Haines Index of Fire Danger

Dry, unstable air increases the probability that wild land fires will become large and/or erratic.  The index here provides an atmospheric reference, Lower Atmosphere Severity Index (LASI), for these fires, based on the environmental lapse rate of a layer of air coupled with its moisture content.

       LASI = a(Tp1 - Tp2) + b(Tp - Tdp)

where T is the temperature at two pressure surface (p1,p2); Tp and Tdp are the temperature and dew point temperature at one of the levels.
In reality, two steps are involved to calculate the index LASI.

First, select the proper index variant based on surface elevation.  There are three categories according to the station altitude.

    i) If the station is at a low elevation (<= 500 m), A is computed by

          (A) 950mb T(C degree) - 850mb T(C degree) factor value:

 less than 4:  1

 4 to 8:  2

 greater than/equal to 8:  3

   and B is computed by:

         (B) 850mb T(c degree) - 850mb Tdp(c degree) factor values:

          less than 6:   1

          6 to 10:   2

           greater than/equal to 10:   3

    ii) If the station is at a middle elevation (500m ~ 1500m), A is computed by

        (A) 850mb T(C degree) - 700mb T(C degree) factor values:

          less than 6:  1

          6 to 11:   2

          greater than/equal 11:   3

         and B is computed by

        (B) 850mb T(c degree) - 850mb Tdp(c degree) factor values:

          less than 6:   1

          6 to 13:   2

          greater than/equal to 13:   3

    iii) If  the station is at a high elevation (>= 1500m), A is computed by

        (A) 700mb T(C degree) - 500mb T(C degree)  factor values:

         less than 18:   1

         18 to 22:   2

         greater than/equal to 22:   3

          B is computed by

         (B) 700mb T(c degree) - 700mb Tdp(c degree) factor values:

          less than 15:   1

          15 to 21:   2

          greater than/equal to 21:   3

 

Second, add the factor values (A + B)

         (A + B)        Class of day (potential for large fire)

         2 or 3            very low

         4                    low

         5                    moderate

         6                    high