Las Vegas: Poster Child for the Urban Heat Island Effect (updated & corrected)

June 26th, 2014 by Roy W. Spencer, Ph. D.

(This post supercedes yesterday’s post, in which I used the unadjusted USHCN temperature data. The conclusion remains the same with the adjusted USHCN data…the official Las Vegas temperature record contains a large urban heat island warming effect which is spurious to the climate signal.)

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.

This can been seen from the raw temperature data, which shows that daytime warming has been modest and nighttime warming has been strong (over 3 deg. F since 1973), whereas the official, adjusted USHCN Tmax and Tmin trends are even warmer than the raw temperature trends, even at night, and are equal to each other:

Fig. 1. Las Vegas temperature trends during 1973-2013 (all months) from raw 3-hrly temperatures versus USHCN adjusted temperatures. Shading represent nighttime hours

Fig. 1. Las Vegas temperature trends during 1973-2013 (all months) from raw 3-hrly temperatures versus USHCN adjusted temperatures. Shading represent nighttime hours.

No matter what instrumentation changes occurred in the raw temperature record, there is no way they cause a nighttime bias that large compared to the daytime (speaking as a former certified aviation weather observer). The USHCN plot provided by Anthony shows 10 deg. F (!) of nighttime warming since the late 1930s, which is simply not a credible representation of the non-urban environment.

The most logical explanation for the raw 3-hourly temperature differences between day and night is related to the dramatic growth Las Vegas has experienced in the last 40 years. 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.

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 just for July.

Fig. 2. As in Fig. 1, but just for July.

It appears that the “homogenization” adjustment performed on the USHCN data has inadvertently used the spurious nighttime warming as truth, and made the daytime warming match it. Given what we know about how urban environments retain heat at night, the exact opposite should have happened. In fact, given that there should also be an urban warming signal during the day, it might well be there has been no real climate-related warming in Las Vegas in the last 40 years.

The net result is that the official (adjusted USHCN) Las Vegas warming trends appear to be dominated by local urban heat island effects.

8 Responses to “Las Vegas: Poster Child for the Urban Heat Island Effect (updated & corrected)”

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  1. Johan says:

    One simply has to admire scientists for so cleverly stating the obvious.

    “These observational results … indicate an increase in the urban heat island amplitude with increase in city size that is consistent among cities across a broad climatic range. This relationship indicates that the energy consumption required for cooling is likely to increase with urban growth, especially during summertime.”

    An excerpt from the abstract of Zhang, P.; Imhoff, M. L.; Wolfe, R. E.; Bounoua, L, The relationship between remotely-sensed surface parameters and urban heat islands in the USA, American Geophysical Union, Fall Meeting 2008.

  2. Mary Brown says:

    Are there any reasonable, nearby rural stations for comparison ?

  3. ossqss says:

    Hummm, just went to a publix food store at 10pm and the recorded temp via the local sensor was 82. My car still showed 91 in that rural parking lot.

    Just sayin, it was a cloudy day too. UHI made me sweat.

    Albeit, I would rather sweat than shiver.

    Keep up the good work Doc!

    Did anyone get a picture of bergs on Superior at Summer Solstice, or at least toss a snowball?

  4. Mac says:

    Has anyone ever graphed electrical power consumption vs temperature over the last 40 years? I wonder if they march hand in hand or if electricity out paces UHI…

  5. Mary Brown, in a BBC ‘Supposed to be scary Climate Program’ called “Earth; The Climate Wars”, Iain Stewart who presented the program, did among all those – (can I say, in my opinion): “idiotic things he did, he did a few things which I kind of liked.

    One of those things was that he measured (an ordinary thermometer was used) the night temperature in Las Vegas and then he took the same thermometer out into the near by desert and found that – if memory serves me right – the desert night was some 5 deg. C cooler than the city.

  6. Well, that was just 2 different measurements made by one man on one night and can hardly be said to be “Scientific” as at a rough guess I will say there are probably 2 different places in any city (including Vegas) that have 2 different temperature values at any given time of the day or night

  7. Christopher Game says:

    The post supersedes its previous day’s post. ‘Supercede’ is a mis-spelling of supersede.

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