I’m going to resist the temptation to try and place the snow of the first few days of February in some kind of long term context. Much of the world knows that London and many other parts of the country have had either heavy or sudden snowfalls over the last few days. It has been widely quoted that London had the heaviest snowfall for 18 years, since February 1991, to be exact.

But to rank the event we have to know how deep the snow was. This should be an easy operation – you stick the ruler into the snow in three representative locations (open ground, away from obstructions and ignoring any drifts), take an average, add a bit for the end of the ruler that is not calibrated and you have your snow depth.

For decades, central London’s snow depth was measured at the St. James’s Park climatological station. The weather station is still there but it is now just an automatic station, sending temperature and rainfall data into the national climatological archive.

Machines cannot measure snow depth – it is one of those meteorological elements that has fallen by the wayside in recent years, along with such things as days of thunder heard, days with sleet or snow falling and days with snow lying. These are all variables that humans can judge but machines cannot. Since 1991 many weather stations have either closed or become automated. Back then, central London had 20cm of level snow. Was there an official measurement earlier this week? No. Lots of people can make unofficial measurements in all sorts of unofficial places, but for consistency we should be measuring the snow depth in the same place in the same way as in the past. This is impossible now – the staff have gone away. There’s a good network of amateur weather enthusiasts and they produce a very valuable monthly publication called the Climatological Observers Link Bulletin (http://www.met.rdg.ac.uk/~brugge/col.html), edited by Roger Brugge of the Meteorology Department of Reading University. This will, no doubt, give us a lot of information about snow depths and much else about the cold winter.  So far, we know that 39cm of level snow was measured close to Epsom racecourse and 33cm nearby at Leatherhead. However, for much of London depths of around 15 to 20cm were more general, placing the event a little behind that of February 1991. However, for the Epsom and Leatherhead area it seems that it may have been the deepest snow since December 1962.

Our ability to place this event in an accurate long term context is challenged by the termination of manual (human) weather observations. This ill-serves our understanding of climate change.

Julian Mayes

4th February 2009.

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