Snow forms if the air in a cloud is below freezing. The water vapour then turns to ice instead of rain and the tiny ice crystals stick together until they form snowflakes. When they get heavy enough to fall, they drop out of the clouds. At this point though, we still don’t know whether they will end up as rain or remain as snow. This depends on the temperature of the air they travel through on the way down to the ground. If it gets warmer, they turn into rain, but if the air stays close to freezing all the way down, then the snowflakes will make it without melting and so fall as snow. If this occurs in a mountain area, it is possible for snow to be falling on the mountaintop while lower down in the valley the air is warmer and so it is raining instead. Once snow covers the ground, it may not melt for a while. Do you know why this is? Do you remember the work we did on the difference between the temperature of light and dark materials? Because snow is white, it is very reflective and so bounces away most of the sunlight which would otherwise warm it up. It the snow partly melts and then re-freezes, the new crisp outer layer will help it last even longer. In this situation, only a warm air mass will be really effective in melting the snow. Snowflakes Snowflakes occur in a huge variety of forms. In fact, no two snowflakes aresnowflake the same – each one is unique, just like each person is unique. All snowflakes have one thing in common though – they all have six sides. If you look at them under a magnifying glass, you will see the different shapes, as well as the fact that they are all hexagons (six-sided). The actual shape of snowflakes depends on the temperature of the air. When air is colder (below freezing) they may look like needles or columns. This snow, formed in freezing conditions, is powdery and dry – and not very good for making snowballs! However, when the air is warmer (just about freezing) the shapes are more complicated and often look like delicate lace. These snowflakes are usually larger and the snow is ‘wet’. It is much easier to squash this snow into heavy, icy snowballs!
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