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A theoretical and observational approach was made to elucidate the phenomena of melting of snowflakes below freezing level in the atmosphere.A main purpose of the theoretical approach was to examine the effect of relative humidity of air on snowflake melting that has never referred so far. It is based on the consideration that latent heat accompanied with sublimation or condensation of water vapor from or on snowflake surface exerts a significant influence on the melting rate of snowflakes in addition to heat transfer due to heat diffusion from the ambient air to snowflakes.In the theoretical calculations, we used an empirical formula of the melting rate of snowflakes proposed previously by Matsuo and Sasyo (1981) as the basic equation. Snowflake diameter, liquid water content, and fall velocity as a function of distance below freezing level were obtained by the calculations using parameters such as relative humidity of air, snowflake sizes, and densities. In saturated air below freezing level, snowflakes commenced melting from just below freezing level and snowflakes with equivalent diameter 1-5 mm in raindrops completed melting within several hundred meters. The width of the melting layer thus formed increased with increasing sizes and densities of snowflakes contained. In subsaturated air below freezing level, on the other hand, melting of snowflakes did not take place as far as a considerable distance below freezing level because of cooling of snowflakes by sublimation of water vapor. The width of the non-melting layer thus formed increased nearly linearly as relative humidity decreased; for example, it was about 120 m for relative humidity of 90%, and 700 m for RH=50%.Under the non-melting layer, snowflakes commenced melting because of increase in heat transfer from the ambient air due to increase in air temperature and water vapor density. The width of the melting layer decreased as the layer became dryer. Fall velocity of snowflakes decreased slightly in the non-melting layer and increased rapidly in the melting layer with increasing distance below freezing level.In the observations, simultaneous measurements were carried out for snowflake water content, fall velocity, mass, and cross-sectional area. The observational results showed that fall velocity and liquid water content of snowflakes as a function of their mass were dependent on surface air temperature above 0 * and relative humidity. Especially at high surface air temperature (*1*), fall velocities were almost constant with respect to their masses and some of them in small mass range were higher than those in larger mass range. This finding shows a different tendency from the results observed by Magono (1953) and Langleben (1954). These observational results were interpreted well by the present theoretical calculations.

Routine surface meteorological data at three weather stations in Japan, i.e., Wajima (sea shore), Matsumoto (inland), and Nikko (inland), were analyzed in order to obtain the relationship between the occurrence of the types of precipitation of snow, sleet, and rain on the ground and surface meteorological elements. The results indicate that the types of precipitation were dependent on surface relative humidity as well as surface air temperature. Even at the same air temperature, the precipitation types varied depending on humidity, and that critically. Two critical humidities were recognized at a given air temperature and the precipitation types were characterized by these critical humidities. Below a lower critical humidity, precipitations were all snow. Within some range above the critical humidity, they could be snow, sleet, or rain. Above a higher critical humidity on the top of the range, only rains were observed. These facts suggest that melting of snowflakes proceeds rapidly at higher relative humidity.From statistical analyses, two critical humidities for snow and rain (%) were obtained as a function of surface air temperature (*).The obtained relations at each station were as follows:, RHcri (snow) = -7.5T+93, RHcri (rain) =46*6.2 -T , Wajima RHcri (snow)=-7.3T+96, RHcri (rain)=39*7.2-T, Matsumoto RHcri (snow)=-6.2T+91, RHcri (rain)=43*6.8-T, Nikkowhere RHcri (snow) is a lower critical humidity and RHcri (rain) a higher critical one. The relations for snow are hardly different among the observation stations, but those for rain are different. The difference of the relations for rain suggests that the size and density of snowflakes are different from station to station and that the difference affects the speed of the generation of raindrops due to melting. The non-difference of the relations for snow, on the other hand, suggests that the, difference in physical properties of snowflakes does not exert any influence on the relations for snow.Sleet was obtained in the transition region surrounded by two relations for snow and rain and its occurrence was associated with high precipitation rate together with air temperature and humidity.These analytical results are explained well using the calculated results previously obtained by Matsuo and Sasyo (1981,a).

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