“But that’s at the airport!”
Since 1995, Denver’s official weather records have been kept at Denver International Airport. While the airport’s roughly 20-mile distance from downtown is mostly insignificant in terms of accurate record keeping, there are tangible differences between temperature, precipitation and snowfall between downtown Denver and the National Weather Service’s official observation site.
Because of the airport’s distance from downtown and a perceived difference in weather conditions, complaints are common when official measurements at the airport vary from what other parts of the metro area may have experienced.
An analysis of the differences between downtown Denver, the old Stapleton Airport’s weather observation site and Denver International Airport, with assistance from the National Weather Service and the Colorado Climate Center, shows that downtown Denver is warmer, wetter and less snowy than both the Stapleton and Denver International Airport sites. A combination of an urban heat island effect, localized climate factors and possible differences in measurements are likely the primary driving factors behind the differences.
This is based on a 20-year comparison (1998 to 2018) between downtown Denver’s observation site at the Denver Water Department, located just southeast of the intersection of Colfax Avenue and Speer Boulevard, and DIA and the old Stapleton Airport.
Based on records at the Denver Water Department’s downtown weather station, temperatures between 1998 and 2018 were 2.8 degrees warmer (53 degrees) than those recorded at DIA. DIA’s average temperature of 50.2 degrees between 1998 and 2018 is almost an identical match with temperatures at the Stapleton Airport site during the same time frame.
The gap in temperature is likely due to the urban heat island effect — the reduction in vegetation and an increase of paved surfaces in city settings. This reduces overall evapotranspiration, creating less cooling processes, and in turn increasing temperatures in urban environments.
“There’s not a lot of trees or grasses (downtown). There’s a lot of pavement. That would definitely contribute to an urban heat island,” said Becky Bolinger, the assistant Colorado state climatologist with the Colorado Climate Center. “It’s going to feel a lot warmer. You’ve got a little bit more evapotranspiration in an area like Stapleton, (where) you’ve got a bit more foliage.”
Downtown Denver also received noticeably more precipitation between 1998 and 2018. DIA averaged 14.03 inches of total annual precipitation between 1998 and 2018, compared to 16.08 inches downtown. That could be due to a combination of factors, but the precipitation gap is likely because of a gradual weakening of summer storms as they move east.
“We might get more rainfall in the summertime as a storm develops over the mountains and moves overhead,” said Dr. Sam Ng, a professor of meteorology at Metro State University. “The (storms) may not have enough of a push to make it to the eastern side of the metro area.”
Downtown Denver also averaged about 10 inches less snowfall between 2006 and 2018. Downtown averaged 37.2 inches of snow for the time period, compared to 49.1 at DIA and 45.2 at Stapleton. DIA began officially tracking snowfall in February 2006, changing the comparison dates for snowfall to the temperatures and precipitation measurements for this analysis.
Warmer temperatures and hyper-local topographic features are likely behind the differences on the snow front.
There are, however, several important asterisks to all of these findings.
For one, DIA’s rain gauge is different from the ones at Stapleton and downtown, different enough that there could potentially be a bit of an “under catch” at DIA, according to Bolinger. The DIA rain gauge is automated, while the Stapleton and downtown gauges are physically measured by a person. Automated stations may slightly evaporate moisture as it’s absorbed by the measuring device.
“I would partially think (the gap in precipitation between downtown and DIA) is due to the tipping bucket,” Bolinger said, in reference to the type of instrumentation used to measure precipitation. “(DIA) has a tipping bucket gauge, which is automated, while at the (other) stations, you go out to measure. There might be a little bit of extra evaporation.”
Another factor could be the range of the individual thermometers assessing temperatures.
“Most thermometers have a 2.5-degree plus-or-minus threshold,” Dr. Ng said. “The (2.8 degree) difference isn’t all that great.”
Why does all of this matter? For one, official climate records are an important part of tracking changes in the climate, and possible variations or trends. Denver’s official weather station has varied from downtown Denver (1882 to 1949) to Stapleton Airport (1950 to 2007) to DIA (2008 to present). Knowing potential local biases can impact everything from simple curiosity about how much rain or snow fell on a certain day to influencing public policy.
Case in point: These statistics appear to strongly suggest the existence of an urban heat island in Denver, which was a large part of the Green Roof Initiative backers’ argument before the 2018 vote.
Chris Bianchi is a meteorologist for WeatherNation TV.
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Updated April 13, 2019 at 8:09 a.m. Due to a reporting error, this story has been corrected to reflect the true date observances began at Denver International Airport.
