Guidelines on the definition and characterization of extreme weather and climate events

The climate is how the atmosphere behaves over a long period of time (typically defined as at least a 30-year period). Descriptions of the climate inform us about the average weather, as well as other aspects of weather patterns and distribution of meteorological parameters – including anomalous, rare and extreme events. WMO plays a critical role in understanding and monitoring climate variability and change and related extreme events. It is therefore crucial for the Commission for Weather, Climate, Water and Related Environmental Services and Applications (SERCOM) to continuously assist WMO Members by providing guidance on various aspects of extreme events, so that efficient monitoring, forecasting and early warning systems are adequately deployed. This constitutes part of the WMO contribution to building resilient societies able to cope with climate variability and adapt to climate change. The guidance provided in the present document focuses on generic definitions and methodologies for the characterization of extreme weather and climate events. This guidance has become timely for ensuring consistent exchange of information that underpins the contents of the WMO State of the Climate reports, climate watches, climate change studies and other emerging applications.