No one from Canada or the northern United States would say that snow “normally” sticks together in a way suitable for making snowballs.
All Canadian and many North American children know that sometimes snow is good for “packing” and sometimes it isn’t.
The relevant variable for making snowballs 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 -4°F, 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’s hands, and it is this ice that makes the snow stick together.
Many of us grew up in the cold parts of North America and 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 United Kingdom generally falls within that band.
Only wet snow, containing up to 50 percent liquid water, is good for making snowballs, and this needs temperatures around freezing point.
Wet snow has a thin film of water on the ice particles, and that this is responsible for gluing them together. Suspending 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.
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 neighboring flakes fit together much better.
With a bigger contact area between flakes, they now stick together more readily.
