Thursday, December 29, 2016

How accurate are temperature forecasts for Dublin

I got a very positive reaction to my presentation about my weather forecast tracking application on Bluemix. Most people who saw this presentation asked which weather forecast service was best. Unfortunately I had to reply to them that I didn't know because I had only just built the app and never used it yet to do any detailed analysis. This post is an attempt start rectifying that error by looking at the accuracy of the various services for forecasting temperature in Dublin over the last month.

At the innovation showcase event a number of people spoke highly of  the Norwegian forecast service and claimed that it was the most accurate in their experience. Therefore, I decided to add this service into my data collection service (it is a testament to the flexible architecture that this was a relatively easy task) and wait until a month had passed before beginning analysis so that we had  enough data from them to compare.

I chose to initially analyse data from Dublin since this is where I live. I analysed temperature readings and forecasts for the period from 22nd of November to 21st of December 2016. I looked at the actual weather reported as well as the forecast for next day and the forecast for 3 days time. Some of the services offered forecasts for further into the future, but I chose to pick the 3 day forecast because this was supported by all services. If you want, you can see the actual data here.

The thing that most people commented upon at the innovation showcase was the fact that actual temperature data reported by each service did not match up exactly. When I analysed this, I noticed that some of this difference was down to data rounding. The temperature data reported by OpenWeatherMap  and DarkSky is given in celcius with 2 decimal places, in contrast WeatherUnderground and only give a single decimal place and both WeatherOnline and the BlueMix weather data are reported in whole numbers.

People asked me which service was actually correct, but the truth is that I have no way to tell. What I decided to do was to compute the average of the temperature reported by all services and then comparing the number reported by each service to this average. As you can see from the table below, the average error was actually quite small - not much more  than the average of 0.25 degrees which would be expected from rounding error. Interestingly, the service has the greatest level of difference from the average.

For assessing the forecasts I looked at the difference between the temperature that was forecast 1 or 3 days earlier and the actual temperature reported on the date in question. I also looked at the forecasts for one week ahead, although this could only be done for 4 of the 6 services since WeatherOnline and OpenWeatherMap don't supply 1 week forecasts.

The average level of error was slightly higher for 3 days in advance  (1.74 degrees) than it was for 1 day in advance (1.41 degrees) and higher again (1.95 degrees) for 7 day forecasts. The most accurate forecasts were from the BlueMix weather service across all 3 timespans. The second most accurate 1 daya forecasts were from the service although the 3 day and 7 forecasts from were definitely worse - the 7 day forecasts from were the worst of all services..

ProviderDecimal PlacesAve ErrFcst1 ErrFcst3 ErrFcst7 Err


Overall, I think the conclusion is that the temperature forecasts are actually quite close to the  actual values, but the details might vary if I looked at a longer time period and/or more cities.

In reality temperature forecasts are relatively easy since temperature doesn't vary widely from day to day in this part of the world. Next I will look at wind speed forecasts since these might be more variable and also because I have been told that they are closely watched by insurance companies since wind speed correlates with weather damage,


Saturday, December 3, 2016

Where the streets have too many names

U2 had a big hit with a song named "Where the streets have no name", but in Ireland many streets have more than one name. Sometimes the change is minor, like when "Baggot Street Upper" becomes "Baggot Street Lower", but other times the streets have totally unconnected names. This is very confusing for visitors and locals alike.

One of the worst examples starts off in Rathmines, where you can travel 3 km along a straight road but officially be walking on 9 different street names:
  1. The journey starts on "Rathmines Road Upper".
  2. As you head north you will see that the road becomes "Rathmines Road Lower" which probably is no surprise.
  3. Continue straight and as you pass over the canal and you will see that for a short time you are travelling on "Richmond Street South". If this was not confusing enough, after a short while you will come to a junction where you can either continue straight or turn right and in both cases you would still be on  "Richmond Street South".
  4. Assuming you keep straight, the road will transform itself into "Camden Street Upper".
  5. Unsurprisingly this road changes its name into "Camden Street Lower".
  6. However, if you continue straight on the same road, you might be a little confused to find that the road transforms again into "Wexford Street".
  7. Many Dubliners know "Wexford Street", but relatively few realize that if you continue walking straight along this street you will be on "Redmond's Hill". If you are looking for this piece of the road, don't be fooled by the name into thinking that there should be some slope in the road - this is a perfectly flat stretch of road.
  8. If you are confused to find yourself on "Redmond's Hill", don't worry - it only lasts about 100 meters or so before transforming into "Aungier Street".
  9. "Aungier Street" is reasonably long, but clearly some city planners decided that it was too long since it changes without warning into "South Great George's Street". Don't be fooled into thinking that walking north on "South Great George's Street" would bring you to "North Great George's Street" - that street is several km away in a different part of town. Instead the road comes to an abrupt end in a T-junction with "Dame Street" (needless to say "Dame Street" also has several names)..
Is it any wonder that tourists get confused when they ask someone what street they are walking along and they are told "Aungier Street" and then 5 minutes later they ask someone  else and they are assured that they are walking along "South Great George's Street" despite the fact that they have been walking straight all the time.