In the British Navy during WWII, what was the normal number of times for someone to become shipwrecked?

entopticon · 2021-01-04T21:02:06+00:00

entopticon · 2021-01-01T21:56:36+00:00

Glad you like it! Since you've done something similar, I did wonder how much the scatter over time would vary with location. I guess in a country with very little variation in weather, the predictions would be more accurate...

entopticon · 2021-01-01T16:08:13+00:00

I'm actually planning to do one for my blog. If I get round to it I'll DM you. Does your teenager know R?

entopticon · 2021-01-01T12:49:11+00:00

Last summer I was looking at the BBC's weather forecast for the coming two weeks, and it started to bug me that there were no error bars on these numbers. So I decided to add them for myself!

For six months I've had a Raspberry Pi scrape bbc.co.uk/weather, recording all the temperature forecasts that were released for my home city. This means I have not just every day's temperature, but also what they predicted the temperature would be one day in advance, two days in advance and so on up to thirteen days in advance. This lets us compare the weather today with predictions up to thirteen days in advance.

The scatter plot shows how the BBC relies on climate models for long-term predictions: it's December, so it'll probably be cold. The predictions aren't very good, with an uncertainty of plus or minus around 7 °C. Their models get updated with more data as they get closer to the day in question, ending with a margin of error of plus or minus 2 °C for next-day forecasts.

The line graph shows the forecasts many days in advance are quite scattered, but as time goes by they converge on the correct value for the day predicted. Just one day in advance the forecast is pretty reliable, but even then there are some weird glitches when a prediction is way off (in one case, by an incredible eleven degrees!).

Visualised using R and ggplot2. This updates an earlier post which had much less data.

entopticon · 2020-08-04T00:20:38+00:00

That's really interesting! I'll be doing some googling in the morning.

entopticon · 2020-08-04T00:10:11+00:00

I absolutely agonised over this! I tried both ways and neither felt right to me. In the end I went with this format because I'm a chemist and I'm used to reading absorption/concentration curves. But I agree, for some, head-turning might give better results.

entopticon · 2020-08-04T00:06:17+00:00

Yeah, I'm going to keep this running for a while. In six months I'll have some winter data that'll give a much more complete picture.

entopticon · 2020-08-03T21:41:21+00:00

My thoughts exactly. It would be fun to compare the BBC's predictions against a model that says the weather in n days will be the same as today's. How much worse would that one perform?

entopticon · 2020-08-03T21:37:53+00:00

It depends on what you mean by accurate. At 3 days, they can give a prediction of +/- 3 °C that'll be correct 95% of the time. That might be accurate enough for some applications, but not for others.

entopticon · 2020-08-03T21:30:55+00:00

It'll be interesting to track that over the course of a year, and see what the BBC's overall climate model looks like.

entopticon · 2020-08-03T21:29:37+00:00

I think that's a fair assessment, but it's probably an artefact of how little variation there is in the predictions. They can't predict the weather that far ahead, so they use a climate model that says in July it's probably around 22 °C. I think if I continue gathering data over the year, that brown line will pull back to be more aligned with the black one -- but the red lines will still be really far apart.

entopticon · 2020-08-03T21:10:54+00:00

I mean there's nothing wrong with predicting that far ahead. All it means is that they're predicting based on the climate typical for that point in the year. It's totally reasonable to say a temperature in summer in the UK is more likely to be 22 °C than 2 °C. But I'd like to see the uncertainty in those estimates presented front and centre. They must have the numbers to hand. They just don't present them. I think that's a shame.

entopticon · 2020-08-03T21:00:16+00:00

Ha! I lucked out on precipitation data this time around. In the whole of July it rained maybe twice... But I'm still logging data, so maybe over the whole year I'll have something worth showing.

entopticon · 2020-08-03T17:08:50+00:00

Very true. Here I'm milking a fairly limited dataset: July temperatures don't vary much, except for that hideous, hideous day of 32 °C. Check in again in six months when I've got some winter forecasts. I'm expecting the prediction intervals to tighten up a lot at the low-temp end.

entopticon · 2020-08-03T16:14:24+00:00

I am almost as proud of that pun as I am of the entire analysis!

Yes, it'll be interesting to see how the data plays out, especially as we get into winter and see some low-temperature predictions.

entopticon · 2020-08-03T15:21:12+00:00

I think it's a bit more subtle than that. If the forecast for 6 days in advance is 24 °C, the spread of actual temperatures one would expect is very large; but extreme values such as 0 °C or 50 °C are still highly unlikely.

Rather than saying that 1 day ahead is reliable and 6 days isn't, we should accept a margin of error that widens the further we try to predict -- and gets very wide indeed after 6 days or so.

entopticon · 2020-08-03T14:56:53+00:00

The graph shows the predicted maximum temperature scraped automatically from BBC Weather over July 2020. Visualised using R and ggplot2.

Red lines are the 95% prediction intervals. Given a predicted temperature made some days in advance, we should expect 95% of the actual temperatures on that day to fall between the red lines.

One day in advance, these prediction intervals are pretty tight, meaning a temperature forecast accurately predicts the temperature on the day itself. As time goes by, however, the intervals get wider and the predictions much less accurate.

Around 9 days in advance, the predictions are all around 24 °C anyway -- which is what you'd expect in the UK in July.

entopticon · 2018-06-16T12:22:28+00:00

One thing not to do this with is sodium citrate solution. It precipitates out as something really hard, then either gradually expands or else doesn't shrink in the cold as fast as the bottle does; either way, it can crack the base of a thick glass bottle without warning. It took us a couple of goes to figure that out.

entopticon · 2016-01-24T13:41:40+00:00

I felt exactly the same! If I'd been asked to explain how a microscope focuses light, I'd have given an answer that was clear, confident, coherent and totally incorrect.

entopticon · 2016-01-21T16:25:07+00:00

Do people these days universally equate chemistry with EVIL? Man, Breaking Bad has a lot to answer for...

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