Time series of an index for the occurrence of cold waves and heat waves

Time series (1949-2010) of an index for the occurrence of: cold waves defined as 4-day periods (blue) and 7-day periods (red) that are colder than the threshold for a 1-in-5 year recurrence (top), and heat waves defined as 4-day periods (blue) and 7-day periods (red) that are warmer than the threshold for a 1-in-5-year recurrence (bottom), for Alaska. Based on data from the National Climatic Data Center First-Order Surface Weather Observing Stations and based on methods from Kunkel et al. (1999).