How do we measure snow?

6 11 2012

Here at the Met Office, we’re already being asked if it’s going to be a White Christmas and there’s always a lot of interest in snow.

It’s too early to give forecasts that far ahead, forecasting snow is – after all – a challenge which requires detailed information. While forecasting snow is one challenge, measuring it when it’s on the ground poses another.

There are several reasons for this. First of all snow is subject to the vagaries of the wind and can be blown into deep drifts, leaving bald patches of earth nearby.

Snow also melts, refreezes, and new snow can fall on top. This makes it difficult to discern how much snow has fallen at different times or on different days.

Another tricky aspect of measuring snow is that it often falls on high ground, away from where the majority of the UK population live – and also away from our observation sites.

Snow often falls on high ground but is less common closer to sea-level.

So what do we do to measure this problematic precipitation? In days gone by a manual observer (ie a human being) would go out with a ruler and measure snow on a flat surface.

But this is time consuming, limits observations (as there were relatively few manual observers) and, apparently, became a tricky operation when snow got particularly deep!

So modern technology has given us automated snow sensors which measure snow depth with a laser signal. A piece of artificial turf is the preferred surface below the laser, as it doesn’t grow and therefore doesn’t complicate readings as grass might.

It’s not all that simple though, as even artificial turf can expand and contract according to temperature, as can the soil below it (which can push the artificial turf up or down). Moles can also cause the same problem! To tackle this, our network is under continual review and calibration to make it as accurate as possible.

These fairly technical pieces of kit can’t be placed everywhere, and until last year there were less than 50 spread out across the UK.

Snow depth sensor

This year we have extended our network with 21 new snow sensors, bringing the total up to 68 – you can see the full network on the map below.

Map showing snow sensor network in 2012

This means we can get snow readings from a wider range of locations, which can help our forecasting and is useful for building records and statistics about UK climate.

It’s worth pointing out that while these additions to our observation network are a valuable step forward, the snow sensor network is still relatively sparse in comparison to our UK land weather observation network, which has 463 stations.

Fortunately this is supplemented by observations supplied to the Weather Observations Website (WOW), where anyone can give an up to date measurement of snow or even upload a picture of how much snow they have.

The very nature of our weather here in the UK means that it’s not possible to give precise information for every location in the country, but our network is being continually improved to provide the most detailed, accurate and up-to-date information available.

You can read more about snow and snow forecasts on our dedicated snow pages.


Actions

Information

7 responses

6 11 2012
Viv

Reminds me of an old Flanders and Swan piece about Stonehenge: “It’s a CALENDER? Bit big. Never fit that on a desk. ….So you can tell if it’s summer? Is it summer? You don’t know? I’ll help you knock the snow of the stones and then we can see…”
Never thought it was so complex, now I know differently!

6 11 2012
Graham Davis

In my first winter observing for the Met Office, a snow-depth sensor such as pictured above, would have recorded a snow depth of about 150cm – assuming it wasn’t buried in snow – whereas a series of samples measured by a human being probably came up with value nearer 15cm. That winter was 1962-3 and the screen enclosure was buried in a five-foot snowdrift – the deepest on the whole airfield.

The wire fence in the photo and other paraphernalia look ideal for creating drifts. The only way to get an accurate snow depth at the moment is via human beings with rulers.

7 11 2012
Dave Britton

Graham, all observing techniques have both strengths and weaknesses of course. No observation is ever used in isolation and is combined with other information both from the site in question and locations near by. This provides us with more information than would be avialable if we were only to use observations taken manually. Once the data is quality controlled it provides greater geograpahical representation than could be achieved otherwise. Of course we will never be able to observe the whole country universally as this is both impractical and not cost effective, but our observations scientists work hard to make sure the UK is represented as effectively as is possible.

6 11 2012
Vinny Burgoo

Pshaw(bury)! When are you going to cover South Shropshire and Mid-Wales properly? Shawbury is on a plain and its weather is not at all representative of the region.

7 11 2012
Dave Britton

We will never be able to observe the whole country universally, but our observations scientists work hard to make sure the UK is represented as effectively as is possible. However our Weather Observations Website allows you to provide more observations, providing additional information that can be shared among the meteorological community and could be used by the Met Office.

7 11 2012
3leftfeet

Interesting to note that the measuring stations are most densely clustered in the areas where snow is least likely, and most sparsely available in the areas where snow is most likely. The hilly parts of Wales and most of Scotland are very poorly served.

7 11 2012
darrog

On your map spot the mistake re Warcop range and Shap.

Yours

Darren Rogers

darrog@btinternet.com

The home of http://www.mauldsmeaburnweather.co.uk

“Wind in the northwest on St. Martins day, there’s a severe winter on the way – Wind in the southwest on St. Martins day, there it will remain ’till February and a mild winter will be had.”

Leave a Reply

Please log in using one of these methods to post your comment:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s




Follow

Get every new post delivered to your Inbox.

Join 6,052 other followers

%d bloggers like this: