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第2次世界大戦末期の昭和18～20年の冬期に,北海道ニセコアンヌプリ山頂の着氷観測所で実験機として使用されたと思われる,「零式艦上戦闘機(通称:ゼロ戦)」の右主翼が1990年8月初旬山頂東側の沢で発見され,その後2004年に回収されて,2005年12月22日から北海道倶知安町の風土館に常設展示されて話題を集めている.それは,60年前のゼロ戦の翼という野次馬的な見方の他に,もはや着氷観測所などといった言葉さえ知らない世代が増えた昨今,そもそも着氷観測所はどんな観測所で,どんな実験をしていて,その結果はどうなったのであろうかという興味とは別に,関連する資料の展示,解説,当時の写真や新聞記事に見られる実験機が,ゼロ戦とは違う「九六式艦上戦闘機(通称:九六式)」のものが多かったからである.どうしてこのようなことになったのか?マスコミ関係や専門家の間で注目されてきたが,幸い「中谷宇吉郎雪の科学館」が所蔵している,当時北海道大学助手で,直接この実験の担当者だった黒岩大助(元北海道大学低温科学研究所長)が撮影していた写真のアルバムから,中谷宇吉郎門下生の一人だった樋口敬二(名古屋大学名誉教授)の考察によって,昭和18～19年の冬が九六式,19～20年の冬がゼロ戦であったという結論が得られた.この報告では,ゼロ戦主翼の発見を契機として着氷観測所ではどのような実験が行われたのか,その実験の主目的であった飛行機の着氷について,使用された実験機の機種とそれに関連するいくつかの疑問について整理したものである.

二重防風ネット内で堆積した乾き新積雪の密度に対する平均気温および平均降水強度の影響に加えて,降雪粒子の卓越結晶形,水平粒度分布,霰寄与率(質量フラックスに占める霰の割合),雪面への運動エネルギーフラックスおよび粒子投映像の円形度との関係を解析した.その結果,新積雪密度への影響として,卓越結晶と平均降水強度の他に,雲粒付樹枝では運動エネルギーフラックスとの相関が大きかった.また,雲粒付立体樹枝では水平粒度分布,円形度,霰寄与率および運動エネルギーフラックスの影響も比較的顕著にみられた.さらに,卓越結晶形の霰(霰寄与率≧70%)では,水平粒度分布の影響が他の因子より大きかった.

The radar echo analysis in which the heavy snowfall intensity stronger than 6cmh-1 in snow depth was recorded on the lower elevation angle of radar scanner was carried out from the viewpoint of traffic transport facilities and localized heavy snowfall phenomena on the Ishikari Plain including Sapporo, Hokkaido, Japan. From the analysis of radar echo, it was clarified that there were high frequency areas. Furthermore, it was understood that there were some areas with sudden changes in snowfall intensity stronger than 10 cmh-1 in a limited few km2 in a horizontal scale and 10 minute duration in calm weather condition. Vertical cross section analyses across the abnormal heavy snowfall areas in which there were recorded with an intensity stronger than 6 cmh-1 by radar showed that the drastic increase of snowfall intensity occurred between 200m and 400m above the ground surface. The change of the maximum value of the radar echo intensity at around 200m level was estimated by the growth of large snowflakes which were aggregated and formed quickly under the horizontal and vertical wind shears at the same level.

Record-beaking heavy snowfalls were observed at Sapporo area in the Ishikari Plain, Hokkaido from December 1995 to January 1996. The total snowfall in December 1995 at Sapporo was 256cm which was more than twice of the monthly average.

Cloud droplets collected on snow crystals were observed by means of microscopic photograph method simultaneously with the measurement of size distribution of cloud particles which were suspended in free air at the top of Mt. Teine, Hokkaido in the winter seasons of 1959, 60 and 61. The falling path of snow crystals of various types ware estimated by the use of the vertical profile of air temperature and humidity and by the use of Nakaya's Ta-s diagram. The mean density and size distribution of cloud particles in the cloud through which the particles fell were assumed to be uniform with those which were observed at the mountain.From the data measured, following conclusions were reached : 1) The size distribution of cloud droplets collected on snow crystals always was larger than that in the free air.2) The efficiency of collection of cloud particles by snow crystals seemed to be much smaller than that calculated by Langmuir method.3) snow crystals of needle type showed fairly good efficiency of collection.The discrepancy in the collecting efficiency is considered to be due to the fact that the size distribution was not observed throughout a whole cloud layer but only near the cloud base.

Continued from the last observation, charges on fairly large natural snow crystals were measured by means of a horizontal electric field before and after their artificial melting during snowfall alternately. It was found that the snow crystals of all sizes obtained considerable positive charge when they were melted. This observation appears to explain the general phenomenon that in steady rainfall negative surface electric field is predominant and raindrops carry positive charge in most cases, while in steady snowfall surface electric field is predominant and most of the snow crystals are charged negatively.