On holiday in Scotland last winter, I found it impossible to make snowballs with that day鈥檚 newly fallen snow. It was an extremely cold day and it would not stick together in the way it normally does. My friend has also experienced this in the Alps. What is the reason for it?
鈥� It鈥檚 not surprising that this question comes from someone in the UK. No one from Canada or the northern US would write that snow 鈥渘ormally鈥� sticks together in a way suitable for making snowballs. All Canadian and many American children know that sometimes snow is good for 鈥減acking鈥� and sometimes it isn鈥檛.
From my memories of a North American childhood, the relevant variable is temperature. When the air temperature is only a little below freezing, as it often is when snow falls in the UK, the snow is usually wet, comes in big flakes and is good for packing. When it is really cold, say about -20 掳C, the snow is usually dry and powdery and is no good for packing. Presumably the moisture content of the snow determines the amount of ice that forms under the pressure of the snowballer鈥檚 hands, and it is this ice that makes the snow stick together.
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A colleague who also grew up in the cold parts of North America reminds me that when the temperature is too far above freezing, snowballs simply turn to slush as you make them. So there is an optimum band of snow temperature for packing, and it just happens that snow in the UK generally falls within that band.
Bob Ladd, Edinburgh, UK
鈥� Only 鈥渨et snow鈥�, containing up to 50 per cent liquid water, is good for making snowballs, and this needs temperatures around freezing point.
In 1842 Michael Faraday suggested that wet snow has a thin film of water on the ice particles, and that this is responsible for gluing them together. He suspended two blocks of ice in a bath of freezing water to show that simply bringing them into contact was sufficient to make them stick together.
Lord Kelvin had a different explanation. Squeezing the snowball brings the points of the ice crystals into contact. Although our hands cannot exert much pressure, the local pressure at the sharp points of the ice crystals can be high enough to cause melting. The instant this pressure is released, the water freezes again. However, the colder the snow, the higher the pressure required.
Our understanding of surfaces is now more advanced. Water molecules on the surface of ice particles are not bound to anything on the air side, so they have excess energy. This energy can be reduced if two surfaces come together, just as Faraday observed.
鈥淲hen the temperature is too far above freezing snowballs turn to slush as you make them鈥�
But if this were the whole story, we would also be able to make snowballs at temperatures well below freezing. At very low temperatures, snowflakes, which come in all shapes and sizes, do not fit together snugly.
However, at temperatures closer to freezing, individual water molecules become more mobile and migrate over the surface to fill the awkward gaps between flakes. This ensures that neighbouring flakes fit together much better. With a bigger contact area between flakes, they now stick together more readily.
Mike Follows, Willenhall, West Midlands, UK