The Physical Investigation of Rime

[摘要] At the weather station on Mt. Fuji the author investigated the various properties of rime deposits and the conditions controlling their types. Basing on the results of observation, he proposed a new theory about the mechanism of rime formation. The results are summarized as follows:_(1) The densities of various types of rime were measured: the glazed frost and the almost transparent hard rime 0.9; hard rime with finny parts on both sides 0.7-0.9; white shelly rime 0.6-0.7; soft rime of finny structure <0.6.(2) The opacity of the hard rime increases with decreasing drop size and liquid water content in fog. It is also closely related to the temperature rise on the front surface of rime, as shown in fig. l.(3) The adhesiveness of rime is the greatest in hard rime with finny sides. The force of adhesion ranges 0-13kg/cm2 and increases with lowering temperature.(4) The shape of cross section of the rime can be classified into 7 types shown in fig. 2. A and B type are formed by flowing of water before freezing, C and Dl type occur when λ>2.5, (λ=2_??_r2/9_??_) D2 and E type when λ<2.5, and F type when λ<1.0. The type depends also upon the length grown. Fig. 3 shows this relation.(5) The finny structure of rime seems to occur to occur when the impinging angle of cloud drops with regard to the front surface is less than a certain velue. we may say, therefore, that the finny structure develops with decreasing λ. When λ is very small, the finny structure becomes isolated and a deep groove is formed on the central line of soft rime.(6) The angle of widening of breadth is also closely related to the value of λ. It is shown in fig. 4.(7) The mean surface temperature of the hard rime was measured with thermojunctuion. It was about 1_??_2°C higher than the air temperature.(8) If we write the mean effective icing time neglecting the heat conduction as τe, and the mean time interval of successive impingements as ε, the mean temperature rise upon the front is theoretically Δθ=τe/εθ, when the air temperature is -θ°C. Using the 1/4 of the Frössling's formula for the rate of evaporation of water drop and the similar one for the convective heat transfer, we obtain the following experession of Δθ:-Δθ=6.12×106M r012/1+2.62 v0.5 r00.6 (1) where M is the rate of icing in gr/cm2 sec, r0 the radius of cloud drop, and v the local wind velocity. Fig. 5 shows the relation between the observed value of Δ_??_/θ and the oaloulsted one, taking v_??_o in the expression (1). The difference is attributed to the neglected wind velocity and heat conduction. This result means that the hard rime is formed by independent freszing of individual water drop, hence the over-supply of water does not occur.(9) Ifτe_??_ε, since the surface temperature must be always 0°C, the impinged dropos unite together before freezing, and the surplus water must flow away. This is the case of glazed frost. Using the formulae of the cooling of s circular cylinder, we obtain the following expression of icing rate for glazed frost: where V0 means the general wind velocity, R the radius of cylinder. The relation between the observed value and the calculated one is shown in Fig. 6.(10) The microscopic structure of rime was studied with polarized light. The isolated finny soft rime is usually a monocrystal, whose oriedtation of optic axis is rather random. In massive soft rime, although each piece of finny sturcture is also composed of a few monocrystal, whose orientation of their optic axes are almost parallel to the direction of growth.

[学科分类] 大气科学