Measurements · Agriculture
Leaf Temperature
Also known as canopy temperature
Leaf temperature (agricultural microclimate).
What it is
What is leaf temperature?
Leaf temperature is the temperature of the plant canopy surface itself, reported in degrees Celsius (°C), and it is not the same as the air temperature around the plant. A leaf is heated by absorbed sunlight and cooled by transpiration (the evaporation of water from its pores) and by the wind passing over it. A well-watered leaf in sunshine can therefore sit several degrees below air temperature, while a water-stressed leaf that has closed its pores heats up above it. The difference between canopy and air temperature is therefore a direct signal of how the plant is managing water and heat. It is a surface temperature, measured across the leaves the sensor sees rather than at any single point.
Because it reflects the plant's own response rather than the surrounding weather, leaf temperature is one of the most direct indicators of crop water stress available.
When a plant runs short of water it closes its stomata to conserve it, transpiration falls, and the canopy warms, so a rising canopy-to-air temperature difference is an early sign of water stress, often before wilting is visible. This is the basis of the crop water stress index, which combines the canopy-air difference with humidity to gauge stress and guide irrigation. Leaf temperature also matters for frost protection, where the canopy can cool below air temperature on clear nights, and for disease and heat-stress management, since surface temperature affects both pathogen activity and photosynthesis.
For growers and researchers, leaf temperature turns the plant itself into the sensor, reporting its condition more directly than soil or air measurements alone.
How it is measured
How Leaf Temperature Is Measured
Two approaches are used. An infrared radiometer measures the thermal radiation emitted by the canopy from a short distance, giving a non-contact reading averaged over the area of leaves in its field of view. It is the method behind most canopy-stress work. A contact sensor, such as a fine thermocouple clipped to a leaf, measures a single leaf directly. Both are professional field sensors connected to a Thiamis gateway for continuous logging. Infrared sensors must be aimed to fill their view with foliage rather than soil or sky, and their emissivity setting matters for accuracy.
An instrument on Thiamis1
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing leaf temperature sensors
An infrared radiometer is mounted above or beside the canopy, angled so that its field of view is filled with representative leaves and not gaps of bare soil, sky, or shadow that would corrupt the average. Both sunlit and shaded readings are informative, and consistent aiming matters because the reading depends on what the sensor sees. Contact sensors are attached to healthy, representative leaves with good thermal contact and without shading the leaf. As with all canopy measurement, the aim is a reading that represents the crop rather than an unusual individual leaf.
Every device and option that measures Leaf Temperature 1
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
 Davis · Davis Wizard III, Davis Wizard II, Davis Monitor… | Weather Stations | Thiamis |
Where it is monitored
Solutions That Monitor Leaf Temperature
Each one links to the solution page showing what that industry measures it for.
Agriculture & Irrigation
Often addedSoil moisture, microclimate, and frost warning
Related
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Questions
Leaf Temperature FAQ
Why is leaf temperature different from air temperature?
Because a leaf makes its own heat balance: it absorbs sunlight, which warms it, and loses heat by transpiration and convection, which cools it. A well-watered leaf transpiring freely can be cooler than the air, while a stressed leaf that has closed its pores heats above it, so the gap between the two is a signal in itself.
Should I use an infrared or a contact sensor?
Infrared radiometers measure the canopy without touching it, averaging over many leaves, which suits whole-canopy stress monitoring; contact sensors read a single leaf directly. Infrared is the more common choice for irrigation and stress work because it represents the canopy rather than one leaf, but it must be aimed carefully at foliage.
How does leaf temperature indicate water stress?
When water is short, stomata close, transpiration and its cooling effect drop, and the canopy warms relative to the air. That rising canopy-air difference, formalized in the crop water stress index, signals stress often before any visible wilting, so irrigation can be timed to the plant's actual need.
How do I monitor leaf temperature?
Pair a Thiamis gateway with an infrared canopy radiometer or a contact leaf sensor for continuous logging of canopy temperature. Use the configurator to combine leaf temperature with air temperature, humidity, soil moisture, and leaf wetness for a full crop-stress and disease picture.
Monitor Leaf Temperature on the Record
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