High values of relative humidity have implications for many processes including human health, animal health and production, and crop disease. Despite its importance, little research has been completed about the persistence of high relative humidity. The goals of this study were to 1) create a baseline climatology of the persistence of high relative humidity (HRH), defined as ≥ 60%, and extremely high relative humidity (EHRH), defined as ≥ 85%, for the Lower Peninsula of Michigan and 2) examine, using persistent EHRH events as a surrogate for leaf wetness duration, the frequency of favorable environmental conditions for apple scab and cherry leaf spot, major crop diseases in the state. Results demonstrate that although overall persistent relative humidity events often occur throughout the state, their frequency appears to be decreasing with time. Temporal trends in the frequency of favorable environmental conditions for apple scab and cherry leaf spot vary by location and disease, but there is a general trend toward fewer occurrence of favorable environmental conditions. The climatological analyses provide Michigan stakeholders with essential information for long-term planning and management to mitigate and/or adapt to persistent high relative humidity and to assess future changes in persistent high relative humidity as expected with climate change.
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