While trying to select typical months to generate a typical meteorological year (TMY) for weather files used in building performance simulation, 5 candidate months are determined at first step according to Sandia Method.

First,We take sum of the absolute differences between long term and candidate month CDFs then divide it by total number of observations. That is called as FS statistics.

After that we calculate the weighed sum of FS statistics and determine the candidate months with lowest weighted sums then months are ranked with respect to closeness of the month to the long-term mean and median.

It is all clear for me until that point.

What i want to ask is why dont we stop here and select the month at the top of the list (the highest ranked month) ?

Procedure goes on.

TMY manual says " The persistence data are used to select from the five candidate months the month to be used in the TMY. The highest ranked candidate month from step 2 that meets the persistence criteria is used in the TMY" .

And for persistence criteria TMY manual says "For mean daily dry bulb temperature, the frequency and run length above the 67th percentile (consecutive warm days) and below the 33rd percentile (consecutive cool days) were determined." What does that mean ? How do we calculate frequency or run lenght ? What is the percentiles? Is it the persentiles of CDFs ? Im really confused. Is there any document that contain examples about this persistence criterias?

Thank you.