What are Smith Periods

Smith Periods are named after LP Smith who was for many years the head of the Met Office agricultural branch. His work on earlier prediction models was carried out in the mid 1950's and was adopted as the main method for predicting blight outbreaks in the UK during the mid 1970's.

Smith Periods are calculated on hourly temperature and relative humidity weather data and use the empirical relationship between weather and blight development determined by Smith in 1956. They represent a tried and tested method for identifying weather conditions conducive to Blight outbreaks. Smith Periods should be used as a guide only and in conjunction with knowledge about the presence of Blight in the local area and the stage of crop development. There is ongoing research in the Potato Industry to understand whether new Smith Criteria are required to better predict the current pathogen population.

 It is also important to be aware that the Smith Periods use weather data representative of postcode areas and not individual fields or allotments. Slope, aspect and the effect of microclimates also have an influence and should be considered when deciding mitigation options. For this reason the information on the Blightwatch pages should not be used in a legal context.

A ‘Full Smith Period’ occurs when the following criteria are met on 2 consecutive days:-

  • Minimum air temperatures are at least 10°C
  • Relative Humidity is 90% or above for at least 11 hours

A ‘Half Smith Period’ occurs when the Full Smith Period criteria are met for 1 day.

A ‘Near Miss’ occurs when minimum air temperatures are at least 10°C for 2 consecutive days but the number of hours with a relative humidity of greater than 90% only totals 10 hours on one or both days.