Accuracy of temperature predictions

Using the weather data I collected previously, I wanted to figure out which of the forecasts services was making the most accurate forecasts.

The way I calculated this was by comparing the temperature predicted in x days time with the actual temperature reported by the service. e.g. if the provider predicted on 1st/Aug that the temperature would be x degrees in 5 days time, then I compare this with their actual reported temperature on 6th/Aug, This absolute error is then averaged across all readings from  all cities

Here is the summary data

Here is a chart of the data for people who prefer visual:

Points to note

• I eliminated one reading from weather underground 6 day forecasts because it would have re-scaled the chart (probably indicates a programming error on my behalf).
• In general the forecast accuracy decreases as the number of days increases.
• Yr.no is best for next day, but bluemix and darksky are equally good further out.
• OpenWeatherMap and Weather underground are significantly worse.

This analysis lumps all cities together for an average result. I might do some analysis later on performance per city.