What is Delta T?
Delta T (ΔT) is the difference between dry-bulb air temperature and wet-bulb temperature — two numbers that together capture how quickly a water droplet will evaporate in current atmospheric conditions. A high Delta T means the air is hot and dry enough that fine spray droplets can evaporate before reaching the target. A very low Delta T means the air is saturated and conditions may favour temperature inversions that carry droplets off-target. Neither variable alone is a reliable spray-condition indicator; their difference is.
Delta T has been used as a practical spray-decision index by broadacre and turf applicators for decades. It is simple, requires only two weather readings, and captures the most important atmospheric driver of droplet evaporation in a single number.
How wet-bulb temperature is calculated
Measuring wet-bulb temperature directly requires a thermometer with a water-soaked wick — a psychrometer. This tool computes it instead from air temperature and relative humidity using the empirical equation published by Stull (2011):
Tw = T · arctan[0.151977 · √(RH + 8.313659)]
+ arctan(T + RH)
− arctan(RH − 1.676331)
+ 0.00391838 · RH1.5 · arctan(0.023101 · RH)
− 4.686035
Where T is dry-bulb air temperature in °C and RH is relative humidity (%). The equation is valid for RH 5–99% and temperatures −4 to 122°F (−20 to 50°C) at standard sea-level pressure (1013.25 hPa), with a mean absolute error of less than 0.54°F (0.3°C) across the valid range. The equation is computed in °C internally as published; all temperatures are displayed in °F. Delta T is then:
Delta T = T − Tw
Reference check: at T = 68°F (20°C) and RH = 50%, the equation gives Tw ≈ 56.7°F (13.7°C) and Delta T ≈ 11.3°F — comfortably in the preferred spray window.
The GRDC spray-window bands
The threshold bands used by this tool come from the GRDC (Grains Research and Development Corporation) Spray Application Manual, §10.3 — Temperature and humidity (Tepper 2012, adapted from Nufarm's Spraywise Decisions chart):
| Delta T (°F) | Band | Guidance |
|---|---|---|
| < 3.6°F | Caution | Too humid; inversion risk; droplets may drift off-target |
| 3.6 – 14.4°F | Preferred | Optimal spray conditions |
| 14.4 – 18.0°F | Transition zone | Acceptable with coarser droplet size; as Delta T approaches 18–22°F, assess condition of the target |
| > 18.0°F | Caution | Evaporation stress; fine droplets prone to volatilisation and drift |
The preferred range (3.6–14.4°F) and the two caution thresholds (<3.6°F and >18.0°F) are the published GRDC boundaries, expressed here in °F. The 14.4–18.0°F range is the manual's stated transition zone — an area where spraying is possible with adjustments, not a separately validated cutoff with its own independently calibrated science. This tool labels it accordingly rather than presenting it as a third equal-status band.
About spray quality — why it's context, not an input
The GRDC manual notes that in the transition zone (14.4–18.0°F), using coarser droplet sizes can reduce evaporation and drift risk. This tool surfaces that guidance as a contextual note when the reading falls in that range. It is not an input that recalculates Delta T or moves the threshold boundaries. Your choice of nozzle and spray pressure affects droplet size; it does not change the atmospheric physics that Delta T measures. Adding a spray-quality selector that appeared to update the thresholds would imply false precision — the same reason this tool has no knobs beyond ZIP code.
How the tool works
- Your ZIP code is geocoded to a latitude/longitude using Zippopotam.us (free, no account required).
- Hourly 2-meter air temperature (°C) and relative humidity (%) for the past day and the next three days are fetched from Open-Meteo (free, no account required).
- For each hour, wet-bulb temperature is computed from the Stull (2011) equation (internally in °C). If the inputs fall outside the valid range (RH 5–99%, T −4–122°F), that hour is flagged rather than returning a potentially unreliable value.
- Delta T = air temperature − wet-bulb temperature. The result is classified into the GRDC band for that hour.
- Current conditions are shown as the headline readout. The next 24 hours are shown as a timeline so you can see when the preferred window is expected to open or close.
