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.

WOW – a new weather observations website for everyone

10 02 2011

The Times has today run a feature – ‘Experts make way for an outbreak of enthusiasm from citizen forecasters’ – about a new project by the Met Office to set up a social network for weather observations.

The weather is a subject that most of us talk about at some point during the day. In fact, many members of the public have an active interest in the weather and enjoy taking their own weather observations.

From spring 2011, the Met Office in partnership with the Royal Meteorological Society and supported by the Department of Education will launch a new web site for weather observers across the UK.

The ‘Weather Observations Website’ – WOW reflects recent advances in technology and how weather observations can be made. At the same time, the growing world of social networking online makes it relatively easy for anyone to get involved and share their weather observations.

By hosting the new website, the Met Office is helping to co-ordinate the growth of the weather observing community in the UK, by asking anyone to submit the observations they are taking. This can be done through using all levels of equipment, so there are no cost restrictions.

On behalf of the UK the Met Office operates a network of over 200 weather observing stations, however by there very nature they can not cover every single corner of the UK.  This project will enable the public to fill in some of those corners, providing valuable information that can be shared with other enthusiasts through the website.

Although the observations submitted to WOW will not be used for input into Met Office weather forecast models, the purpose of the website is to provide a platform for the sharing of current weather observations. This will be regardless of where they come from, what detail of information or the frequency of reports.

It is hoped that users will then use the data to explore the British weather, looking at how weather varies from place to place, moves across the country and how height or your location can make a difference to the weather you see. Over time WOW will build up an historical record of weather observations for sites across the UK.

Involvement can include submitting ad-hoc information such as ‘it is snowing here’, or uploading a photograph of the weather you have observed. It also means you can submit routinely taken data from manned or automatic weather stations of high standards.

It is hoped that the website will encourage further growth in the UK’s amateur weather observing community, and help educate children about the weather. It is hoped that this will become the UK’s largest source of weather observations.

Observations sent to WOW will be monitored in two ways. Firstly, a series of ‘quality flag’ thresholds will be established across the UK to prevent obvious extremes from being entered. For example, this means that a maximum temperature of 25 Celsius cannot be entered during the winter months. In fact, quality data will be used to monitor very localised weather variations as and when they happen.  Secondly, the web site will be self-regulatory with other users being encouraged to query and ultimately remove false information.

WOW is an exciting initiative from the Met Office and the Royal Meteorological Society, joining up the thousands of observations that are being taken across the UK, enabling them to be shared in real time and joined with the UK’s national observing network.

You can find out more information and register your interest at


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