Snow

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 are 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!

Too cold to snow?

You may have heard people say that it is "too cold to snow". However, this expression is not strictly true, as snow could fall from air of any temperature. In reality though, snow rarely falls from air that is very cold. This is because cold air cannot usually hold enough moisture for any type of precipitation. So in this way, the expression does have some truth in it.

In fact, the heaviest snowfalls occur when the temperature is at freezing point. More snow falls in a year in the southern part of Canada and the northern part of the United States than at the North Pole for exactly this reason.

We can therefore see why snow is so difficult to forecast. It snows the most when the temperature is around freezing, but a rise of just a degree or two can result in rain instead.

Good snow

– cold, snowy weather can be fun! Skiing and tobogganing on snow are popular activities in the winter. Have you tried either of these sports? Or have you built a snowman in your garden – or had a snowball fight with your friends?

Bad snow

– as well as being fun, snow can also be dangerous. In spring, when snow starts to melt, it can slide down the mountainside as an avalanche. Tons of snow and ice crash down into the valley below and bury anything in their path. Trees are uprooted by the snow, buildings flattened and many people are killed each year by avalanches. Did you know that snow travels at an average speed of 100 miles per hour in an avalanche? That’s faster then cars travelling on a motorway in England!

Snow can also be dangerous in the form of a blizzard. Here, snowfall is combined with strong winds and it often becomes impossible to see anything. This can effect transport and communications in both rural and urban areas, as well as piling snow up against objects such as cars or buildings, often trapping people inside.

Did you know that snow takes up much more space than the same amount of water? You can test this by collecting snow in a container such as a glass.

Snow fact: In general, snow takes up to ten times the amount of space that rain does. So 10 cm of snow would equal about 1 cm of rain!

Now it's time to move on to look at a third form of precipitation.  Click on the sun icon and together we'll discover more about hail.