Processing of ASHRAE Data
From frequencies of measured values, a weighted mean of the data and a standard deviation from the weighted mean are computed for each month. For the temperature, more data are available and the weighted mean and standard deviation are also computed at each hour. Figure 3-4 shows an example of weighted mean computation for the diurnal temperature fluctuations from the initial data given as frequencies of observations.
Figure 3-4: Computation of weighted mean from frequencies of observations for the diurnal fluctuations of temperature.
These values are used for the definition of different conditions, as detailed in Ambient Variables and Conditions.
All the monthly averaged observations except the solar irradiance are supposed to be made at the middle of each month. This time depends on the number of days in the month:
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Months with 31 days (January, March, May, July, August, October, December): data at the 16th at noon
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Months with 30 days (April, June, September, November): data at the 16th at midnight
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Months with 29 days (February, leap years): data at the 15th at noon
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Months with 28 days (February, other years): data at the 15th at midnight
In addition, the temperature observations are supposed to be made at the beginning of each hour (00:00 a.m. to 11:00 p.m.).
Finally, the solar irradiance observations are made at the 21st of each month at noon. Depending on the number of days in the month, this date corresponds to 68% (for months with 31 days), 70% (for months with 30 days), or 75% (for February) of the month. The leap years are not considered and the 21st of February always corresponds to 75% of this month.