Calculator
Forest fire danger index calculator: the Angström index from humidity and temperature.
Enter the day’s lowest relative humidity and highest air temperature to get the Angström fire danger index and its class, worked out in your browser.
How it is calculated
The Angström index is a fire danger index developed in Sweden that uses only two weather readings. It is calculated as I = R ÷ 20 + (27 − T) ÷ 10, where R is relative humidity in per cent and T is air temperature in °C. Regional studies that apply it daily use the day’s minimum relative humidity and maximum air temperature, the driest and hottest point of the day, and this calculator follows that practice. A lower index means higher fire danger: dry air and heat both pull the number down.
| Angström index (I) | Class |
|---|---|
| Above 4.0 | Fire occurrence unlikely |
| 2.5 to 4.0 | Conditions unfavourable for fire |
| 2.0 to below 2.5 | Conditions favourable for fire |
| Below 2.0 | Fire occurrence very likely |
The formula and the four classes are as published in peer-reviewed regional studies, for example the comparison of Angström fire index values for Nalchik and Telavi. The thresholds come from that literature and have not been calibrated for Indian forest types, so treat the class as illustrative; confirm with the official source before acting on it.
Where ForestWatch fits
A single index for a single station tells you how dry and hot the day is, not where a fire is or who should go. In a fire hotspot alert system such as ForestWatch, each satellite or field detection carries the fire-weather index for its station along with wind and fuel load, and the incident queue is ranked by spread risk, the settlements and wildlife corridors in the path and how long the incident has waited.
Worked example (illustrative)
Suppose a station in a dry deciduous forest records a minimum relative humidity of 30% and a maximum temperature of 35 °C on an afternoon in the fire season. These are illustrative readings, not data from any real station.
- Humidity part: 30 ÷ 20 = 1.5.
- Temperature part: (27 − 35) ÷ 10 = −0.8. Above 27 °C this part turns negative.
- Index: 1.5 + (−0.8) = 0.7.
- Class: 0.7 is below 2.0, so fire occurrence is very likely.
If the same station recorded 60% humidity and 20 °C after rain, the index would be 3.0 + 0.7 = 3.7, which is in the unfavourable band. The swing shows how quickly the index responds to a dry, hot spell, and why it is read daily rather than once a season.
What the index does not cover
- Fuel moisture. It does not track how dry litter, grass and dead wood have become over previous days, so a single humid day after a long dry spell can look safer than it is.
- Wind. Wind drives spread, and it is not in the formula.
- Rain. Recent rainfall is not carried over from day to day.
- Local calibration. The class thresholds were published for other regions, not for Indian forest types.
The Canadian Forest Fire Weather Index (FWI) System is more complete: it adds wind and rain and keeps daily moisture codes for fine fuels, duff and deep organic layers that carry over from one day to the next. That carry-over is why it cannot be worked out from one day’s readings alone, and why this tool uses the simpler Angström index.
Official forest fire alerts in India
This calculator is not an official fire danger rating. For official forest fire alerts in India, use the Forest Survey of India forest fire portal and follow your state forest department’s instructions. Read what a forest fire danger rating system is and how to use satellite forest fire hotspot alerts.
Questions
What is the Angström fire danger index?
A simple fire danger index developed in Sweden that uses relative humidity and air temperature: I = R ÷ 20 + (27 − T) ÷ 10. A value below 2.0 means fire occurrence is very likely and above 4.0 means it is unlikely.
How is a fire danger index calculated from humidity and temperature?
With the Angström index, divide the day’s minimum relative humidity by 20, add 27 minus the day’s maximum temperature in °C divided by 10, and read the result against the four classes. A lower number means higher danger.
What is the difference between the Angström index and the Fire Weather Index?
The Angström index uses one day’s humidity and temperature. The Canadian Fire Weather Index adds wind and rain and keeps daily fuel moisture codes that carry over between days, so it needs a continuous daily weather record.
Where can I get official forest fire alerts in India?
From the Forest Survey of India forest fire portal and your state forest department. This calculator gives a simple published index, not an official alert or rating.
See it on your own data. ForestWatch — Every hotspot, ranked by risk. Book a 30-minute working session with an engineer.