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Daylight Saving Time

The Effect of Daylight Saving Time on Climate Data

The most significant effect of Daylight Saving Time (DST) is on the nominal 9am observation, particularly on elements such as temperature and humidity. During this period of the day the temperature is rising quickly, and there can be a significant difference between the temperature taken at 8am (standard time, by the sun) and that taken an hour later. Because of this the relative humidity will also be affected. A typical example is shown in the graph below.

 
Change in temperature over time
A graph of air temperature (Melbourne, March day) showing a typical rapid rise
between 8am and 9am (standard time)
 
Problems occur when trying to evaluate monthly mean statistics in the months of changeover between DST and standard time, generally October and March. There may not be sufficient days within the month at either 8am or at 9am to calculate an adequate mean (normally about 20-25 values are required). The date of the change varies from month to month, leaving differing number of records for the different hours. Partly for this reason the two hours are usually combined and a nominal 9am mean value of, for example, air temperature is given.
 
Care also needs to be taken when examining continuous data (for example pluviograph records used to determine rainfall intensity) near the time of changeover to or from DST.

 
Last modified 23 November 2005
 

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