Wednesday, 15 February 2012

Extreme Weather (Snow) - 4th/5th February 2012

This blog post is for Year 12 Geography students at Bethany School who are studying Extreme Weather for Edexcel AS level Geography.   It explains the meteorological processes which produced the only significant snowfall of winter 2011/12 and suggests some practical ways to create an ‘extreme weather log book’.  The snowfall has a relatively small social and economic impact and was of a much lower magnitude than the snows of December 2010.  On average most parts of Kent have10-20 days per year when snow or sleet falls. This suggests that this event was not extreme weather at all, but was instead ‘normal’ winter weather. Click here to download a copy of this post. 
Snow being cleared at Ely Station, Cambridgeshire, Sunday 5th February
Snowy Fieldwork  
The best way to observe weather is to look out of the window or go outside.  I had been tracking the progress of the band of snow and knew it would arrive at about 8pm.  Soon after 8pm I noticed the first flakes of snow.  By 11pm there was a good 2cm of snow and by 6.30am there was 10cm.  I used a tape measure to gauge the depth of snow.  I was also monitoring the temperature with my weather station.  Temperatures had been around 0.0°C but as the weather front arrived, temperatures rose to 0.9°C.  The temperature remained above freezing for the rest of the day on Sunday 5th Feb, meaning that the snow was slowly melting.  This gradual thaw continued all week. 

BBC Video Forecasts
The BBC broadcasts weather video forecasts in association with the Met Office; these can be viewed online at  The video clip below describes how this snowfall was the result of a battleground between two contrasting air masses.  Air masses are large scale ‘masses’ of air with similar temperature and humidity.  The screenshot below shows what was happening in the atmosphere and the map below clearly shows where these two air masses were located.  Click here to view the actual weather forecast. 

Why did we get snow?
The diagram below shows a cross section across a warm front. This warm front was the dividing line between a very cold, dry Polar Continental air mass and a slightly milder but more humid Polar Maritime air mass.   This more humid air was forced to rise over the colder air.  As it did so the air cooled, water vapour condensed into ice crystals which collided forming snowflakes.   Once these snowflakes were heavy enough they fell as snow. In many places 10cm of snow fell but in some places 15cm fell.
Cross-section across a warm front - Notice where the snow falls

Met Office Rain Radar maps
The Met Office produces rain radar maps which also display falling snow.  These maps, updated every 30 minutes, are a great way of watching extreme weather. The slides below clearly show the Easterly progression of the band of snow as it crossed Eastern England.

Surface Pressure Charts
The Met Office produces surface pressure maps, also referred to as synoptic charts. The map below shows the ‘synoptic situation’, or big picture of the weather.  You can clearly see a weather front draped across England producing heavy falls of snow to the East. Can you link this to the warm front cross-section above?

Twitter is also a useful way to monitor extreme weather.  Twitter users included the hashtag #uksnow in their tweets which were combined into a mashup map showing the distribution of the snow (shown below).

Severe Weather Warnings
The Met Office has responsibility for warning the public about extreme weather and released 'Severe Weather Warnings'.   These are a good example of a 'non-structural' responses.  The screenshots below show the actual severe weather warning issued at noon, 8 hours before the snow arrived.  The warning was very accurate and was broadcast widely.