A tale of a statistician without an umbrella

You should have seen our office today – our cycle to work resembled swimming rather than biking, so wet clothes were hanging everywhere. Well, I can blame only myself, since a normal human being would assume that 98% probability of rain means “it WILL rain, take a bus”. However, being an incurable optimist, I counted on these 2%. In the end, improbable things happen a lot, as Jordan Ellenberg (a mathematician, of course) says. But how improbable was a bit of sunshine in London today? And why on Earth do I try to squeeze some maths even in the weather prediction?!

If you live in the UK, chances are that you begin your day checking your favourite weather app. Based on the information you gained, you know what to wear – unless you’re a proper Briton, then you wear shorts, no matter what, don’t you? Regardless of the website you use, the forecast is probably provided either by Met Office or ECMWF, who constantly compete to produce the most reliable weather prediction in the world. Despite, or maybe because of this rivalry, their forecasts are more and more accurate. So again, why can’t they just get it right, why do they give us percentages?

Weather is chaotic. You might associate chaos with the butterfly effect, a term coined by Edward Lorenz. If you think that it means that a butterfly in Asia can create a tornado in America, then please, please forget this concept; or, even better, read an excellent book by Ian Stewart, Does God Play Dice?, or request an article about it in the comments. The bottom line is that weather forecasting centres will never be able to predict the weather with the accuracy of 100%, no matter how hard they try. Never. Ever.


A chaotic system: a slight change in the intial conditions can lead us to a different “wing” (by Computed in Fractint by Wikimol [Public domain], via Wikimedia Commons)

Instead, they calculate their confidence in a particular weather forecast. For example, 98% PoP (probability of precipitation) between 8 am and 11 am in London means that there is 98 in 100 chance that in this period we’ll get at least 0.1 mm of some precipitation. Basically, if you kept going back in time and went for a walk in London today between 8 and 11 one hundred times, twice you would be lucky and came home completely dry.

What?! It doesn’t make any sense! We can’t go back in time! But computers can. Even better, they can look into the future. Because weather is chaotic, small changes in the initial conditions (so the temperature, pressure, clouds etc. in the moment when we start simulations) can lead to big changes in the outcome. This is why Met Office and ECMWF use so-called ensemble forecasting. They run many simulations starting from slightly different conditions and look at their outcomes; it’s a bit like in Groundhog Day, but less creepy. And this means that today I believed in these 2 measly forecasts out of 100 – and that’s why my neighbours asked me if I cycled into the Thames on my way home (not funny).

Learn your probabilities and figure out your chances for a dry day. Or take an umbrella, just in case.



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