How to tell if it will snow or not!


Satellite image from the University of Dundee, January 2010

Usually I end up writing my ‘In Defence of Meteorologists’ posts after negative incidents – namely, when people complain about them failing to have performed their duty as magical future-telling wizards with 100% accuracy…

But this time I’m writing it following a happy incident. It’s snowed in the UK, yes, snowed, in November, and a lot of people are pretty pleased about this, because who doesn’t love snow? However, the inevitable response is actually pretty dismissive of the entire field of meteorology as a result, and it goes something like this: ‘I can’t believe they *actually* got something right for once!’

It seems that even when a good thing happens, meteorologists are still sidelined. What would it take for people to treat the utterly impressive work they do with any kind of respect? I see others rise to defend other areas of public service, such as the NHS, when somebody has had a bad experience and insists on tarring the whole service with that brush, but I don’t see as much of this in meteorology, and it’s probably because people don’t have as much of a personal connection with the field, nor enough knowledge. So, my intent is to change that.

Weather forecasting systems are the supercomputers of our time – they deal with more variables than anything else, with greater levels of uncertainty even than the most tricky field of insurance, and they are awe-inspiring bits of tech. I mean, check out the Cray High-Performance Supercomputer plans that are taking up a huge portion of the Met Office’s investments at the moment.


Here’s an existing Cray supercomputer in the ECMWF offices

The fact that in this day and age we have any kind of way to predict the future at all is stunning. And let me make a distinction – it’s not actually ‘prediction’ – it is ‘forecasting based on probabilities’, because actually knowing what’s going to happen in the future is impossible.

Why is it impossible? You might think that, provided we had all the correct equations ready and all the possible variables of the earth-atmosphere system input into a machine, that we would be able to solve for all future configurations of that data?

Well, no. It’s not that simple. See, especially when it comes to fluid dynamics, there are some non-linear equations, such as the Navier-Stokes equation, with are unsolved in certain areas and this is a pure mathematics problem. Despite the issues with turbulence and flow interactions, however, the weather forecasting services still manage to make truly impressive efforts. And in the future, who knows? In fifty years I imagine we will have advanced even further, and I look forward to returning to this article when that happens, to compare how much better things have gotten. Endless possibilities!

There is much misconception about weather forecasting because of the sheer number of illegitimate reporting being done. The Daily Express is well noted for this, for instance, and other news outlets have been known to issue scaremongering reports rather than reasoned and interesting reports on the actual science being done, which is a shame, because it means that when the Met Office releases something with scientific legitimacy, it is often overload for the non-scientific layperson, who ends up classing it ALL as bogus.

Here’s some starter info from the Met Office on how snow forecasting works. But in addition, I would urge everyone to learn to read synoptic charts (the lovely surface pressure charts with all the cool lines and stuff going on over them). If you do this, it would be well worth your while and allow you to have a clearer idea of what weather forecasters are talking about. Synoptic charts are beautiful, and once you understand thickness lines, you yourself can more accurately understand whether reports of snow are likely to result in actual snow or not.

Met Office synoptic chart for Sunday 22nd November

Look for the dotted lines. They will be either blue or red (note – these are not the lines with the blue spikes or the red semicircles – these are much smaller, thinner dotted lines that may not be apparent on first glance). These lines don’t show up on every graph. If and when you find them. then look for the tiny number reading around this. These numbers tell you what the air pressure is at the top of the air column. In general if it’s above 528millibars, it means you’re probably on a warm patch of air (because warm air rises). If it’s below, you’re likely to get snow (and remember, likely does not equal inevitable). So, based on the above chart:


This means it’s likely that the part of the world that I live in is likely to experience the snow that those in the UK are experiencing today (since the air mass is moving from west to east). And remember too that thickness lines are one thing, but to get snow you also have to have the right temperature range and the right amount of moist polar sea air coming in.

Thickness lines are a complicated subject. But you can gain a lot just from looking at the areas of highs and lows. In general, areas of Low pressure bring cyclonic weather, where air converges and rises. This is responsible for the tropical thunderstorms around the Equator, and also for the cold and wet batterings the UK gets from the North Atlantic. Areas of High pressure bring cloudless summer weather to the UK, but they also can bring ‘Siberian’ weather in the winter. Cold, dry and icy. Almost the whole inner area of Antarctica, for example, is a constant area of high pressure, and it is the driest place on earth (yes, even drier than the Sahara – it is officially classed as a desert).

So just knowing a.) what season it is and b.) what kind of pressure zone you have overhead can go a long way in aiding you with knowing what sort of weather to expect. Then there’s knowing where the prevailing weather fronts and air masses are currently sweeping in from. Knowing this gives you a real sense of connecting with the complicated and chaotic earth-atmosphere system once you get into it!

On a related note, the UK has a very interesting convergent trough approaching this noon (check out the flecked black line over Cornwall and South Wales – this is cool because usually the circular centres of lows are convergent, so having a convergent line is extra awesome):


I hope more people get interested in the cool charts available (for free!) from reputable weather agencies. NOAA – the American Oceans and Atmosphere Administration – is also excellent for this, as is the ECMWF – the European Centre for Medium-Range Weather Forecasting, based in Reading.

And if you’re not already using the free Met Office app, you really should have it on your smartphone at all times. It’s great – not only does it give you UK weather forecasts and world forecasts, it also provides synoptic charts (aka surface pressure charts) and satellite data of cloud coverage, temperature readings and rain cover.

Another thing that complicates our weather research is government policy. The Met Office currently gets £117 million to run its service every year. It’s peanuts compared to many other services, such as the NHS (£115 billion), and even the Church of England (£170 million). That’s right. The government spends more money each year on the upkeep of old Church of England buildings than it does on the Public Weather Service… And to make matters worse, because of budget cuts, the BBC has dropped the Met Office’s contract starting next year. The mere fact that they’re opting for an even cheaper service signals the importance of ‘money savings’ over rigorous science, and that is a real shame.

Imagine if, instead of wasting energy dissing meteorologists, people with platforms spent that energy exploring the actual state of the tech, the actual state of the field of weather forecasting? We would surely all become more knowledgeable, more aware, and maybe even lead to more improvements in the field! That sounds like a much better use of time to me.

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