Dr Naomi Lavelle: What exactly is snow and will we have a white Christmas?

We often associate snow with Christmas, even though white Christmases are not actually common in Ireland (only eight in the last 50 years).

Dr Naomi Lavelle: What exactly is snow and will we have a white Christmas?

We often associate snow with Christmas, even though white Christmases are not actually common in Ireland (only eight in the last 50 years).

But while we conjure up dreams of snow for the big day, here is a little of the science behind it.

What exactly is snow?

Snow is made up of ice crystals and forms in a similar way to rain. It is not, however, just frozen raindrops. Snow forms under specific conditions when water vapour freezes directly into ice crystals (missing out on the liquid phase).

Snow crystals form around a small spec of dirt or dust, creating a six sided crystalline shape, a hexagon. As a snowflake falls through the air, water (vapour) molecules stick to the six edges of the hexagonal shape and the snowflake grows bit by bit.

The final shape of the snowflake as it touches the ground will depend on the journey it took to get there; the humidity, movement and temperature of the air it passed through along with other atmospheric conditions all have a big influence on the design of the snowflake.

What is the difference between a snow flake and a snow crystal?

A snow crystal is a single crystal of ice that can come in many different shapes. A snowflake is the term we are more familiar with — it can refer to one snow crystal or a number of snow crystals all grouped together. Some snowflakes can contain a clump of hundreds of tiny snow crystals.

Why is snow white?

When we observe the colour of an object we are actually seeing the colour of light that is reflected back off that object. Green grass, for example, absorbed all light expect green light, which it reflects; so the grass appears green. The crystals of snow reflect all the colours of visible light, which, combined together,appear white.

Is it true that no two snowflakes are the same?

The simple answer to this is yes. Although we cannot vouch for every single snowflake that ever existed, science does suggest that the exact shape and structure of each natural snow crystal is unique meaning that no two snowflakes are identical.

This is not so surprising if you consider the many different factors (temperature, moisture, time, etc) that are involved in creating each snowflake.

However, it can sometimes be difficult to tell two snow crystals apart under a microscope, especially those of more simple structure.

Designer snowflakes have been created under identical laboratory conditions that appear similar when viewed under a microscope, but even these are not identical at the molecular level. Snowflakes that appear identical can still have different forms (isotopes) of hydrogen atoms within their structure.

Can it ever be too cold to snow?

In theory, no, snow can be made at very low temperatures, once there is enough water vapour around to make snow and enough circulating air to transport it. However, as warmer air can hold more water vapour we tend to get most snowfall at temperatures around freezing point or a bit below.

Will we have a white Christmas this year?

According to the Irish Meteorological service’s current forecast, it would appear that conditions around Christmas will be quite mild this year. Whether white or not, I’d like to wish you all a very Merry Christmas and a Happy New Year.

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