Snow Clump Formation
Date: Spring 2012
We are having some very unusual snowfall. It is not flakes, but clumps of 100-700 flakes. Most of the individual flakes making up the clumps are fern dendrites, with a few stellar dendrites. How do these clumps form? They range 1.0 to 5.2 cm across.
What you are observing is not too unusual. Snowflakes can agglomerate to form large clumps if the conditions are correct. The “correct conditions” are temperatures not too far below 0 C., and not a lot of wind to break the clumps apart. Under these conditions, the individual flakes tend to form these “super flakes”. They are not very stable and tend to settle onto the ground or other substrates. The conditions are sufficiently rigid that this type of snow fall does not usually last very long. The clumps form when the flakes make contact under quiet conditions.
When the temperature is near freezing, 32 degrees Fahrenheit 0 degrees Celsius, the snowflakes tend to clump together because they are wetter (have a higher water than ice content).
Sometimes the air temperature changes to a little above and a little below the freezing point and sometimes it is a little above AND a little below in the same local area.
That is when the snowflakes clump together.
When the air temperature drops to below the 32 F/0 C point, the snowflakes contain more ice and less water and have a lesser tendency to clump.
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Update: June 2012