Rice planthoppers are major pest for rice plants in Asian countries. They immigrates from overseas to Kyushu in Japan in the Baiu season every year. To predict the long distance migration of rice planthoppers to northern Kyushu, a migration model was developed with a hypothesis that rice planthoppers were transported by low-level jet stream at 1,000-2,000 m heights. The weather conditions for the migration period (June to July) were analyzed with the 850 mb chart at 21 h in 1980-1985 and at 09 h and 21 h in 1986, and possible migration days were decided. On the other hand, actual migration flight waves were decided with the number of catches by two nets. The possible migration flights decided by the weather chart agreed approximately with the actual flights. The accuracy of this prediction method was improved by using the 850 mb chart at 09 h additionaly. Finally, the prediction procedure was described.

A equation of precipitation-element size distribution will be proposed in this paper . It is represented by the normal distribution of liquid water content per unit volume of air with size . The conventional distributions (e.g. Marshall-Palmer (1948) and Best (1950)) and the characteristic distributions observed and computed as reported in the writer's previous paper (Shiotsuki , 1974) are fitted by this equation.Furthermore, the melted drop-size distributions for snowflakes and graupels are also represented by such normal distribution. The most. usefulness of the proposed equation is that it is applicable to almost all kinds of size distribution of various types of precipitation.The new equation is determined by the space liquid water content (M) and the mean diameter (D) and the standard deviation of i t (a) of the normal distribution of liquid water content.Thus the present equation also needs some parameters which may be dependent on precipitation type, as well as the conventional equations . But those three parameters of the present equation can be easily determined by observing the data of radar reflectivity (Z), precipitation intensity (R) and number flux of precipitation particles (N).

The flat size distributions of riandrops were observed in heavy rainfalls during the last stages of Baiu at Hitoyoshi in southern Kyushu. Most flat distributions are fairly frequently seen especially in the beginning stage of shower rain, but such flat shape remained as to appear in the medium size range of the average distribution of each individual convective rainfall.Generation processes of those characteristic spectra were investigated by numerical experiments. Cloud models were considered with some assumptions from the results of the radar observations which were simultaneously made in the raindrop sampling. Intermittent supply of liquid water with a certain drop size spectra, which was assumed to be corresponding to the observation fact of combination of cell echoes by radar, was introduced in the well mixed cloud model by the process of drop disintegration and coalescence. The results represent well the average size distribution as observed in each rainfall at Hitoyoshi, which is characterized by the numerous space density of drops smaller than 1 mm diameter, the high population of large drops, and the flat shape of the distribution curve in medium size range. Furthermore, the very flat distribution was obtained by the one dimensional sedimentation model in which the rain with the abovedescribed average distribution was released at the upper level.The Z-R relation derived from the computational results may be well represented by Z=360 R1.6 which was observed at Hitoyoshi.

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