Frosty Morning In dayboro

Dayboro Frost Risk Calculator

Live frost predictions powered by my own weather station. Local sensor data, not Brisbane guesses.

Frost in the Dayboro valley does not follow Brisbane rules. My station at 130 metres elevation in the D'Aguilar foothills has recorded overnight minimums as low as 3.9°C, which is cold enough to kill tomatoes, beans, and anything else that does not appreciate a cold snap. I've lost plants to that reading myself. This calculator uses live readings from my soil temperature sensor, dew point measurements, wind speed, and barometric pressure to compute a frost risk score updated every five minutes.

Unlike generic frost calculators that work off Bureau of Meteorology data from Archerfield or Brisbane Airport (both coastal, flat, and nothing like a valley), my tool uses data from a weather station sitting in the same microclimate as your garden. The Dayboro Model forecast I run provides 7 night predictions so I can plan ahead, while live sensor data refines tonight's risk as conditions evolve.

Frost Risk Calculator

Live frost predictions for the Dayboro valley, from my own sensor network.

Frost risk varies by location and altitude
Dayboro (default)
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Understanding frost in the Dayboro valley

I sit in a valley at approximately 130 metres elevation in the D'Aguilar foothills, about 35km north west of Brisbane CBD. This geography creates a microclimate significantly different from coastal Brisbane. Cold air drains from the surrounding ridges into the valley floor on calm, clear nights, creating frost pockets that generic weather forecasts simply cannot predict. I've watched my station drop 4°C in two hours on a still night when Brisbane was sitting steady.

The Bureau of Meteorology's nearest automatic weather station is at Archerfield, a flat coastal site at 12 metres elevation. When the BoM forecasts a minimum of 8°C for "Brisbane", my Dayboro valley can be sitting at 4 to 5°C. A critical difference for anyone with frost sensitive plants in the ground.

Ground vs. screen temperature: weather stations measure air temperature at approximately 1.5 metres above ground (screen height). On calm, clear nights, the air layer at ground level can be 3 to 5°C colder than screen height. My station reading 5°C could mean close to 0°C at plant level. That is why "light frost" on the ground can occur even when the official minimum does not reach freezing. I learned this the first winter I lost a row of beans on a 4.5°C reading.

How the Frost Risk Score Works

The calculator computes a frost risk score from 0 to 100 using five weather factors, each weighted by its importance in frost formation:

Temperature (0–40 points)

The single largest factor. Score scales by proximity to 0°C: temperatures at or below freezing score the maximum 40 points, while anything above 13°C scores zero. The scaling is non linear. the difference between 5°C and 2°C matters far more than the difference between 12°C and 9°C.

Dew Point Depression (0–20 points)

The gap between air temperature and dew point determines whether conditions favour radiative frost or fog. A gap of 3–5°C is the "sweet spot" for frost formation (maximum 20 points) because the air is dry enough for strong radiative cooling but not so dry that frost cannot form. Very small gaps (under 1°C) indicate fog, which actually provides some frost protection by trapping heat.

Wind Speed (0–15 points)

Calm conditions allow a stable cold layer to form near the ground. Wind speeds below 2 kph score the maximum 15 points; moderate winds (10+ kph) mix warmer air downward and largely prevent frost. The Dayboro valley is naturally sheltered, so calm nights are common during winter high pressure systems.

Cloud Cover (0–15 points)

Clear skies allow maximum radiative heat loss to space. Overcast conditions act as an insulating blanket, scoring zero. Cloud cover is parsed from the Dayboro Model forecast text: clear = 15, partly cloudy = 10, mostly cloudy = 4, overcast = 0.

Barometric Pressure (0–10 points)

Stable high pressure (above 1020 hPa and rising) creates the ideal conditions for radiative frost: clear skies, light winds, and dry air. Falling pressure typically indicates an approaching weather change with cloud and wind, reducing frost risk.

Crop Frost Vulnerability

The calculator includes 19 common Dayboro garden vegetables grouped by frost tolerance. These thresholds come from horticultural data calibrated against local growing experience:

Tolerance Crops Damage Starts Kill Temp
None Tomato, Beans, Cucumber, Capsicum, Zucchini, Basil 5°C 2°C
Light Lettuce, Potato, Coriander, Pumpkin 2°C −2°C
Moderate Carrot, Broccoli, Cabbage, Parsley, Silverbeet 0°C −5°C
High Spinach, Kale, Garlic, Asparagus −3°C −10°C

The crop grid shows three status badges based on the forecast minimum temperature: At Risk when the minimum is at or below the crop's damage threshold, Watch when within 3°C of the threshold, and Safe otherwise. Remember: ground temperatures can be 3–5°C colder than screen height forecasts.

Limitations and Honest Reporting

No frost calculator is perfect, and this one has specific limitations you should know about:

  • Frost is rare at Dayboro. the lowest we've recorded is 3.9°C. True sub zero screen temperatures are not in our observational record. This calculator will correctly show "Negligible" for most of the year (October through April).
  • Dew point not in forecast data. the Dayboro Model forecast does not include a direct dew point prediction. We estimate overnight dew point using the fog indicator as a proxy: "moderate fog" means dew point equals the minimum temperature; no fog means dew point is approximately 5°C below the minimum.
  • Cloud cover is text, not numeric. parsed from the forecast conditions string ("partly cloudy," "overcast," etc.). Edge cases in forecast wording may occasionally be misclassified.
  • Hyper local variation. the weather station is at a single point in the valley. Your property's frost exposure depends on its specific elevation, aspect, proximity to creek lines, and surrounding vegetation.

Frequently Asked Questions

Does Dayboro actually get frost?
At screen height (1.5m), Dayboro's coldest recorded temperature is 3.9°C. True meteorological frost (0°C at screen height) has not been recorded at our station. However, ground level temperatures can be 3–5°C colder on calm, clear nights, meaning ground frost does occur in low lying areas of the valley. particularly near creek lines and in frost hollows. Longtime locals recall hard frost on the grass and ice on windscreens during cold winters.
When is frost season in the Dayboro valley?
The frost risk window runs from approximately late May to mid September, with the highest risk in July and August. The valley's topography channels cold air into low lying areas overnight, creating localised frost pockets. The first cold nights typically occur in late May, with the last frost risk nights by mid September.
What should I do when the score is above 50?
A score above 50 (Moderate to High risk) means you should take action: cover frost sensitive plants with frost cloth or old sheets draped over frames (not directly on foliage), move potted plants under cover or against north facing walls, and water your garden in the afternoon so the soil retains heat overnight. The protection tips section of the calculator provides specific advice for each risk level.
Why does the score change during the evening?
The calculator blends forecast data with real time sensor readings from the Dayboro weather station. As the evening progresses, actual temperature, humidity, wind, and pressure readings refine the forecast based risk score. If conditions are calmer or clearer than forecast, the score may increase; if cloud or wind arrives unexpectedly, it may decrease. The score updates every time the weather station pushes new data (approximately every 5 minutes).
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