Ten Day Temperature Forecasts: Their Accuracy

Created 2004/04/21, modified 2004/12/23
Feedback welcome; email daveclarkecb@yahoo.com

Note that this page is probably out of date. The forecasting methods and accuracy of uk.weather.com have probably changed since this study was done.
The page has been left here for archival reasons.

This is an independent study into the accuracy of ten-day weather forecasts as published on the Internet by uk.weather.com.

Huge numbers of people access these weather forecasts, but who knows or cares how accurate they are?

This page records the maximum daily temperatures forecast for Adelaide, South Australia, and compares them with the actual temperatures recorded there. It covers the period from 13^th April 2004 to 31^st May 2004 and is a current project.

Also see 7-day forecast which considers the 7-day forecast data as broadcast on South Australian TV.

For this period the averages of the absolute errors are given in the table below. All temperatures are in degrees Celsius.

Days ahead 1 2 3 4 5 6 7 8 9 10
Ave. error 1.3 1.3 1.4 1.3 1.9 2.2 2.2 2.2 2.1 2.0

That is, for example, the one-day forecast was on average 1.3 degrees in error, either too high or too low, etc. One would expect that the greater the time span of the forecast, the lower the accuracy is likely to be. This trend is beginning to show up.

Days ahead	1	2	3	4	5	6	7	8	9	10
Ave. error	1.3	1.3	1.4	1.3	1.9	2.2	2.2	2.2	2.1	2.0

To place these forecasts in some perspective one can consider how they compare to a forecast of the average maximum daily temperature for the time of year. The far right column in the table below shows the absolute difference between the actual temperture and the average maximum temperature for each day. If one was to forecast these average temperatures then the average absolute error of the forecasts would be 2.5 degrees indicating that the further reaching forecasts are little better than forecasting the average maximum daily temperatures.

Data

The data used to produce the above accuracies are given in the table below. Note that the table contains gaps because there were days when I was unable, or forgot, to record the forecast temperatures.

Weather forecasts - daily maximum temperatures
			Forecasts: No. of days ahead										Absolute errors Difference between forecast and actual temperature										Dif. from average
Day	Date	Actual temp.	1	2	3	4	5	6	7	8	9	10	1	2	3	4	5	6	7	8	9	1
Tuesday	13/04/2004	31.5	28										3.5										9.5	Wednesday	14/04/2004	32.2	29	27										3.2	5.2									Thursday	15/04/2004	21.1		22	23										0.9	1.9									Friday	16/04/2004	20.4			20	22										0.4	1.6							1
Saturday	17/04/2004	21.5	21	21		21	22						0.5	0.5		0.5	0.5					Sunday	18/04/2004	20.6	21	22	22		22	22						0.4	1.4	1.4		1.4	1.4				Monday	19/04/2004	25.0	23	22	24	24		21	22					2.0	3.0	1.0	1.0		4.0	3.0			Tuesday	20/04/2004	27.8	25	26	24	24	24		22	20				2.8	1.8	3.8	3.8	3.8		5.8	7.8	Wednesday	21/04/2004	30.3	26	28	26	26	26	26		22	21			4.3	2.3	4.3	4.3	4.3	4.3	8
Thursday	22/04/2004	28.7	28	26	24	28	26	26	26		22	22	0.7	2.7	4.7	0.7	2.7	2.7	2.7<
Friday	23/04/2004	18.4	19	19	19	21	26	23	22	22		23	0.6	0.6	0.6	2.6	7.6	4.6	3.6		Saturday	24/04/2004	16.7	18	18	18	19	21	25	24	19	19			1.3	1.3	1.3	2.3	4.3	8.3	7.3<
Sunday	25/04/2004	16.5	19	19	19	20	21	22	25	24	21	21	2.5	2.5	2.5	3.5	4.5	5.5	8.5<
Monday	26/04/2004	18.4	20	20	20	20	21	22	23	23	22	19	1.6	1.6	1.6	1.6	2.6	3.6	4.6<
Tuesday	27/04/2004	17.8	20	19	19	20	20	22	22	22	21	20	2.2	1.2	1.2	2.2	2.2	4.2	4.2
Wednesday	28/04/2004	19.1	20	20	19	20	22	22	22	22	22	20	0.9	0.9	0.1	0.9	2.9	2.9	2.9	Thursday	29/04/2004	16.3	17	19	20	19	19	21	21	21	22	21		0.7	2.7	3.7	2.7	2.7	4.7	4.7	Friday	30/04/2004	18.0	16	16	17	18	18	18	19	19	20	22		2.0	2.0	1.0	0.0	0.0	0.0	1.0<
		Average of absolute errors for April data											1.8	1.9	2.0	2.0	3.0	3.9	4.4	4.7	4.5	3.5	4.2

