# Thermal inversion and spraying

> What a thermal inversion is, when it forms, the signs to look for in the field, why spraying under one moves product off target and how to detect it.

- Canonical URL: https://pulvecast.com/guides/thermal-inversion
- Language: en
- Last updated: 2026-09-19
- Path: Home › Spraying weather guides › Thermal inversion and spraying
- Site guide for agents: https://pulvecast.com/llms.txt

A thermal inversion — also called a temperature inversion — is one of the most common reasons for spray ending up far from where it was applied. It is easy to miss, because inversion conditions often look like perfect spraying weather: still air, clear skies and cool temperatures.

## What an inversion is

Normally air gets cooler with height. Warm air near the ground rises and mixes with the air above, which disperses anything suspended in it. In an inversion the pattern flips: a layer of cool, dense air sits at the surface under warmer air. The cool air cannot rise, so vertical mixing stops.

Spray released into that layer does not disperse upward and settle out the way it normally would. Fine droplets and vapour stay concentrated in a thin layer near the ground and can move sideways with the slightest air movement, sometimes over long distances, before settling on whatever is there — a neighbour's crop, a garden, a water body.

## When inversions form

Surface inversions form when the ground loses heat quickly by radiation:

- **Clear, calm nights** — the classic case. Without cloud cover, the ground cools rapidly after sunset and chills the air in contact with it.
- **Late afternoon and dusk** — they can start to form before sunset as the ground begins to cool.
- **Dawn and early morning** — they are usually strongest around sunrise and break up as the sun warms the ground and the air starts to mix.
- **Low-lying ground** — cool air drains downhill and pools in valleys and hollows.

Cloudy, windy nights are less prone, because cloud limits the cooling and wind keeps the air mixed.

## Signs in the field

- Smoke or dust that hangs in a layer or drifts sideways instead of rising and dispersing.
- Ground fog or mist, especially in low areas.
- Heavy dew on the crop.
- Very light or no wind, particularly after a windy afternoon.
- Sounds and smells carrying further than usual.
- Air noticeably cooler near the ground than a few metres up.

Many labels prohibit application during an inversion, and some require operators to check for one before starting — typically with a smoke test or temperature readings at two heights. Follow what your label specifies.

## Why a low wind is not the safe option it seems

Operators often wait for the wind to drop to avoid drift. Under an inversion, calm air is exactly the problem: there is not enough mixing to bring droplets down, and a light drift can carry a concentrated cloud off the field. Most guidance recommends a light but steady breeze (see [wind speed for spraying](https://pulvecast.com/guides/wind-speed-for-spraying)) and no inversion. A low [Delta T](https://pulvecast.com/guides/delta-t) in the early morning is another hint that an inversion may still be in place.

## What to do

- Plan applications for when the inversion has broken: the air has warmed, the breeze is steady and smoke rises freely.
- Be especially careful near sensitive crops, homes and water — see [spray drift](https://pulvecast.com/guides/spray-drift).
- For night work in hot climates, check the inversion status hour by hour rather than assuming conditions stay the same.

## Further reading

- [Air Temperature Inversions: Causes, Characteristics and Potential Effects on Pesticide Spray Drift — NDSU Extension](https://www.ndsu.edu/agriculture/extension/publications/air-temperature-inversions-causes-characteristics-and-potential-effects)
- [Introduction to Pesticide Drift — US EPA](https://www.epa.gov/reducing-pesticide-drift/introduction-pesticide-drift)

## In PulveCast

PulveCast estimates inversion risk for each hour by comparing the forecast air temperature at 80 m with the temperature at 10 m, both from Open-Meteo. If the upper level is warmer, it shows caution; if it is warmer by 1.5 °C or more (the default, adjustable), it flags an inversion.

## More guides

- [Delta T for spraying: what it is and why 2 to 8 °C](https://pulvecast.com/guides/delta-t)
- [Delta T chart: how to read it](https://pulvecast.com/guides/delta-t-chart)
- [Spray drift: causes and how to reduce it](https://pulvecast.com/guides/spray-drift)
- [Wind speed for spraying](https://pulvecast.com/guides/wind-speed-for-spraying)
- [The best time of day to spray](https://pulvecast.com/guides/best-time-to-spray)
- [Humidity and dew point for spraying](https://pulvecast.com/guides/humidity-and-dew-point)
- [Rain and spraying: rainfast periods and forecasts](https://pulvecast.com/guides/rain-and-spraying)
- [Temperature and spraying](https://pulvecast.com/guides/temperature-and-spraying)
- [Droplet size and nozzle selection](https://pulvecast.com/guides/droplet-size-and-nozzles)
- [Weather for drone spraying](https://pulvecast.com/guides/drone-spraying-weather)
- [Kp index, RTK and GNSS for spray drone operators](https://pulvecast.com/guides/kp-index-rtk-gnss)
- [Pre-spray checklist](https://pulvecast.com/guides/pre-spray-checklist)

## Frequently asked questions

### What time of day do thermal inversions happen?

Surface inversions usually form in the late afternoon or evening on clear, calm days, strengthen overnight and are strongest around sunrise. They normally break up in the morning once the sun warms the ground and the air begins to mix.

### How can I tell if there is an inversion?

Look for smoke or dust that hangs in a layer or moves sideways, ground fog, heavy dew and very light wind. Temperature readings at two heights, with the upper one warmer, confirm it.

### Can I spray during an inversion if I use coarse droplets?

Coarser droplets reduce the amount of fine spray, but they do not remove it, and many labels prohibit application during an inversion regardless of droplet size. Wait until the inversion has broken.

## Check the spray window before you go to the field

PulveCast gives a green, yellow or red verdict for every hour from Delta T, humidity, wind, gusts, drift risk and thermal inversion. Free on Android and iOS, no account needed.

- Google Play: https://play.google.com/store/apps/details?id=com.pulvecast.app
- App Store: https://apps.apple.com/app/pulvecast/id6779816829