Example data table
| Scenario | Ta (deg C) | Tr (deg C) | v (m/s) | Emissivity | ERT (deg C) |
|---|---|---|---|---|---|
| Dense shade under canopy | 28 | 29 | 0.30 | 0.95 | ~ 28.6 |
| Full sun near reflective wall | 32 | 52 | 0.20 | 0.95 | ~ 45.0 |
| Greenhouse aisle with fans | 30 | 36 | 0.60 | 0.95 | ~ 33.0 |
Formula used
This calculator combines air temperature and radiant temperature using heat transfer weights. It also offers a globe-temperature option to estimate mean radiant temperature.
How to use this calculator
- Measure air temperature near where you work in the garden.
- If you know mean radiant temperature, choose that mode.
- If you have a globe thermometer, choose globe mode.
- Enter a realistic air speed for your shaded spot.
- Keep emissivity at 0.95 unless you have data.
- Click Calculate to see ERT above the form.
- Download CSV or PDF to share site conditions.
Why radiant heat matters in gardens
Direct sun, hot soil, stone paving, and nearby walls can raise radiant exposure far above the shaded air reading. Effective radiant temperature (ERT) blends air temperature with mean radiant temperature, producing a single comfort indicator that better matches how gardeners feel while pruning, harvesting, or scouting pests. It is especially useful on calm days when sunlight dominates heat stress.
Inputs that drive effective radiant temperature
Air temperature (Ta) describes the bulk air, while mean radiant temperature (Tr) represents the combined effect of surrounding surface temperatures and solar load. Air speed increases convective cooling through the convective coefficient (h_c). Emissivity affects the radiative coefficient (h_r), which becomes larger when Tr is high and surfaces strongly exchange longwave radiation. For outdoor work, a default emissivity of 0.95 fits clothing, leaves, and soil. If you measure globe temperature, enter globe diameter and coating emissivity to estimate Tr consistently across different beds and seasons.
Interpreting ERT for shade and work scheduling
Use ERT to compare locations at the same time of day. For example, a shaded bed might show Ta 28°C and Tr 29°C, giving an ERT close to air temperature. A reflective wall can push Tr above 50°C even when Ta is 32°C, raising ERT sharply and increasing perceived heat load. Record notes on cloud cover, soil moisture, and shade type to explain day-to-day differences.
Greenhouse and hoop house applications
Inside greenhouses, Tr can rise due to warm glazing, benches, and dense foliage, while fans increase air speed. Increasing air speed from 0.2 to 0.6 m/s can reduce ERT by shifting weight toward Ta. Combine ERT checks with ventilation settings, shade cloth percentage, and misting schedules to keep worker comfort stable during peak sun.
Design actions to reduce radiant stress
Practical controls include adding shade cloth, using light-colored mulches, wetting hardscape surfaces, and increasing airflow corridors. Planting tall hedges or trellises can block low-angle sun, lowering Tr near work zones. Use ERT as a before-and-after metric when testing canopy density, windbreak placement, reflective ground covers, or changing work height and orientation.
FAQs
What does effective radiant temperature represent?
It represents a weighted blend of air temperature and surrounding radiant heat, adjusted by convection and radiation coefficients. It helps compare comfort between sunny beds, shaded paths, and greenhouse aisles using one consistent temperature-like value.
How is ERT different from air temperature?
Air temperature ignores hot surfaces and direct solar load. ERT includes mean radiant temperature, so it rises near reflective walls, warm paving, or glazing. On calm sunny days, ERT can be much higher than the shaded air reading.
How can I estimate mean radiant temperature without sensors?
Start with a practical estimate: in deep shade, set Tr close to air temperature. In open sun, use Tr about 8 to 20°C above air temperature depending on surfaces, cloud cover, and reflections. Refine with spot checks later.
Why does air speed affect the result so much?
Higher air speed increases the convective coefficient, which pulls the weighted result toward air temperature and improves perceived cooling. Fans in greenhouses or breezes outdoors can reduce effective heat load even when surface temperatures remain high.
What emissivity value should I use?
If you do not have measured data, use 0.95. This is a reasonable default for vegetation, soil, and typical clothing. Only change it when you have surface material data that clearly differs from this range.
How can I use ERT to choose shade solutions?
Calculate ERT before and after changes such as shade cloth, trellis placement, or reflective mulch. A lower ERT confirms reduced radiant load. Combine it with airflow improvements for the biggest comfort gains during midday work.