Time series of an index for the occurrence of heat waves (top) and cold waves (bottom), defined as 4-day periods that are hotter and colder, respectively, than the threshold for a 1 in 5-year recurrence, for the Northeast region. The dashed line is a linear fit. Based on daily COOP data from long-term stations in the National Climatic Data Centers Global Historical Climate Network data set. Only stations with less than 10% missing daily temperature data for the period 1895-2011 are used in this analysis. Events are first identified for each individual station by ranking all 4-day period mean temperature values and choosing the highest (heat waves) and lowest (cold waves) non-overlapping N/5 events, where N is the number of years of data for that particular station. Then, event numbers for each year are averaged for all stations in each 1x1 degree grid box. Finally, a regional average is determined by averaging the values for the individual grid boxes. This regional average is the index. The most intense heat waves occurred in the 1910s and 1980s, with moderate values in recent years. There is no overall trend in heat waves. Cold wave events were most intense in the 1910s and the 1970s/80s and have been low during the last few years. The overall trend in cold waves is not statistically significant.