Day	Date	Actual temp.	1	2	3	4	5	6	7	8	9	10	1	2	3	4	5	6	7	8	9	1
Saturday	1/05/2004	16.4	16	16	16	16	18	18	18	18	18	19	0.4	0.4	0.4	0.4	1.6	1.6	1.6
Sunday	2/05/2004	17.8	17	17	17	17	17	18	18	18	18	18	0.8	0.8	0.8	0.8	0.8	0.2	0.2	Monday	3/05/2004	18.3	19	19	19	19	19	18	18	19	18	18		0.7	0.7	0.7	0.3	0.7	0.3	0.3	Tuesday	4/05/2004	19.3	18	18	18	19	19	18	18	19	19	19		1.3	1.3	1.3	0.3	0.3	1.3	1.3<
Wednesday	5/05/2004	18.5	18	18	18	18	20	19	17	17	18	21	0.5	0.5	0.5	0.5	1.5	0.5	1.5	Thursday	6/05/2004	19.7	20	19	18	19	19	19	19	18	18	18		0.3	0.7	1.7	0.7	0.7	0.7	0.7
Friday	7/05/2004	19.4		21	21	20	20	19	19	21	18	19		1.6	1.6	0.6	0.6	0.4	0.4	1.6
Saturday	8/05/2004	21.3	21	21	21	21	21	21	20	19	21	19	0.3	0.3	0.3	0.3	0.3	0.3	1.3
Sunday	9/05/2004	19.5		20	21	21	22	22	19	21	19	19		0.5	1.5	1.5	2.5	2.5	0.5	1.5
Monday	10/05/2004	20.5			20	20	22	21	21	20	20	20			0.5	0.5	1.5	0.5	0.5	0.5	Tuesday	11/05/2004	21.3	20			21	21	19	19	18	20	19		1.3			0.3	0.3	2.3	2.3	3.3	Wednesday	12/05/2004	22.2		19			19	19	19	19	19	20			3.2			3.2	3.2	3.2	3.2<
Thursday	13/05/2004	20.4	17	17	17			19	19	20	19	19	3.4	3.4	3.4			1.4	1.4	0.4	Friday	14/05/2004	19.7	20	19	20	19			18	18	21	21		0.3	0.7	0.3	0.7			1.7	1.7	Saturday	15/05/2004	18.4	18	18	19	19	19			18	18	19		0.4	0.4	0.6	0.6	0.6			0.4	Sunday	16/05/2004	19.2	21	19	19	22	22	19			16	16		1.8	0.2	0.2	2.8	2.8	0.2			Monday	17/05/2004	17.6	17	17	17	18	21	21	21			16		0.6	0.6	0.6	0.4	3.4	3.4	3.4	<
Tuesday	18/05/2004	17.4	16	17	20	18	18	21	21	17			1.4	0.4	2.6	0.6	0.6	3.6	3.6
Wednesday	19/05/2004	17.7	17	18	19	21	18	18	18	18	17		0.7	0.3	1.3	3.3	0.3	0.3	0.3
Thursday	20/05/2004	19.2	20	19	19	19	20	19	19	19	19	17	0.8	0.2	0.2	0.2	0.8	0.2	0.2	Friday	21/05/2004	20.4	21	21	21	20	19	18	18	21	20	20		0.6	0.6	0.6	0.4	1.4	2.4	2.4<
Saturday	22/05/2004	23.4	19	19	20	18	19	17	19	20	22	22	4.4	4.4	3.4	5.4	4.4	6.4	4.4	Sunday	23/05/2004	18.7	18	18	19	19	18	21	19	20	19	20		0.7	0.7	0.3	0.3	0.7	2.3	0.3<
Monday	24/05/2004	18.4	17	16	17	17	15	18	18	19	21	18	1.4	2.4	1.4	1.4	3.4	0.4	0.4<
Tuesday	25/05/2004	17.5	17	17	17	17	17	16	18	19	16	20	0.5	0.5	0.5	0.5	0.5	1.5	0.5
Wednesday	26/05/2004	15.4	16	15	17	16	16	16	16	17	18	16	0.6	0.4	1.6	0.6	0.6	0.6	0.6	Thursday	27/05/2004	14.7	16	15	16	15	16	16	16	16	18	16		1.3	0.3	1.3	0.3	1.3	1.3	1.3	Friday	28/05/2004	17.5	16	17	17	16	15	15	17	17	17	18		1.5	0.5	0.5	1.5	2.5	2.5	0.5<
Saturday	29/05/2004	16.1	17	18	17	17	17	14	16	17	17	17	0.9	1.9	0.9	0.9	0.9	2.1	0.1	Sunday	30/05/2004	18.2	19	20	20	19	18	17	15	17	14	14		0.8	1.8	1.8	0.8	0.2	1.2	3.2<
Monday	31/05/2004	19.3	19	19	19	20	19	18	18	17	18	14	0.3	0.3	0.3	0.7	0.3	1.3	1.3<
		Averages of absolute errors for May data											1.0	1.0	1.1	1.0	1.3	1.5	1.4	1.3	1.3	1.5	1.6
		Average of all absolute errors											1.3	1.3	1.4	1.3	1.9	2.2	2.2	2.2	2.1	2.0	2.5

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Ten Day Temperature Forecasts: Their Accuracy

Note that this page is probably out of date. The forecasting methods and accuracy of uk.weather.com have probably changed since this study was done. The page has been left here for archival reasons.

This is an independent study into the accuracy of ten-day weather forecasts as published on the Internet by uk.weather.com. Huge numbers of people access these weather forecasts, but who knows or cares how accurate they are?

Data

Note that this page is probably out of date. The forecasting methods and accuracy of uk.weather.com have probably changed since this study was done.
The page has been left here for archival reasons.

This is an independent study into the accuracy of ten-day weather forecasts as published on the Internet by uk.weather.com.

Huge numbers of people access these weather forecasts, but who knows or cares how accurate they are?