Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather
By: Dr. Jeff Masters , 1:41 PM GMT on February 13, 2009
The lowest temperature ever recorded in the state of Maine, a -50°F reading taken on January 16, has been confirmed as real, according to a press release issued by the U.S. Geological Survey (USGS) and National Weather Service this week. The new record occurred at 7:15 a.m. Jan. 16 at a remote river gauge in Big Black River (see USGS image at right), about four miles from the Canadian border. It ties the record set in 1933 for New England's lowest temperature, set at Bloomfield, Vermont. The old Maine record was -48°F, set in 1925 at Van Buren. All-time state records are difficult to break. The last time a state record low was set occurred January 5, 1999, when Congerville, Illinois recorded -36°F. Only one state record high temperature has been set in the past the decade--the 120°F temperature measured in Usta, South Dakota on July 15, 2006.
All-time record lows are inconsistent with global warming, right?
An impressive cold wave hit the northern and eastern portions of the U.S. January 11-18, with 17 states reporting record daily lows. In addition to the coldest temperature ever measured in Maine, one station, Waterloo, Iowa, tied its 1962 record for all-time coldest temperature, when the mercury hit -34°F on January 16. If global warming is occurring, we should not expect to see very many all-time city or state records being set. The nation's January-December average temperature has increased at a rate of 0.12°F per decade since 1895, and at a faster rate of 0.41°F per decade during the last 50 years. This 2°F rise in temperature has undoubtedly allowed more high temperature than low temperature records to be broken. However, this is a low enough amount of warming that there should still be a few cold temperature records being set, since the weather is so highly variable.
The statistics support this position. The Waterloo, Iowa mark was only the second time this decade that an all-time record cold temperature has been set at a major U.S. city. The cities I consider are the 303 cities author Chris Burt tracks in his excellent Extreme Weather book. The cites chosen were selected based primarily on their length of weather records (all the records go back to at least 1948, with most going back to the 1800s), and include all the largest cities in the U.S. The only other all-time coldest temperature record set at these cities this decade was the -44°F recorded in Grand Forks, North Dakota on 1/30/2004. By contrast, 49 all-time high temperature marks have been set this decade (Figure 1).
Perhaps a better judge of the impact of global warming on extreme temperatures, though, is to look at record warmest and coldest months. Month-long records are more reflective of the climate than an extreme event lasting just a few days. No all-time coldest month records were at any U.S. cities during January 2009, and it was not even close. Despite the cold blast of Jan. 11-18, the month of January finished out above average in temperature for the lower 48 states. So far this decade, no U.S. major city has set an all-time coldest month record. The last time a coldest month record was set occurred in January, 1994 when Caribou Maine and Bayfield, Wisconsin recorded their coldest month. By contrast, there have been 61 all-time warmest month records set in those same 303 cities between 2000 and 2008 (Figure 1). The summer of 2007 alone saw 42 all-time high (or warmest month ever) records. Just one record was set in the summer of 2008.
Figure 1. Minimum and maximum temperatures records for the U.S. for 303 major stations. The image has been updated through January 2009 to include the one record low set that month. The original version of this image was for 2007, and I modified it to update it for four changes made in the 2008 data. The numbers for the decade of the 2000s are correct, but there are four (out of 606) records that need to be subtracted off some of the earlier decades. Note the the 1930s were the most extreme decade for total number of records set, but the 1920s were the least extreme. U.S. weather has a high degree of variability from decade to decade. Image credit: Chris Burt, Extreme Weather.
Are the pattern of U.S. temperature records due to the Urban Heat Island effect?
There have been 110 all-time high temperature or all-time warmest month records set at the 303 major U.S. cities this decade, and only two such low temperature records set. Is this disparity due to global warming, or the Urban Heat Island effect? The Urban Heat Island (UHI) effect occurs when development of former natural areas into pavement and buildings allows more heat to be trapped in cities, particularly at night. During the day, the UHI effect often leads to a slight cooling, since it can increase the amount of turbulence, allowing cooler air to get mixed down to the surface. For example, Moreno-Garcia (1994) found that Barcelona, Spain was 0.2°C cooler for daily maxima and 2.9°C warmer for minima than a nearby rural station.
However, temperature records are typically taken in parks and airports removed from the main heat-trapping areas of cities, and are not as strongly affected as one might expect. There are several reasons for this. One is that when tall buildings are present, they tend to block the view to the sky, meaning that not as much heat can escape upwards. In addition, the presence of moist vegetation keeps the atmosphere moister in park-like areas (which include the grassy fields near airports where temperature measurements are taken). This extra moisture helps cool the atmosphere on a local scale of tens of meters, due to latent heat effects (the energy required to convert liquid water to water vapor). Peterson (2003) found that "Contrary to generally accepted wisdom, no statistically significant impact of urbanization could be found in annual temperatures." The study used satellite-based night-light detection to identify urban areas. Recent research by Spronken-Smith and Oke (1998) concluded that there was a marked park cool island effect within the Urban Heat Island. They found that parks in typical cities in the U.S. have temperatures 1 - 2°C cooler than the surrounding city--and sometimes more than 5°C cooler. While the Urban Heat Island effect probably has contributed to some of the reduction in record low temperatures in the U.S. in the past decade, research by Parker (2004, 2006) and Peterson (2003) theorizes that Urban Heat Island effect is a factor ten or more less important than rising temperatures due to global warming.
Is the Urban Heat Island effect partially responsible for global warming?
Global warming is affecting the entire Earth, including rural areas far from cities, and the 70% of the world covered by ocean. Thus, the Urban Heat Island effect--if not corrected for--can cause only a small impact on the global temperature figures. Since the Urban Heat Island is corrected for, the impact on the observed global warming signal should be negligible. For instance, NASA uses satellite-derived night light observations to classify stations as rural and urban and corrects the urban stations so that they match the trends from the rural stations before gridding the data. Other techniques (such as correcting for population growth) have also been used. Despite these corrections, and the fact that the Urban Heat Island effect impacts only a relatively small portion of the globe, global warming skeptics have persistently used the Urban Heat Island effect to attack the validity of global warming. There are no published peer-reviewed scientific studies that support these attacks.
Parker, D.E., 2004, "Large-Scale Warming is not Urban", Nature 432, 290, doi:10.1038/432290a, 2004.
Parker, D.E., 2006, "A Demonstration that Large-Scale Warming is not Urban", J. Climate 19, pp2882-2986, 2006.
Peterson, T.C., "Assessment of urban versus rural in situ surface temperatures in the contiguous United States: No difference found", Journal of Climate, 16, 2941-2959, 2003.
Spronken-Smith, R. A., and T. R. Oke, 1998: "The thermal regime of urban parks in two cities with different summer climates. Int. J. Remote Sens., 19, 20852104.
The surface temperature record and the urban heat island, realclimate.org post, 2004.
My next post will be sometime Tue-Thu.
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