Formula used
This tool estimates cooling using a practical night-time rate model: Cooling = Rate × Hours. The rate starts from a baseline and is adjusted by sky, wind, terrain, moisture, and canopy.
- Rate = 1.20 × Sky × Wind × Terrain × Ground × Canopy (in °C per hour)
- Estimated Min = Start − Cooling
- Min ≥ DewPoint + 0.5°C to reflect saturation limits
Dew point is computed from temperature and humidity using the Magnus approximation.
How to use this calculator
- Select Estimate when you lack a forecast low.
- Enter your current temperature and the hours until sunrise.
- Set sky and wind from what you observe or expect.
- Use dew point when available; otherwise enter humidity.
- Review frost and chill risks, then follow actions listed.
Example data table
| Start (°C) | Hours | Sky | Wind (km/h) | RH (%) | Estimated Min (°C) | Drop (°C) | Frost Risk |
|---|---|---|---|---|---|---|---|
| 12 | 10 | Clear | 3 | 55 | 1.5 | 10.5 | Moderate |
| 9 | 9 | Partly | 8 | 70 | 2.8 | 6.2 | Low |
| 7 | 11 | Cloudy | 12 | 80 | 4.6 | 2.4 | Very low |
| 6 | 8 | Clear | 1 | 65 | 0.8 | 5.2 | Moderate |
| 4 | 10 | Overcast | 6 | 90 | 2.2 | 1.8 | Low |
Examples assume typical garden exposure; your microclimate may differ.
Night Cooling Mechanics in Garden Microclimates
Outdoor temperature drop comes from radiative heat loss to the night sky plus exchange with air and soil. Clear skies boost radiation, while clouds reduce it. Cold air drains into low spots, so beds in hollows can finish cooler than slopes. Moist soil stores and releases heat more steadily than dry soil, moderating the overnight minimum.
Interpreting Sky and Wind Inputs for Better Decisions
Sky condition and wind shape the cooling rate. On calm, clear nights, leaf surfaces can cool below the measured air temperature, causing “radiation frost” before the forecast low is reached. Light winds may mix warmer air down or increase convective loss in exposed beds. Stronger winds usually limit surface frost by mixing, yet they can raise chilling stress and plant water loss.
Humidity, Dew Point, and Frost Formation
Humidity influences how far temperatures can fall before saturation. Dew point is the threshold where condensation begins; once reached, latent heat released by condensation resists further cooling. The calculator therefore limits the minimum near dew point. When dew point is low, the air can cool much further, increasing frost potential on clear nights. When dew point is high, frost risk drops but disease pressure may rise.
Using Drop Rate and Risk Labels for Plant Protection
The average drop rate helps schedule protection. Faster drops call for earlier frost cloth, low tunnels, or moving containers beside walls that re‑radiate heat. Install covers before sunset and seal edges to trap rising warmth. Many seedlings and tropicals slow growth below 10°C, even without frost. Combine the estimated minimum with crop thresholds and exposure duration to decide where to act first.
Field Calibration and Recordkeeping for Reliable Results
For professional use, validate results with a min‑max thermometer near plants. Compare nights, then refine inputs for terrain, canopy, and soil moisture. Log start temperature, hours, and observed minimum, and export CSV or PDF for reviews. With consistent records, you can create triggers for covers and irrigation timing.
FAQs
1) What does the calculator estimate?
It estimates an overnight minimum temperature and the expected drop over your chosen hours, using sky, wind, humidity or dew point, and site exposure factors. It also labels frost and chill risk to support quick protection decisions.
2) Should I use Estimate mode or Forecast mode?
Use Forecast mode when you have a trusted official low and want the drop and rate for planning. Use Estimate mode when forecasts are uncertain or your garden is a known microclimate, such as a hollow or courtyard.
3) Why is dew point important for night cooling?
As air cools toward saturation, condensation releases latent heat that slows further cooling. Dew point therefore limits how low temperatures can realistically fall under similar conditions, especially on humid nights.
4) How accurate are the results?
Accuracy depends on microclimate and sensor placement. Treat outputs as planning guidance, then calibrate by comparing several nights against a thermometer near plant height. Consistent inputs and records improve reliability over time.
5) What settings reduce frost risk most?
Cloud cover, tree canopy, and nearby structures reduce radiative heat loss. Slight mixing winds can also reduce surface frost. Moist soil earlier in the day stores heat, while dry, exposed beds cool faster overnight.
6) How can I protect tender plants quickly?
Cover plants before sunset with frost cloth, trap air at the soil edge, and avoid touching foliage directly. Move containers to walls or under eaves, and group pots together. Remove covers after sunrise to prevent overheating and disease.