Las Vegas: Poster Child for the Urban Heat Island Effect

June 25th, 2014 by Roy W. Spencer, Ph. D.

(This post is being superceded, as John Christy has pointed out I used the unadjusted USHCN data for this.)

As many of you are aware, Heartland’s 9th International Conference on Climate Change (aka the “skeptics conference”) will be held in Las Vegas, Nevada during July 7-9, 2014.

Anthony Watts has already posted some July temperature statistics for Las Vegas…basically, it’s really hot there in July.

What is notable about the “official” surface temperature record there is the strong urban heat island (UHI) effect which still remains in the USHCN data. Daytime warming has been modest, but nighttime warming has been spectacular….10 deg. F or more since the 1940s.

How does something like this spurious warming still remain in the USHCN data? The “homogenization” adjustment procedure that NOAA uses in USHCN apparently does not effectively remove the spurious warming. Anthony has posted extensive evidence regarding this issue in the past.

I examined the raw 3-hourly temperatures (from NOAA Integrated Surface Hourly [ISH] data) collected at McCarran Airport in Las Vegas, as well as at Nellis Air Force Base, since 1973. It clearly shows how nighttime temperatures have increased in the last 40 years compared to the daytime temperatures:

Fig. 1. Las Vegas temperature trends 1973-2013 as a function of time of day (dark shading is approx. nighttime hours).

Las Vegas temperature trends 1973-2013 as a function of time of day (dark shading is approx. nighttime hours).

What is rather remarkable, though, is that the USHCN temperature trends for the same period (1973-2013, the red stars), while agreeing with the ISH 3-hourly data for Tmax, are twice as warm as the ISH data for Tmin.

How can this be?? How can the USHCN adjustment procedures actually magnify the nighttime warming, rather than reduce it? I have no explanation for this. (Nellis AFB data are not available in the USHCN data after 1970 for some reason).

If we just focus on the month of July, the results are roughly similar to the full-year results:

Fig. 2. As in Fig. 1, but for the month of July.

Fig. 2. As in Fig. 1, but for the month of July.

Las Vegas has seen dramatic growth over the time period represented above. The number of visitors has skyrocketed from 8 million in 1973 to about 40 million today, a factor of 5 increase. The population has increased by a factor of about 6 or 7.

All of this translates into more waste heat from air conditioning, plus more artificial surfaces which warm faster than natural surfaces. If you doubt this for even the natural desert surrounding Las Vegas, look at the Landsat IR thermal imager data in this report.

During the day, the extra heat can mix convectively through a pretty deep layer of the atmosphere, which limits the daytime warming. But at night, the nocturnal inversion traps heat, magnifying the UHI effect.

The bottom line is that the USHCN data still seems to contain significant UHI effects, which are inflating warming trends, possibly even in the daytime. So, at this point, I don’t think we know if there has been any “global warming” experienced in Las Vegas.

And when it comes to the UHI effect, I doubt that the old adage “what happens in Vegas stays in Vegas” applies.

11 Responses to “Las Vegas: Poster Child for the Urban Heat Island Effect”

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  1. JohnKl says:

    Hi Roy,

    “I dont think we know if there has been any global warming experienced in Las Vegas.”

    Or anywhere else recently either. Not to worry though the data can always be adjusted, the peer review strengthened and fortified to remove dissenters and ever-increasing restrictions imposed on individual freedoms by our government to ward us from any danger real or IMAGINED!

    Have a great day!

  2. Johan says:

    Maybe all those visitors should paint the town white instead of red ?

  3. Anthony Watts says:

    Thanks Roy,

    The reason the UHI is still in the data is that there has not been any breakpoints in the data because the stations have USHCN 2 looks for such breakpoints as a test for a temperature offset.

    This has been so gradual, nothing they have will detect it short of human eyeballs, but they don’t use those at NCDC.

  4. Anthony Watts says:

    …breakpoints in the data because the stations have

    should have read:

    breakpoints in the data because the stations have been in one place with no large station moves.

  5. Streetcred says:

    “How can the USHCN adjustment procedures actually magnify the nighttime warming, rather than reduce it? I have no explanation for this.”

    LOL … Steve Goddard might know the answer 😉

  6. yonason says:

    We used to live in Philadelphia, and sometimes in the summers we would go 50 miles nearly due West to go camping. During the day, the temps in the woods would get quite warm, though never quite as high as in the city. But at night the woodland temps would plunge. While it remained in the upper 90’s (Degs F) in Philly at night, we were shivering in our tent.

    Having personally experienced UHI and it’s magnitude relative to surrounding land, I will never believe temperatures extrapolated for the country as a whole based on readings taken almost exclusively in cities.

  7. Norman says:


    I agree with your observation. My vehicle has a thermometer that measures outside air and the city environment in summer and winter is usually a few degrees (sometimes more especially in winter and at night) higher than around 10 miles outside the city limits.

    But regardless of UHI effect, other evidence is used in support of proof the overall is warming somewhat. One of the biggest is the Arctic sea ice extent. The problem is Antartic sea ice extent is now at the highest measured in satellite era for May. So you have less ice in the Arctic but more in the Antartic so is the globe actually warming overall? I am wondering if most of the Arctic low ice is more due to the position of the warm gulf stream rather than conclusive proof that Carbon Dioxide is melting the sea ice. From my observations of Arctic sea ice it seems the low ice extent is always around Barents Sea. That could be just that the warm gulf ocean is moving there for now but may change over time and not proof that the lower ice extent is carbon dioxide based.

    I still wish the debate was more scientific based (empirical evidence) than religious. I don’t blame AGW advocates for their passion. They are certain that in 100 years enormous ocean flooding will take place, the Earth will be about 10 F hotter. Many place will be unliveable because they are too hot. Crops will not grow and billions of people will die as the Earth will be able to sustain a very small population.

    The problem with the passion is it will certainly interfere with objective evaluation of all the available evidence and trying to get a valid determination of true direction of global tempratures and the resulting effects of such.

  8. ri says:

    The Satellite data has a huge error because it cant see inside the acres of casinos that are all 69 deg.
    If we want a true picture I would say the temperature has gone down not up.
    Plus who goes outside in Vegas anyway.


  9. There are some interesting differences in NCDC and Berkeley homogenizations for the Las Vegas MCC station.


    Berkeley’s homogenization reduces the trend ~50% more than NCDC. It does certainly seem like NCDC is not catching some of the urban-correlated bias in this particular case.

  10. Mary Brown says:

    Searchlight, NV (267369) has a population of less than 1000 and very similar geography. It is part of the USHCN. How to the temps compare?

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