When they appoint the mean date of the last frost days as the standard day for frost injury protection, it frosts very frequently after this day. The author proposed the day corresponds to M+2PE as the new standard day for frost injury protection, where M is the mean of the last frost days and PE is their probable error. As the so cold spring that it frosts after this day, occurs about 10% in probability, so this standard day is useful for the economical frost injury protection. (Fig. 1).
Then the author calculated M+2PE of various weather stations in Fukuoka Prefecture (Table 1), and drew their isogram (Fig. 2).
Fig. 3 shows the daily frequency of frosting at Fukuoka. It frosts more frequently on 15-17, 22, 27 and 28, April, than the other days. These characteristical days are recognised at Iizuka, Saga and Hita. So, they must take care of frost injury on these days.
