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Dreaming of a White Christmas              

                                     December 2010

Dreaming of a White Christmas

snowmanIt’s that time of year again, and there will be many songs and carols depicting that traditional white Christmas. Snow at Christmas is a very traditional part of our culture – Christmas cards, A Christmas Carol by Dickens – so many aspects rely on snow to create mood and atmosphere. Connecting Christmas with snow is thought to have started in the 17th and 18th centuries when Britain had a minor “ice age” – until 1813/14 the River Thames used to freeze over regularly and hard enough to hold “frost fairs” on the surface.

Today we are more likely to have snow in January or February as the cold deepens although in 2004 Britain officially had a white Christmas when snow was widespread especially across Northern Ireland.

Without doubt snow makes landscapes look pretty; winter trees with each of their bare branches delicately balancing a couple of inches of soft white snow are a photographer’s dream. Snow is formed from the same basis as rain. In a simple definition, when warm, wet air rises away from the earth’s surface, it cools, and the air can’t carry so much moisture. As the air cools the water it is holding condenses to form clouds. When these droplets of water that form the cloud get large and heavy enough to counterbalance the upward pressure of the warm air rising, they will float around or begin to fall back to earth as rain.

If the cloud has risen high enough, as the air cools further away from the earth the water can turn into ice crystals. Interestingly, each ice crystal develops its own unique shape depending on the surrounding temperature and the amount of water vapour in the air. If it is below freezing and there is a lot of water vapour, the crystal will grow evenly with six spaced branches. These can develop into prisms, plates or stars, with very definite individual shapes and patterns. As the air and the droplets it contains move around, more and more water vapour collides and then sticks onto these branches, making the ice crystal grow bigger and bigger and of course heavier. Eventually, this crystal is too big to “float” in the air and will leave the cloud and begin a downward descent back to the earth. It will continue to pick up more water vapour as it descends.

Different things happen depending on the temperature of the air the crystal enters as it falls to the earth. Nearer the earth, the air can vary in pockets of different “weather” so there can be layers of hotter and colder air as you go up or down. Generally, if the ice crystal enters warmer air as it reaches the earth, it will begin to melt and the crystal will begin to soften from the outside inwards, turning it into snow. The soft edge means as it collides with other melting crystals it may stick together to form the large lovely snowflakes that are the favourite of Christmas cards. Sometimes the crystals will begin to melt and then refreeze as they hit colder temperatures again nearer the Earth’s surface, and then this turns it into sleet.

Ideally, for snow to reach the ground’s surface in pristine condition, the air temperature must be no more than 2°C. The heaviest falls of laying snow in the UK generally occur when the air temperature is between zero and 2°C. Interestingly, snow contains less water than rainfall; 30cm of fresh snow has about the same amount of water as 25mm of rainfall. Nevertheless in quantities snow can become very heavy and has been known to bring down branches and even roofs.

Luckily here in the UK we rarely if ever experience freezing rain and ice storms. These are incredibly damaging and can cause serious damage to power lines, trees and other structures. Freezing rain occurs when the earth and objects on the earth are below 0° Celsius. Falling snow super cools the snowflakes so they are just on freezing point; when they hit the colder items on the ground, they instantly turn to a film of ice which can coat everything including cars, anything that is outside at a temperature of less than 0° Celsius. When everything including road surfaces is covered in a sheet of ice, you can imagine how dangerous it is.

But heavy snowfalls themselves can cause major disruptions including blocking roads and rail lines, and while we may be dreaming of a White Christmas, in reality if the country becomes heavily snowbound two or three days before the festive season, it would also cause a lot of problems as well as looking pretty!

 

 


 

Nutricentre Discount for laterlife visitors If in any doubt about any of the information covered in health and nutrition related articles and it's relevance for you, consult your GP.

 

 



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