What does NEC4 clause 60.1(13) mean?

Clause 60.1(13) replaces argument about bad weather with a measurement. The event is that a weather measurement is recorded within a calendar month, before the Completion Date for the whole of the works, at the place stated in the Contract Data, the value of which by comparison with the weather data is shown to occur on average less frequently than once in ten years. The Contract Data states the place where weather is measured, the measurements to be recorded, such as cumulative rainfall or days with snow lying, and the source of the weather data against which they are compared. To establish the event the Contractor obtains the recorded measurement for the calendar month and the ten year value for that measurement and that month from the weather data, and shows that the recorded value is worse. Weather at a different place, a measurement the Contract Data does not list, or a bad fortnight that is not bad over the month, are outside the paragraph. The measurement is only known when the month's record is available, so the Contractor becomes aware then and notifies the event under clause 61.3 within eight weeks of that. The change to the Prices is assessed under clause 63.1 as the effect on actual Defined Cost by the date of the notification and forecast Defined Cost afterwards, plus the Fee, and clause 60.1(13) itself limits the assessment to the difference between the weather measurement and the weather the data show to occur less frequently than once in ten years, so the Contractor is compensated only for the excess over a one in ten year month. What most often goes wrong is that the Contractor claims for every wet day, or cannot separate the cost of the excess days from the days it should have allowed for. Any delay to the Completion Date is assessed separately under clause 63 by the programming expert.