Surface Hoar Growing for Several Days Akihiro Hachikubo and Eizi Akitaya Institute of Low Temperature Science, Hokkaido University, Sapporo 060, Japan Tel: (81) 11-706-5476, Fax: (81) 11-706-7142, e-mail: hachi@hhp2.lowtem.hokudai.ac.jp Mountain Weather and Snowpack Key words: surface hoar, weak layer, radiative cooling ABSTRACT Surface hoar growing for several clear and humid days were observed. In the daytime, the air and the snow sur- face temperature increased and the relative humidity de- creased, hence evaporation (sublimation) occurred at the snow surface. The amount of evaporation calculated with a bulk method was enough to evaporate out the surface hoar crystals which grew previous night, but they were observed to survive on the snow surface even in the day- time. In the following nights new hoar crystals formed on the old ones and developed largely. This result suggested that the surface hoar crystals were cooled by outgoing ra- diation even in the daytime and kept their size, while snow grains underneath the surface were warmed by solar ra- diation and evaporated. The layer composed of the sur- face hoar and the depth hoar crystals showed a very weak shear strength. INTRODUCTION Surface hoar crystals are formed by the deposition of wa- ter vapor onto the snow surface during the clear night. They have long been of interest to avalanche researchers (e.g. Perla and Martinelli, 1976), because after being bur- ied by a subsequent snowfall, they often form a weak layer due to their lack of intercrystalline bonding and weak at- tachment to the original snow surface. Besides, a layer composed oflarger hoar crystals is hard to metamorphose. Lang et al. (1984) reported that the shear strength remained too low to measure for extended periods of time. In this paper we report the surface hoar crystals survived on the snow surface even in the daytime and developed largely for several days. OBSERVATION RESULTS The following two cases of surface hoar growth are de- scribed. Case one on December 26 to 28, 1994 It was almost clear during this period except for the mOrn- ing Dec. 27 and the midnight Dec. 27-28, and the surface hoar was formed in the two nights. Figure 1 shows the time variations of the air temperature To at 1m high and the snow surface temperature T s ' T s was always lower than To and the difference between To and T s became larger during two nights. T s increased in the cloudy conditions due to the decrease in the radiative cooling. Figure 2 shows the time variation of the latent heat flux. From 18:30, Dec. 26 to 3:00, Dec. 27 the total condensation of surface hoar was 74 g/m 2 , while the total evaporation from 3:30 to 15:00 on Dec. 27 was 62 g/m 2 • Therefore, the surface hoar crys- tals formed in the first mght were expected to be elimi- nated mostly. However, they were survived on the snow surface and the new hoar crystals developed on them in the second night. Figure 3a shows a microscopic photo- graph of surface hoar crystals formed on a snow particle at the surface in the first night. Their size was less than 1mm in diameter. Figure 3b shows the "survived" crystal even in the daytime though it was rounded by the evapo- ration. New crystals in the second night developed on the old ones and reached to 3mm in diameter (Fig. 3c- e). Case two on February 23 to 26, 1995 The surface hoar growth was observed for three clear and humid nights ofFeb. 23 to 26. The time variations of To and T s are shown iri Fig. 4. T s was 5 to 10°oC lower than To at each midnight due to the radiative cooling. Figure 5 shows the time variation of the latent heat flux. Large amount of Fig. 3 Microscopic photographs of surface hoar crystals formed on December 26 to 28, 1994.. (al surface hoar crystals fonned in the first night. (b1a surface hoar crystal survived in the daytime and rounded by the evaporation. (cl- (e) new crystals in the second night developed on the old ones. b c OBSERVATION SITE AND OBSERVED ITEMS Observations were carried out in the winter of 1994- 95 in the Teshio Experimental Forest of Hokkaido University located in Toikanbetsu, northern Hokkaido, Japan. The sta- tion (45 oo N, 142 oo E) is close to the Japan Sea (about 20km in distance), and the southwest is the predominant wind direction in winter. Air temperature, humidity, wind speed, snow surface temperature and vapor condensation were measured. Observation methods and the instrumen- tation used were same as Hachikubo et al. (1995) and de- scribed in detail by Hachikubo and Akitaya (1997). The latent heat flux at the surface, which corresponds to the vapor condensation rate or evaporation rate, was obtained with the weighting- type evaporimeter in nighttime, while it was estimated with a bulk method (Takeuchi and Kondo, 1981) in daytime using the data of air temperature, sur- face temperature, humidity and wind speed. In the calcu- lation, the value of 2.9 ¥ 10- 3 obtained from field observa- tions (Hachikubo and Akitaya, 1997) was used as the bulk transfer coefficient of water vapor. 86