3.2.1.2 Statistical calculation based on ASHRAE WDVIEW 7.0 data
The ASHRAE weather datasets allow these external design temperatures to be based upon a statistical treatment. This treatment analyses the weather dataset and returns the design external temperatures that are only exceeded by the percentage of the year or month that the user specifies.
The Outdoor winter design temperature is the hourly average temperature exceeded on the given percentage (Annual percentile for Heating Loads design weather) of occasions in the historical dataset. This dataset is stored as a set of binned temperature statistics. Linear interpolation is applied between consecutive bins.
The peak design dry bulb temperature for cooling load calculations is the hourly average temperature that is exceeded on the given percentage (Annual or Monthly percentile for Cooling Loads design weather) of occasions in the historical dataset. This peak temperature appears in the table as the maximum dry bulb temperature for the warmest month. The percentage of the time that this temperature is exceeded in the peak month is then calculated. This monthly percentile is then applied to the other months to derive the design maximum dry bulb temperatures for those months on the same statistical basis. Minimum dry bulb temperatures are then obtained for each month by subtracting from the maximum dry bulb temperature the daily temperature swing calculated for each month. In the case of the peak month the temperature swing is provided in the database. For the other months the swing is estimated by deriving the amplitude of the first Fourier component of the clear sky horizontal global solar radiation flux for the month in question and using this to scale the temperature swing recorded for the peak month.
The wet bulb temperature coinciding with the maximum dry bulb temperature (Twb at max Tdb) for each month is calculated as the mean wet bulb temperature coincident with the maximum dry bulb temperature, using the historical binned temperature data.
The peak dry bulb and wet bulb temperatures obtained by this method are in close agreement with those published in the ASHRAE Handbook of Fundamentals for the annual percentiles 0.4%, 1% and 2%.
The Handbook also publishes monthly dry bulb and mean coincident wet bulb temperatures for monthly percentiles 0.4%, 2%, 5% and 10%. There is no fixed relationship between monthly and annual percentiles in general, but for a given temperature the monthly percentile will be greater than the annual percentile (by a factor of about 12 for very low percentiles). The monthly percentile is displayed following retrieval of the design weather data, and a desired value for monthly percentile can be achieved by applying successive adjustments to the annual percentile.
For further information about ASHRAE weather data, see Chapter 14 of the ASHRAE Fundamentals 2021.
