Consensus Assessment of a Climate Extreme – the July 2005 Heat Wave in Colorado

In July 2005 Colorado had a heat wave. At the time, there was considerable debate in the media on whether this was an extreme, unprecedented event. The difference of views were highlighted in the Climate Science weblog of July 28 2005 entitled “What is a Record Heat Wave, or a Record in Any Climate Metric?”

In the July 28th weblog, I wrote,

“The discussion on the significance of the recent heat wave in eastern Colorado continues (see the Rocky Mountain News article “Hot streak has experts divided“). This article illustrates an important issue in climate science: What measures do we use to identify a heat wave (or other climate extreme) as an all-time record? Can we make such claims from a record at an individual location?”

The “experts” are now not divided. A paper on the July heat wave by Pielke, R.A. Sr., K. Wolter, O. Bliss, N. Doesken, and B. McNoldy, 2005 entitled “July 2005 heat wave: How unusual was it” has been submitted to the National Weather Digest.

The abstract of the paper states,

“In mid July 2005, very hot temperatures developed over the Rocky Mountains and western Great Plains. This paper summarizes this heat wave and places it in historical perspective. The core of the heat wave was centered near Denver where several weather stations approached or exceeded their all-time record high temperatures on July 20-21st. Denver International Airport rose to 105°F July 20th and two NWS Cooperative stations in the Denver metro area reached 108°F exceeding any previous records for the city. For July as a whole, the month was not the warmest on record, however, due to sharply cooler weather shortly after the heat wave. Based on 5-day running mean temperatures, this heat wave ranks first for Denver, Edgewater/Lakewood and Fort Collins. An alternative method for evaluating heat waves, moist enthalpy which combines temperature and humidity, provides a markedly different perspective and shows the Denver heat wave to be less extreme due to very low humidity accompanying the event. ”

The conclusion of the paper states,

“Several different metrics are shown here for comparing heat wave severity and ranking the recent July 2005 Colorado heat wave: monthly temperatures, frequency of temperatures above specified threshold values, daily temperature extremes, 5-day running means and moist enthalpy. Based on daily temperature extremes, July 2005 was a record heat wave for several locations. New records included 108°F at Northglenn and Denver Water Department, 105°F on July 20, 2005 at DIA (tying an old record previously set in 1878), and 103°F at Fort Collins on July 21st. The heat wave was persistent, and record or near-record 5-day running means were observed near Denver. Also, several stations reported the greatest number of days of 100°F or higher.

However, in terms of frequency of monthly temperatures and daily temperatures of 90°F or above, July 2005 was not exceptional. Cooler weather earlier in the month, and a notable cold front in late July were responsible. Finally, a new metric for assessing heat, moist enthalpy, showed significantly different results. The low humidities contributed to the higher observed temperatures. Cooler but more humid locations (like Fort Collins) actually have greater heat (effective temperature) when compared to the relatively hot but dry conditions observed at DIA. Based on moist enthalpy considerations, the 2005 heat wave was not exceptional.

The diversity of observations of the heat wave suggests that we need to address the question, should we record temperatures that register the full impact of heat waves that affect people both in terms of mortality and electrical power consumption (cooling) inside the ‘urban heat island’, or should we keep the observing sites out at airports where we get a more objective record of regional temperatures that are often substantially lower than those affecting the majority of the population? Over the past 50 years data tends to be collected at airports, but the value of also retaining urban observation sites is clear.”

This paper shows not only can we document a weather event using a variety of climate metrics, but colleagues can work in good faith to produce a truly consensus assessment. This is the model that the global climate change community should adopt.

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