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The BBC weather serviceoffers countless weather forecasts on their web site. One of themost useful features being their localised weather reports. Thisgives a five day prediction of the weather for any location inBritain. For example, here's what the weather inCambridgeis prophesised to be.
Problem is, just how good are these predictions? After once using theBBC's weather page to book myself a nice sun-drenched Friday off work,only to find it pissing down with rain on the day, I was inspired(well.. ok I got bored sometime later on) to write a small computerprogram to check up on the BBC. Are their five day weatherpredictions useful, or are they no good at all? Is the super computer inthe MET office accurately modelling the world's climate, or is it resting itsbrain and picking out sun and rain symbols at random? We will find the answer!
So how do we tell if they're right?
Weather Guru 2 1 – Accurate Weather Forecasts 7 Days Ahead
Well, rather than scurry around the whole of the UK in order to check theirweather divinations, I've chosen to take the easy route and use the BBC's ownweb pages. The five day weather predictions also include today's weather.I'm making the assumption that predicting today's weather is dead simple,so the BBC couldn't possibly get this wrong. So every day, a computer scriptreads the latest five day forecast, and compares today's weather with what theBBC said it would be in their previous forecasts.
Over time, as this program collects more and more of the weather predictions,we'll be able to see how often the weather forecasts were right.
What are the results?
The tables below show the accuracy of the BBC's weather forecasts, listedby time. E.g. as I write this, the table below shows that the weatherforecast for Cambridge one day ahead was 53% accurate. In other words, theBBC's guess about tomorrow's weather in Cambridge was right roughly half ofthe time.
It's not so useful to just show the numbers for the BBC's weather accuracy ontheir own. You need something to compare them to. So, the tables also showthe accuracy of another really simple weather predictor - guessing thattomorrow's weather will be the same as today's. Hopefully the BBC weatherwill be better than this?
Stats last updated at 2020-10-31 12:25
Bournemouth
Days monitored: 5851
Days ahead
1
2
3
4
5
6
7
8
BBC accuracy
58 %
49 %
45 %
41 %
39 %
37 %
33 %
32 %
BBC (yearly)
72 %
66 %
62 %
57 %
58 %
49 %
44 %
43 %
My accuracy
33 %
28 %
28 %
26 %
26 %
26 %
25 %
24 %
Cambridge
Days monitored: 5870
Days ahead
1
2
3
4
5
6
7
8
BBC accuracy
58 %
49 %
45 %
40 %
43 %
38 %
37 %
37 %
BBC (yearly)
67 %
64 %
59 %
50 %
47 %
36 %
35 %
35 %
My accuracy
34 %
29 %
28 %
28 %
27 %
26 %
27 %
26 %
Cardiff
Days monitored: 5887
Days ahead
1
2
3
4
5
6
7
8
BBC accuracy
59 %
49 %
44 %
39 %
36 %
34 %
33 %
32 %
BBC (yearly)
67 %
65 %
59 %
52 %
49 %
43 %
45 %
37 %
My accuracy
33 %
28 %
27 %
26 %
25 %
24 %
24 %
24 %
Durham
Days monitored: 5819
Days ahead
1
2
3
4
5
6
7
8
BBC accuracy
55 %
46 %
41 %
38 %
39 %
37 %
35 %
36 %
BBC (yearly)
63 %
58 %
52 %
46 %
42 %
38 %
36 %
33 %
My accuracy
33 %
30 %
28 %
26 %
26 %
26 %
26 %
26 %
Ely
Days monitored: 5903
Days ahead
1
2
3
4
5
6
7
8
BBC accuracy
59 %
50 %
45 %
40 %
41 %
37 %
36 %
36 %
BBC (yearly)
66 %
62 %
59 %
48 %
44 %
36 %
36 %
32 %
My accuracy
35 %
29 %
28 %
27 %
27 %
26 %
26 %
25 %
Glasgow
Days monitored: 5895
Days ahead
1
2
3
4
5
6
7
8
BBC accuracy
57 %
47 %
44 %
38 %
36 %
34 %
34 %
32 %
BBC (yearly)
65 %
65 %
57 %
53 %
51 %
44 %
41 %
39 %
My accuracy
33 %
28 %
25 %
26 %
25 %
24 %
24 %
24 %
London City
Days monitored: 5908
Days ahead
1
2
3
4
5
6
7
8
BBC accuracy
59 %
50 %
46 %
41 %
42 %
37 %
36 %
36 %
BBC (yearly)
64 %
59 %
56 %
52 %
48 %
38 %
38 %
34 %
My accuracy
34 %
29 %
28 %
27 %
26 %
26 %
27 %
26 %
Nottingham
Days monitored: 5907
Days ahead
1
2
3
4
5
6
7
8
BBC accuracy
58 %
48 %
43 %
39 %
39 %
36 %
35 %
35 %
BBC (yearly)
65 %
64 %
54 %
47 %
40 %
37 %
33 %
37 %
My accuracy
33 %
29 %
28 %
26 %
25 %
26 %
25 %
25 %
Southampton
Days monitored: 5908
Days ahead
1
2
3
4
5
6
7
8
BBC accuracy
59 %
51 %
45 %
41 %
38 %
37 %
32 %
31 %
BBC (yearly)
68 %
64 %
58 %
52 %
48 %
44 %
37 %
38 %
My accuracy
34 %
29 %
28 %
26 %
26 %
26 %
26 %
24 %
York
Days monitored: 5907
Days ahead
1
2
3
4
5
6
7
8
BBC accuracy
57 %
48 %
43 %
39 %
41 %
38 %
37 %
36 %
BBC (yearly)
65 %
60 %
54 %
48 %
43 %
38 %
33 %
33 %
My accuracy
32 %
30 %
28 %
27 %
26 %
27 %
26 %
26 %
How do I measure 'accuracy'?
Very simply! I take the BBC's weather icons and compare them, using a bit ofleeway. So if the prediction is 'Partly Cloudly', then 'Sunny Intervals' isalso considered equivalent. Likewise, 'Light Showers', 'Light Rain' and'Drizzle' are all considered close enough to be an accurate forecast.
The 'yearly' accuracy is based on the last 365 days of results, toshow if the BBC is getting better or worse over time.
Weather Guru 2 1 – Accurate Weather Forecasts 7 Days 7
I'm working on adding a better scoring system, since this accuracymeasurement is a bit rubbish really..
So? Are weather reports prophetic or pathetic?
So what have we learned? Can we fault the prescience of the BBC's five dayforecasts? Yes. The weather predicted by the BBC for four days time was just30-40% accurate - i.e. they get it wrong around two times out of three.
My alternate weather prediction scheme isn't stunning either - it gets afairly consistent 25-35% accuracy, for all lengths of time. This is surprising- it means that the weather in four days time is just as likely astomorrow's to be identical to today's. (If you can follow that and figure out abetter way to phrase it, please let me know!) It is reassuring that the BBCdo better than this simple prediction technique!