To obtain the Heat Zone of a region it is necessary to record the number of days per year during which
the temperature will go above 30° C [86° F]. The Zone-Index is given by the average of
those days according to the following table:
Average number
of days per year
above 30° C [86° F]
Zone
Below -1
1
1
to
7
2
> 7
to
14
3
> 14
to
30
4
> 30
to
45
5
> 45
to
60
6
> 60
to
90
7
> 90
to
120
8
> 120
to
150
9
> 150
to
180
10
> 180
to
210
11
Above 210
12
This Heat Zone Index was designed by the American Horticultural Society (AHS). The average is calculated over a period of 12 years (7 years in some cases, when data are missing).
The temperature is one limiting factor among several others such as humidity, soil
conditions, insolation, exposure to wind.
