Boulder Weather Summary for 2009 and Decade

Steve Jones and George Oetzel

Boulder's 2009 weather year was much snowier, slightly wetter, and slightly cooler than average [but see caveat in paragraph 2]. The official weather station at the National Institute of Standards (NIST) South Broadway labs recorded 124" of snow, about 41" more than the long-term recorded average. The 22.14" of precipitation recorded at NIST was about 3" more than the 1897-2008 Boulder average. The mean daily temperature was -0.5° F below the 112-year average. The local conditions contrast with the global weather for 2009 and the decade, which a NASA report describes as the second warmest year and the warmest decade on record. (Brief summary of NASA report, with link.)

However, don't forget that the weather station has moved several times since record-keeping began (from downtown Boulder to University Hill to downtown Boulder--including the roof of a downtown firehouse--to NIST), and the current NIST site may be snowier and wetter, and possibly cooler, than previous sites. Also, snow measuring techniques have changed over the years, resulting in slightly higher snowfall amounts being recorded. In addition, temperature-reporting techniques have varied (reporting high and low temperatures from midnight to midnight, for example, results in different mean daily temperatures than you get if you report high and low temperatures from 5 p.m. to 5 p.m.).

Until October 10, this was one of the most pleasant weather years anyone could remember. A warm (though snowy) winter was followed by a mild, wet spring and a cool summer. Then wintry weather struck in spades. We received 30" of snow during the last three weeks of October, and the mean daily temperature for the month was more than 8° below average. An additional 26"of snow fell in December, when the mean daily temperature ran more than 7° below average. The temperature bottomed out at -12° on December 9, and we experienced a total of four sub-zero nights during the year. The year's high temperature was a relatively mild 96° F, and there were only 13 days when the temperature reached 90° or higher.

2009 was only the third year this decade with reported below-average temperatures in Boulder. Mean daily temperatures from 2000-2009 were about 0.4° F above the 1897-1999 average. Mean annual precipitation was 19.1", roughly equal to the 1897-1999 average and 1.7" below the 1990-2009 (NIST weather station) average.

So what about droughts and global warming? Boulder did experience a significant drought, from 2000-2002, when total precipitation was 17% below the 1897-1999 average. However, this drought was more severe in southwestern Colorado, extensive areas of the Great Basin, and areas of the northern plains, and the media tended to overgeneralize its effects locally, leading to exaggeration of the drought's severity in eastern Colorado. (Also, the Denver media continue to compare DIA data with historic Denver data without noting that the referenced weather stations are 20 miles and half a life zone apart). Nevertheless, some recording stations in Boulder County were the driest they've been during any three-year period since record-keeping began.

As for global warming, the recent Climate Change in Colorado Report for the Colorado Water Conservation Board, based on the best available data from weather stations throughout the state, concluded that the state's mean daily temperature for 2000-2008 was 1.5° F warmer than the 1930-1999 average. Every year from 2000-2008 was warmer than average. The report projected an additional 2.5° F of warming by 2050. The jury is still out on future precipitation trends.

The graphs compare monthly means over the perios 1961-2009. Weather recording was on the grounds of the Boulder Central Fire Station, north of downtown, from 1958 to 1989. There was no recording station from 8/1/89 through 3/31/90. Beginning 4/1/90, the current station on the NIST campus was established. Correspondingly, the graphs use different symbols for the two periods, and there are two lines that reflect the averages.

January has a generally increasing trend since the NIST station was established; 2007 was the coldest during that time. July also shows an increasing trend through the 90's, but it has leveled off since 2000. Of course, these trends should not be regarded as indicators of the effect in Boulder of global warming. Eighteen years is much too short an interval to provide evidence of a long-term trend.

The adjacent figure shows Boulder's annual precipitration for the period 1961 through 2009. As in the above figures, the two horizontal lines represent averages for the periods of the two different measuring stations. It is likely that the NISTstation would have recorded more precipitation than the downtown station if they had operated in the same years, because it is closer to the uplift of the Flatirons. The graph shows that the decade of the '90s was generally wetter than average, while the current decade has seen more dry than wet years. Sources: NOAA Earth System Research Laboratory: http://www.esrl.noaa.gov/psd/boulder//getdata.html Colorado Water Conservation Board . 2009. Climate Change in Colorado.

B.C.N.A.
P.O. Box 493
Boulder, CO 80306