Angström index


The Angström index is a very simple fire index developed in Sweden and designed in order to be calculated mentally. It only requires relative humidity and temperature as input variables (Chandler et al. 1983).

It has been used in some parts of Scandinavia for indicating expected fire starts on a given day (Chandler et al. 1983).

A drawback of the Angström index is the inaccurate reflection of the relationship between relative humidity, temperature and fuel moisture (Chandler et al. 1983).


The Anström index \(I\)
is calculated as follows (Chandler et al. 1983, Langholz & Schmidtmayer 1993):

\[ I=\bigg(\dfrac{H_{13}}{20}\bigg)+\bigg(\dfrac{27-T_{13}}{10}\bigg)\]

where \(H_{13}\) relative air humidity [%] and \(T_{13}\) air temperature [°C] at 13:00.

The Angström index is supposed to be calculated on a daily basis. No particular values or conditions are required when starting the index calculation.

Index interpretation

The Angström index is interpreted as follows:

Index values Interpretation
\(I\) > 4.0 Fire occurrence unlikely
4.0 < \(I\) < 2.5 Fire conditions unfavorable
2.5 < \(I\) < 2.0 Fire conditions favorable
\(I\) < 2.0 Fire occurrence very likely


Chandler et al. (1983)
Langholz & Schmidtmayer (1993)