Monday, January 18, 2010

Fermi Contest Solution…

Thank you to everyone who entered our Fermi Estimation Contest this past January. Congratulations to winner Alex D, who received a free copy of How Many Licks? Several contestants asked for my solution, so I’m posting my results below:


How many snowflakes are in this snowman?


I assumed the snowman was made of three spheres of radii R1=30 cm, R2=45 cm, and R3=65 cm. The volume of the snowman is then given by:

V = (4Pi / 3)*(R13 + R23+R33) ~ 1.6m3

From, there are roughly 100 crystals per snowflake and 1018 molecules per crystal. One water molecule weighs 2.992x10-26 kg. From these facts, we can calculate the mass of a flake:

(100 crystals per flake)

*(1018 molecules per crystal)

*(2.992x10-26 kg per molecule)


3.0x106 kg per snowflake

According to Wikipedia, the density of snow varies considerably from 8% the weight of water for newly fallen snow, to 30% after it settles, to 50% in late spring. Since the density of water is about 1000kg/m3 and the snowman is pretty densely packed, I chose 500 kg/m3.

Using these numbers, we can calculate the number of snowflakes:

(1.6 m3)*(500 kg per m3) / (3.0x106 kg per snowflake) ~ 2.7x108 snowflakes

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