The Angstrom Ignition Index is a simple fire danger rating created in Sweden. It is simple enough to be calculated mentally from observed current conditions.

It can be calculated from:

A = (R/20) + ((27-T)/10))

where A is the Angstrom Ignition Index Value, R is the Relative Humidity in percent (%), and T is the  air temperature in ° C.

For "A"	Fire Occurance
A > 4.0	Unlikely
4.0 ≤ A <2.5	Unfavorable
2.5 ≤ A ≤ 2.0	Favorable
A ≤ 2.0	Likely

Note that the Angstrom Ignition Index works inversely from the other fire indexes, smaller values equating to higher fire dangers.