Visual observation on the natural snow surface shows that the phenomenon of blowing snow changes by an index P_b. The phenomenon is related strongly to wind velocity V(m/s) and air temperature T(°C). The index was set up by way of experiment as follows:

(1)

Where a(s/m) and b(°C^_1) are the coefficients, and the author assumes a = b = 1. Then, the dimentionless equation P_b called “blowing snow index” is defined:

On the snow surface on Shonai Plain (Honshu, Japan) 152 visual observations of the phenomenon were carried out at 9:00 every day. Observation points were set up on the plain at Tsuruoka C, Hirata T., Uza T. and Amarume T. Results from these observations were classified in 3 groups, namely, no blowing snow, slightly blowing snow and heavy blowing snow.

Figure 1 shows the obtained numbers n of each group. In the group of no blowing snow, the mean P_b. is 3.1. In the slightly and heavy groups, P_b is 11.4 and 14.2 respectively. From the tendency shown on figure 1, it could be assumed that blowing snow phenomena do not happen in P_b7, slightly blowing snow happens in 7≤P_b12, and heavy blowing snow happens in 12≤P_b.

Fig. 1. Grouping Of Observation Numbers Against P_b. A: No Blowing Snow, P_b = 3.1, B: Slightly Blowing Snow, P_b -11.4, C: heavy blowing snow, P_b = 14.2.

At the same places, the amount of blowing snow transport Q (g/m·s) was measured 367 times. Values of Q were measured using a box blowing snow gauge. Up to this time, Q had been written as follows:

Now, the author attempts to write as:

the mean values of Q = 16.4 g/m·s and P_b = 12.1 were obtained from those measurements.

The obtained α from those measurements have a wide range of fractuations. Then, the coefficient α is estimated as follows:

>The phenomenon of blowing snow depended remarkably on wind velocity, air temperature, degrees of hardness of a snow surface and diameters of snow particles. Therefore, it seems that to describe the tendency of the phenomenon by only P_b is inadequate.

The obtained Qs are grouped in accordance with P_b, namely, Q<20 g/m-s to 7≤P_b<12, Q=20 g/m.s to 12 ≤P_b. Figure 2 shows the distribution of Q as parametered by P_b

Fig. 2. Distribution Q against P_b; white small circles: Q<20 g/m-s, white large circles: Q=20 g/m·s.

The attempts to describe the initiation condition of blowing snow and the blowing snow transport Q by the index P_b are studied. The effectiveness of P_b on blowing snow are summarized in the following: (1) On the condition P_b<7, the snow surface has no blowing snow, 7≤P_b<12 has slightly blowing snow and 12≤P_b has heavy blowing snow. (2) On the estimation of transportation, Q could be expressed as follows:

but, the coefficient had wide ranging values.

In the particular case 7≤P_b<12, the amount of snow transportation showed Q<20 g/m·s, and the other case, 12(P_b, showed Q=20 g/m·s. For these reasons, the index has a simple but useful characteristic on initiation condition of the blowing snow, but has little usefulness for the transport Q.