The Interpretation of Smith Periods. Smith Periods for blight control are calculated from hourly temperature and relative humidity values. The weather records on which the post coded Smith Periods are based are supplied by the Met Office.
A full Smith Period has occurred if, on each of 2 consecutive days:
- the minimum air temperature was at least 10°C, and
- there were a minimum of 11 hours with a relative humidity of at least 90%