One of the trickiest challenges facing an enthusiastic young meteorologist is forecasting snow. It's also one of the most crucial, because of the disruption it causes – a snow forecast that doesn't come through means money wasted on prevention, but an unexpected snow shower often means travel chaos.
So how do you get the white stuff right? Here's WIRED's guide to forecasting snow at home, without a supercomputer to help you.
First off, remember that due to the limitations of computing power and the effects of chaos theory, it's only really possible to forecast a week at most in advance in any kind of detail. Any errors (and there are always errors) in your recording of the initial conditions, or compromises taken to make sure that your weather model actually runs in a reasonable timeframe, get increasingly magnified as the forecast goes forward, until the predictions are incredibly detached from reality.
Even at the cutting edge of numerical weather prediction, like the ensemble forecasting approach taken by researchers at the European Centre for Medium-range Weather Forecasting (ECMWF) you can't get much beyond ten days with any real accuracy. As a result, you should look very sceptically on any predictions from commercial forecasting companies of the chances of a white Christmas. Normally, it's just a desperate plea for press attention.
If you want to know what the chances are for the coming weekend, though, then you've got more chance of getting it right. Your first stop should be the synoptic charts published by the Met Office, usefully compiled and published by a Swiss site called Westwind. These charts show the surface pressure (with solid lines known as isobars), any centres of high and low pressure, and any fronts or organised bands of showers (see the Met Offices' site for a good key to different symbols). Look closely, however, and you'll also see dotted lines, often at right angles to the isobars.
These dotted lines are known as "thickness" lines, and represent the distance in decametres between the area in the atmosphere where the pressure is 1000hPa (usually close to the surface) and where it's 500 hPa. This interval is where precipitation-forming clouds reside, and the warmer the air in that gap, the more it'll expand, and the larger that number will be. For snow, you want cold air, so a good rule of thumb is to look below 528 decametres. If the 528 line is below where you live, and there's precipitation coming in, there's a good chance it'll be snow.
However, that's only a rough guide. Sometimes it'll snow in the 530s, or rain or sleet as far down as the mid-teens. If you want to be more accurate, you need to look at a few more variables.
Firstly, snowclouds actually reside in the very bottom part of the atmosphere, between 1000hPa and 850hPa, so knowing the thickness between those two levels can be more illuminating. You can also look at how cold it is at 850hPa. It usually needs to be at least -5 degrees celsius for snow.
The wet-bulb temperature is also important. The wet-bulb temperature is the lowest temperature that can be reached by the evaporation of water only. It's the temperature that you feel when your skin is wet and is exposed to moving air. It gives a good indication of how much moisture there is in the air, which is necessary for the growth of snowflakes. This really needs to be below freezing where clouds are sitting, at 850hPa, for snow to form.
But it's no good for snow to fall if it's just going to melt as soon as it hits the ground. You also need freezing temperatures at the surface if you want the snow to stick about long enough for your kids (or you) to build that snowman. That can often be a problem, as the cloud cover associated with snowfall also traps heat at the surface. As a result, it's a good idea to look at forecasted surface temperatures before buying a scarf for Mr Frosty.
Not just for the snow-day in question, either. You want it to stay cold for a few days after the snowfall if things are to stay deep and crisp and even for long.
Finally, most of the above refers to extratropical storms, and frontal snowfall. There's another mechanism for snow production that can deliver huge amounts of snow in a relatively short space of time. It's called lake-effect snow, which refers to the Great Lakes of North America, but in the UK it more often applies to the North Sea.
When very cold air passes over considerably warmer water, it picks up energy and water vapour. When that air is then forced to rise as it hits land, that vapour freezes and falls out as snow.
The uplifting, particularly if there's hills close to the water's edge that the wind is coming up against, is often strong enough to deposit several inches of snow per hour over many hours.
In the UK, that situation occurs most often when we get cold, dry air coming from Scandinavia and Siberia over the North Sea. At the start of winter, the North Sea is about 13 degrees, so when very cold air sweeps over it, it can dump significant amounts of snow on Britain's east coast. The best known example of this occurring is probably the snowfall in Southeast England in January 1987.
So in summary, if you keep an eye on the meteorological charts then there's no reason why you can't deliver a forecast that's just as good as the professionals. In order, you want to look out for: the dotted 528 line, temperature at 850hPa, wet-bulb temps at 850 hPa, and freezing temperatures at the surface. Keep an eye out for cold dry winds coming from Europe, too, especially if you live on the east coast.
Oh, and if you've taken any photos of snowfall in your area, we'd love to see them. Upload the photo on the web and drop us a link in the comments below, then we might even compile a gallery from the best of them.
Update 13:13 26/11/2010: The Met Office got in touch with us to point us to its dedicated snow forecast page. If all the above sounds too much like hard work, then head on over to the website.
This article was originally published by WIRED UK
