Will it be a white christmas

Are you Dreaming of a White Christmas?

By Margaret Franklin

While the notion of ‘ A White Christmas’ or ‘Walking in a Winter Wonderland’ may sound romantic, for most of us in Ireland, snow is rather an inconvenience, if not a downright hazard. This is of course, because we are unaccustomed to snow. An ocean current from the Gulf of Mexico keeps our little island relatively warm, making snow in December something of a rarity. A few years ago, we didn’t get snow until January and then it was more prolonged than we had been used to in recent years. More recently, we got our first snowfalls before the end of November, which is most unusual and unexpected, so the country practically ground to a halt for two weeks. This was due to a pincer movement of icy Arctic air coming directly from the north, while at the same time, the country was blasted by cold winds from Siberia to the east. There were worries that supplies of salt to treat the roads were running low, so that local authorities had to mix in sand to stretch it out. But then, the wind direction changed and the snow suddenly melted away. Things got back to normal. Then, just when we thought it was all over, the snow returned. To add to our problems, the ships bringing fresh supplies of salt from Egypt and Turkey were delayed by storms in the Mediterranian Sea and there were worries that they would not reach us before the supplies ran out.

So what are the odds of snow on Christmas Day this year? The weather is rather mild and unsettled at the moment, so it looks unlikely. Bookmakers’ odds are changing by the day.

The weather forecasts are only accurate for about three days ahead and there is a greater uncertainty in the five-day forecast. If the Polar Front shifted to the south of Ireland within the next few days, theoretically it is a possibility, but it is highly improbable. In order to qualify as a ‘White Christmas’, snow must be officially recorded between 9.00 a.m. and 11.59 p.m. on December 25th. On this basis, Ireland’s last official White Christmas was in 2004. Bookmakers offer separate odds for Belfast, Dublin and Cork airports, with snow at Belfast being the most likely, as it is the most northerly, while snow in Cork is least likely.

In order for snow to form, we need the right conditions of pressure, temperature and humidity in the atmosphere. We know that pure water freezes at zero degrees on the Celsius scale at normal atmospheric pressure. This is an arbitrary temperature scale, which defined zero degrees as the temperature at which pure liquid water and solid ice are in equilibrium at a pressure of one atmosphere. Similarly, the temperature at which pure water boils at atmospheric pressure was defined as 100 degrees on the Celsius scale.

The Fahrenheit scale, still in use in America, seems even more arbitrary, with the freezing point of water defined as 32 degrees and its boiling point 212 degrees. But at least it recognises that we might need to measure temperatures below the freezing point of water and avoids the use of negative numbers for most practical everyday purposes. But the lowest temperature possible, absolute zero, is actually 273 degrees below the freezing point of water on the Celsius scale. So this means that on the absolute temperature scale, which is known as the Kelvin scale, pure water freezes at 273 degrees, provided the pressure is at normal atmospheric pressure.

Pressure variations will change the temperature at which pure water freezes. However, naturally occurring water is never pure, it contains dissolved solids and this actually lowers the freezing point. This is the reason for using salt on the roads, as it allows water to remain in the liquid state at temperatures lower than zero degrees Celsius (or 273 K).

During the day, the sun melts some of the snow and the pressure caused by traffic on the roads assists this process. Salt dissolves in the melt and provided the temperature does not drop too far below freezing, will prevent the formation of ice. Sand and grit provide additional friction to prevent vehicles from skidding.

Were it not for the fact that water expands on freezing, increasing pressure would not have the effect of lowering the freezing point. The reason why water expands on freezing is due to the particular shape of water molecules and the type of forces holding them together in ice and snow. As most people know, the chemical formula for water is H2O. This means that its molecules contain one atom of oxygen chemically bonded to two hydrogen atoms. Water molecules are V-shaped, with the oxygen atom at the apex of the V and a hydrogen atom at each end. Because of this shape, water is a polar molecule.

This means that it behaves as the electrical equivalent of a tiny magnet. The oxygen atom at the point of the V is slightly negatively charged compared to the hydrogen atoms. In water vapour, each water molecule is isolated from its neighbours, but when water condenses to form a liquid, the molecules are brought closer together, so that the

hydrogen atoms from some of the molecules are attracted to oxygen atoms in neighbouring water molecules. Thus, water forms little clusters of several water molecules, held together by these hydrogen bridges, which as known as ‘hydrogen bonds’. As the temperature drops, molecules move more slowly, so that the molecules can come even closer together and more hydrogen bonds are formed. As the molecules come closer together, the volume of a given mass of water decreases, so that it contracts until at 4 degrees Celsius, water reaches its maximum density. This is due to extensive hydrogen bond formation. But as the temperature drops lower still, an interesting thing happens: water begins to expand and this expansion is even more marked when it freezes to form solid ice.

The structure of ice is such that all of the water molecules in an ice crystal are strongly bonded together by hydrogen bonds in a rigid three-dimensional framework. But the hydrogen bonds have a definite length and therefore hold the water molecules somewhat apart, so that they are not as closely packed as in liquid water above 4 degrees Celsius.

This makes ice less dense than water, so that it floats. When pressure is exerted on ice, this puts a stress on it, which causes some of the ice to melt, allowing the water molecules to crowd closer together.

This is helpful to skaters on an ice rink. The blades of the skates exert pressure on the ice, causing some of it to melt. There is thus a thin film of liquid water between the ice and the skates, providing lubrication. As the skater passes on, the pressure is released and the ice freezes again over the tracks. The same phenomenon allows us to make snowballs.

When we gather up a handful of snow and press it between our hands, the pressure causes some of the snow to melt. It is not just the heat of our hands that does this, as it works even when we are wearing padded gloves. Then, when we release the pressure the water freezes again, acting as a sort of cement to keep the snowball together. Water is quite an extraordinary substance. But it is so ubiquitous that we take it for granted.

But to get back to the question of whether we will have a White Christmas. Well, I am not going to bet on it, as the position is still so uncertain. With a change of wind direction, the weather can change overnight. But whatever the weather, let us hope that everyone who is travelling will be able to get home safely in time for Christmas.