Honest caveats
Delta T is necessary but not sufficient
In-range Delta T is a prerequisite for good spray conditions, not a guarantee. Delta T captures evaporative stress but does not measure wind speed or detect temperature inversions — both independent, critical factors for droplet drift (GRDC §10.2 and §10.4 respectively). A Delta T of 9°F with a 30 km/h gusting wind is not safe to spray. Always check wind speed and inversion conditions separately before making the call to spray.
Thresholds are from Australian broadacre cropping
The GRDC bands were developed and validated for large-scale cropping applications in Australian agricultural conditions. They have not been independently calibrated for residential turf applications. The underlying atmospheric physics generalise — a Delta T of 4 is a Delta T of 4 everywhere — but users should treat these thresholds as well-established professional field guidance rather than turf-specific science.
Stull assumes sea-level pressure
The Stull (2011) equation is valid at standard sea-level pressure (1013.25 hPa). At significant elevation, the computed wet-bulb will carry a small additional error beyond the paper's stated mean absolute error of <0.54°F (<0.3°C). For most residential users at moderate elevation this is inconsequential in practice; it is flagged here for completeness.
Grid weather is not your microclimate
Open-Meteo provides model-based weather for a grid cell centred on your ZIP code's coordinates. Your lawn's actual temperature and humidity can differ — especially in sheltered spots, low-lying areas, or where reflected heat from structures, pavement, or walls affects conditions near the application site. On-site readings from a thermometer and hygrometer are the most accurate basis for a spray decision.
The product label always governs
Many herbicide, fungicide, and pesticide labels carry their own temperature, humidity, and wind-speed application restrictions that are independent of and may be stricter than the general Delta T guideline. This tool is a general spray-conditions index. The label is the legal and agronomic authority for every product you apply — read it and follow it.
Sources
GRDC Spray Application Manual §10.3 — Temperature and humidity. Tepper G (2012), adapted from Nufarm Spraywise Decisions (2012). grdc.com.au — §10.3 Temperature and humidity — Primary source for the Delta T definition, preferred/transition/caution bands, and spray-quality guidance in the transition zone.
Stull, R. (2011). “Wet-Bulb Temperature from Relative Humidity and Air Temperature.” Journal of Applied Meteorology and Climatology 50:2267–2269. DOI 10.1175/JAMC-D-11-0143.1 — Wet-bulb derivation equation, validity bounds (RH 5–99%, T −4–122°F), and mean absolute error (<0.54°F).
Open-Meteo (https://open-meteo.com) — Free, keyless weather API providing hourly 2-meter air temperature and relative humidity.
Zippopotam.us — Free, keyless ZIP code to latitude/longitude geocoding.
Frequently asked questions
What is Delta T and why does it matter for spraying?
Delta T is the difference between dry-bulb air temperature and wet-bulb temperature — a measure of how quickly spray droplets will evaporate in current atmospheric conditions. A high Delta T means conditions are hot and dry enough that fine droplets evaporate before reaching their target. A very low Delta T means the air is near saturation, increasing inversion and drift risk. The GRDC preferred spray window is a Delta T of 3.6–14.4°F.
What Delta T range is safe for spraying herbicides?
The GRDC Spray Application Manual identifies 3.6–14.4°F as the preferred spray window. Below 3.6°F, inversion risk increases and droplets may drift off-target. Above 18°F, evaporation stress becomes significant and fine droplets are prone to volatilization. The 14.4–18°F range is a transition zone where spraying is possible with coarser droplet sizes.
Does Delta T tell me everything I need to know about spray conditions?
No. Delta T captures evaporative stress but does not measure wind speed or detect temperature inversions — both independent, critical factors for droplet drift. A Delta T in the preferred range with a 20+ mph gusting wind is not safe to spray. Always check wind speed and watch for inversion conditions (dead calm air with smoke or mist hanging flat) separately from Delta T.
Does the Delta T threshold change for different types of pesticides?
The GRDC Delta T bands apply to spray applications generally and are not product-specific. However, individual herbicide, fungicide, and insecticide labels may carry their own temperature and humidity restrictions independent of Delta T — and those label requirements are the legal authority for that product. Always read the product label for application condition restrictions before mixing.