spray records

During Application

• Monitor weather every 30-60 minutes
• Watch for changing wind direction or speed
• Note temperature trends
• Check for approaching weather systems
• Be prepared to stop if conditions deteriorate

End of Application

• Record final weather conditions
• Note any condition changes during application
• Document total application duration
• Complete spray records while details are fresh

When to Stop Spraying

Immediately cease application if:

• Wind speed exceeds 15 km/h (or label maximum)
• Wind drops below 3 km/h (inversion risk)
• Delta T falls below 2 or rises above 10
• Temperature exceeds label limits
• Signs of inversion appear (smoke layering, extreme calm)
• Wind direction changes toward sensitive areas
• Rain begins or appears imminent
• Visibility deteriorates (fog, dust)
• Any unsafe conditions develop

Professional applicators know when to stop is as important as when to start. Stopping during unsuitable conditions protects your operation, neighbors, and the environment.

Conclusion

Weather conditions are the single most important factor determining spray success and safety. Understanding temperature, wind, humidity, Delta T, and their interactions allows you to identify ideal spray windows and avoid dangerous conditions.

Modern technology, including portable weather stations and spray record apps like Spraybook, makes weather monitoring easier and more reliable than ever. By combining proper equipment with sound knowledge and conservative decision-making, you can maximize spray effectiveness while minimizing drift risk and environmental impact.

Remember: weather conditions can change rapidly. When in doubt, don't spray. The best spray day is the one where conditions remain ideal from start to finish.

Published 15/12/2025