What this calculator works out
This calculator sums hourly temperatures above a developmental threshold to give a total in accumulated degree hours, and reports how many observations were used.
Hourly resolution matters for the early stages of development, which can complete in less than a day. A daily mean would smooth away exactly the variation that determines whether an egg has hatched. These tools are teaching aids. They are not validated for casework, and the reference values that make any result meaningful must come from published species-specific literature.
Hourly data
Before you calculate
- Use the species-specific lower developmental threshold from published literature.
- Hourly data should reflect the body's location. Weather station records are corrected to the scene by placing a logger there and correlating over a comparable period.
- Values can be separated by commas, spaces or line breaks, so pasting from a spreadsheet column works.
- Check the observation count in the result. A missing hour is easy to overlook and understates the total.
Egg hatch in a common blowfly may require only a few hundred degree hours — under a day at summer temperatures. A daily mean cannot resolve that, and the difference between a warm afternoon and a cold night is precisely what determines whether hatching occurred before or after a given time.
How this calculator works
A sum of hourly contributions:
Contribution for each hour = max(0, hourly temperature − threshold)
Total ADH = sum of all hourly contributionsThe same clamping to zero applies as for degree days: hours below the threshold contribute nothing rather than subtracting. Converting to degree days is a division by 24.
Worked example: twelve hours
Using the default figures — a 10°C threshold with hourly readings of 15, 15, 16, 17, 18, 19, 20, 20, 19, 18, 17 and 16°C:
- Contributions: 5, 5, 6, 7, 8, 9, 10, 10, 9, 8, 7, 6
- Total: 90 accumulated degree hours from 12 observations
90 ADH over 12 hours is a mean contribution of 7.5 degree hours per hour, equivalent to a constant 17.5°C. If a published egg hatch requirement for the species were 180 ADH, this half-day accounts for exactly half of it — which is the kind of statement entomological evidence is actually built from.
Common mistakes
- Pasting a daily series into an hourly tool. Check the observation count against the period.
- Using a threshold from a different species. The single largest source of error.
- Forgetting to convert when comparing with ADD figures. Divide by 24.
- Using uncorrected weather station data. Sun, shade, shelter and indoor conditions all differ.
- Assuming the ambient temperature applied to the larvae. A maggot mass generates substantial heat.
Frequently asked questions
How many hours of data do I need?
Enough to cover the whole period of interest without gaps. Missing hours silently reduce the total and therefore underestimate development. Where a gap is unavoidable, interpolating between adjacent readings is preferable to omitting them, and the interpolation should be documented.
Should I use air temperature or something else?
Air temperature at the body's position is the usual input, measured with a logger placed at the scene. Ground surface, inside a vehicle, under a covering and within a maggot mass can all differ from ambient by several degrees or more, and the relevant temperature is the one the larvae actually experienced.
What if temperatures fluctuate around the threshold?
This is where linear thermal summation is weakest. Development at fluctuating temperatures does not always match development at the equivalent constant mean, particularly near the threshold. More sophisticated models exist, and a forensic entomologist would note the limitation rather than ignore it.
How is scene temperature reconstructed retrospectively?
By placing a data logger at the scene after the fact and recording alongside the nearest weather station for a period of days to weeks, then building a regression between the two. That relationship is applied to the station's records for the period of interest. It is standard practice and it is what makes weather data usable.
Is what I enter stored?
No. Temperature data is processed entirely in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — Forensic Science Regulator guidance on method validation and expert evidence
- Met Office — hourly and historic temperature records for scene correlation
Sources are checked at publication and can change — how I choose and check references.