The Physics
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Mass of a Snowflake

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"Are All Snowflakes Shaped Alike?" Science and Technology Desk Reference. Gale Research Inc.1993 "The many forms that a snow crystal can have are explained by the number of water molecules involved (10−18) [sic] and the different ways that molecules can arrange themselves in a three-dimensional hexagonal pattern." [see below]
Snowflakes. Owashtanong Islands Audubon Society. "Large snowflakes can measure up to 2"across and contain hundreds of individual crystals." [see below]
Natural Snowflakes. Ken Libbrecht. California Institute of Technology. "Since a typical small snow crystal might contain 1018 water molecules, we see that about 1015 of these molecules will be different from the rest." [see below]
Physical Properties of Ice. Ken Libbrecht. California Institute of Technology. "Mass of a water molecule:  mH2O = 2.992 × 10−26 kg" [see below]

Snowflakes are made up of many ice crystals clumped together that form when water vapor changes directly from a gas to a solid. In very cold air, water molecules cling on to particles, forming tiny ice crystals. As more water molecules freeze onto the crystal, they join at angles to form a six sided structure. Nearly all snow crystals have six sides, but they vary in shape. As the crystal grow, they fall with increasing speed. They may collide with one another to create snowflakes. A snowflake is made of up to a hundred snow crystals. The particles of snow may vary in size, from crystals almost invisible to the unaided eye to snowflakes one inch or more in diameter. The largest snowflake ever seen was 8 by 12 inches and was reported to have fallen in Bratsk, Siberia in 1971. A typical small snow crystal might contain 1018 water molecules scattered throughout the snow crystal. The mass of a water molecule is 2.992 × 10−26kg. Thus, a typical snow crystal may have a mass of 2.9 × 10−8 kg. A typical snowflake made of 100 snow crystals was calculated to be about 3 mg.

crystals per snowflake 100
water molecules per crystal 1018
mass of a water molecule × 3 × 10−26 kg
 = 3 × 10−6 kg

It has been said that no two snowflakes are alike. Factors such as humidity, wind, and temperature affect the growth of each snowflake as it falls. The pattern or structure may change as it falls from the sky. A crystal needs atmospheric conditions of no hotter than −15 °C to grow. These six sided hexagonal crystals are shaped in the high clouds. In the middle height clouds, ice crystals are shaped in needle-like or flat, six sided forms. And a wide variety of six sided shapes are formed in low clouds. The colder the temperature, ice crystal tips are sharper. At warmer temperatures, the ice crystals grow slower and smoother. When a snowflake melts, its intricate design is lost forever in a drop of water. It may also vanish by means of changing from ice to vapor.

Judy Moy -- 2001