The trouble with automatic weather stations in cold weather

Figure 1 – Courtesy of the Met Office and Weather for Schools

Automatic weather stations now make up nearly all the network of observing sites in the UK, and they all have an inerrant problem when it comes to reporting precipitation totals in very cold weather, and that is they can’t!

Precipitation from AWS

Have a look at the 12 hourly totals [18-06] from 06 UTC this morning from across the country (fig 2) and you’ll see that almost all stations are reporting no more than a ‘trace’ of precipitation. These accumulations are obviously incorrect because most of the precipitation that’s fallen in the form of snow will still be stuck, frozen in the funnel of the gauge. Some AWS sites do have snow depth sensors, but these are not the answer, and can be easily fooled as we have seen this winter, by drifting when the wind is strong enough to lift any snow that has fallen.

Figure 2
Weather Radar

Here’s are my estimates from the radar network of the precipitation that has fallen since 06 UTC on Saturday morning (fig 3). If this is anything to go by then, the deepest of the snow looks to have fallen across Norfolk and Suffolk in some kind of convergence zone, with precipitation totals as high as 16-24 mm. I estimate that there was around 6.9 mm from around High Wycombe, which I realise is not enough to account for the 27 cm of snow there at the moment. In my defence I offer the “spoking” that’s evident from the Chenies radar. Chenies lies not so many miles E’NE of High Wycombe, and I did speculate last year that the “spoking” that occurs there at times is caused by the radar being blocked by some of the large trees that surround the site.

Figure 3
One solution

I don’t know enough about why this is such a problem in the 21st century, you would have thought that it would be easy enough to detect when temperatures are close or below freezing and turn on heating elements inside the gauge to melt any snow that it catches. For all I know this might be already happening, because the SYNOP format is not how AWS report their readings these days, and much more detailed one minute or even more frequent observations from all the sensors are being taken and transmitted back to Exeter, which may include the water equivalent of any snow that has fallen, who knows…