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The white track is crystal clear

I am a keen reader of Object Lesson on page 4 of Friday, a cornucopia of apparently useless facts which, none the less, I use regularly in our science club, if not in mainstream lessons.

But I was surprised to read Stephanie Northen's assertion that "snowflakes form from super-cooled water droplets" (December 8). I always understood that frozen rain is hail and that snow is formed by the sublimation of water vapour directly to ice at low pressure high in the atmosphere. I always use the difference between snow and hail as an example when dealing with phase chage.

I was also surprised at Ms Northen's assertion that "not all are the same". I realised that she must be referring to the various isotopes of hydrogen and oxygen giving rise to molecules of subtly different mass, and presumably, bond length. However, the abundance of isotopes other than 1H and 16O are surely too small to make any difference to the statistical probability of two identical snowflakes forming.

I hope I haven't been too nerdy by being over-serious, but accuracy matters.

Noel Jackson, Head of science, Belmont school, Durham

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