Freezing Rain v Snow 9th Feb 12

10 Feb

Thursday 9th of February produced quite a varied amount of precipitation types from good’ol rain across Scotland and Ireland to a wintry mix of freezing rain, sleet and snow across parts of England and Wales.  One distinct variation was the difference in precipitation type across Northern England and generally over relatively short distances.

North-west England in particular, including Lancashire and Cumbria for example picked up a large amount of freezing rain which in some places lasted all day.  Yet just over the Pennines and the afternoon produced quite a different setup with snow falling across many parts of Yorkshire, so why was there such a difference and why didn’t North-west England get snow?…

…the answer lies up in the atmosphere.  Before I advanced further on this, for those that have not really heard of freezing rain are unsure how it forms and develops then I recommend taking a look at this video from the BBC’s Nick Miller which explains what freezing rain is…

http://www.bbc.co.uk/weather/feeds/16956208

In association with this explanation the following is what is called an atmospheric sounding.  It allows meteorologists to view what is ‘going on’ within the upper atmosphere.  Now what is important here is how the temperature actually rises with height.  Note that at the surface level the temperature (red line) is at or even below freezing, yet if you following that line up the temperature rises (the line progresses to the right).  Precipitation was thus falling through this warmer layer, melting but then not having enough time to re-freeze before it reached surface levels which were at or below freezing and hence causing the freezing rain across North-west England where this atmospheric sounding is from yesterday afternoon.

Now across Yorkshire the atmosphere was different.  This rise in temperature wasn’t evident or not to the extent that was present across Lancashire and Cumbria for example.  As a result when yesterday’s frontal systems reached Yorkshire the precipitation through the day was primarily of snow, particularly during the afternoon simply because of the lack of this rise in temperature.  The precipitation was able to fall through the atmosphere without melting and thus at surface levels was snow.

The above atmospheric sounding is a text-book example of a freezing rain ‘environment’, something which doesn’t happen often in the UK, particularly on the scale of yesterday and lasting for a large portion of the day as well. So if you ever see freezing rain being mentioned again within the forecast think of surface temperatures at or below 0C, but as one rises up through the atmosphere the temperature actually rises.

Regards to all,

M.

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