Is it going to snow?!
By James Dale-Adcock
05 Dec
James Dale-Adcock is the author of our Geography for Common Entrance textbooks and Academic Deputy Head at Cranleigh Prepatory School. 

“Is it going to be a white Christmas?”
“Are we going to get snowed in so we can’t get to school!?”
Yes, it is the time of year when teachers have to pull the blinds down to stop the class hurling themselves towards the classroom windows any time a piece of Clingfilm is blown through the air in the hope that it might be a snowflake, and parents are bombard with the inevitable questions about whether it will snow. Your answer should really be “you tell me....”.
The study of weather and climate (meteorology) is an important area of the Common Entrance Geography syllabus and covered by all pupils at Key Stage 3. It is a topic which is, of course, very relevant to our daily lives and is best understood when layered, beginning with an understanding of three areas; the difference between ‘weather’ and ‘climate’, the water cycle and air masses. We will focus on the latter: an air mass is a large body of air stretching hundreds of miles in length and width, all of which has similar moisture and temperature. The air mass gets its moisture and temperature from its source regions which are usually flat areas of water or land. Air masses are defined by their origin and the course they travel.  
Air masses influencing Britain
How does this helps us know if it is going to snow?
Look at the air masses diagram. If the wind is blowing from the north during the winter months it will be bringing cold air from the Arctic. The Artic maritime air masses travel over the far Northern Atlantic picking up some moisture but retaining a very cold temperature. This moisture may be precipitated as snow over highland in Scotland, northern and central Britain. Another air mass which may bring snow in winter may be the Polar Continental air mass. This originates in Siberia and travels over frozen Scandinavia before picking up moisture in the North Sea and depositing snow along the eastern coast of Britain, typically in Lincolnshire and East Anglia.
Of course it is not as straight forward as that...
Higher atmospheric winds called the Jet Stream blow from west to east across the Northern Hemisphere which pull the Polar Maritime and Tropical Maritime air masses beneath them towards Britain; these prevailing winds are why we receive mostly mild and wet weather. However, the Jet Stream can also move weather systems (areas of high and low pressure). If an area of high pressure is moved to the north west of Britain in the winter, it forces winds around the top of it which causes air to plunge down from the Arctic and Scandinavia, resulting in snowfall.
It is useful to watch the weather forecast with your children on the television or internet and listen carefully to what the presenter is saying. You will hear them talk about where the wind is coming from, high or low pressure systems and possibly movement of the jet stream, in relation to what sort of weather we are experiencing. If they forecast snow, see if you children can understand the reasons why it is being predicted based on the information in this blog.
So, is it going to snow? Use the activity sheet to record and predict snow fall over your Christmas holiday and see how successful you are at analysing the weather conditions based on your meteorological knowledge. Let’s hope Santa is good at Geography!!
Core terminology

  • Meteorology – the study of the processes that cause particular weather conditions

  • Air mass - a large body of air stretching hundreds of miles in length and width, all of which has similar moisture and temperature.

  • Prevailing wind - main wind direction (from the south-west in Britain)

  • Jet stream - higher level atmospheric winds which blow from west to east across the northern hemisphere
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