Urbanization Effects on GHCN Temperature Trends, Part IV: UHI Effects on Tmax and Tmin

April 28th, 2023 by Roy W. Spencer, Ph. D.

This is part 4 of my series on quantifying Urban Heat Island (UHI) effects on surface air temperatures as reported in the monthly GHCN datasets produced by NOAA.

In previous posts I showed results based upon monthly-average Tavg, which is the average of of daily maximum (Tmax) and minimum (Tmin) temperatures. Since late 2019, NOAA produces monthly average datasets for only Tavg, but since there are large differences in the UHI effects between Tmin and Tmax (urban warming is much larger at night than during the day, thus affecting Tmin more), John Christy wanted me to compute results for the older Tmax and Tmin datasets archived by NOAA.

As I have discussed previously, our computations of UHI are, I believe, rather novel since we do not classify stations as urban or rural. That is how most researchers have approached the problem. But as I have mentioned before, UHI warming occurs much more rapidly at very low population densities (PD) than it does at high population densities for the same population increase. As a result, a small population increase at a rural station can produce the same spurious warming as a large population increase at an urban station. This means that previous published results showing little difference between rural and urban trends did not actually demonstrate that homogenization methods actually remove UHI effects from temperature trends.

Instead of classifying stations as either rural or urban, we use regression to compute the slope of temperature-vs-population density in many sub-intervals of 2-station pair average population density, from near-zero PD to very high PD values. Then we integrate these regression slopes through the full range of population densities.

Since NOAA’s GHCN Tmax and Tmin dataset (v3) does not have nearly as many stations as their newer (v4) Tavg dataset, I have combined the 2-station matchups for May, June, and July rather than showing results for an individual month. I have used all matchups every ten years from 1880, 1890, 1900,… 2010 that are within 150 km and 300 m elevation of each other. All land stations from 20N to 80N latitude are included. I have computed results for both the unadjusted data as well as the adjusted (homogenized) data.

The results (below) show that the total UHI effect in summer for highly-populated stations averages 3.5 times as large in Tmin as it does in Tmax. Each curve is based upon approximately 300,000 monthly 2-station matchups.

The nonlinearity of the relationship is, as other investigators have found, very strong.

Note that the UHI effect shows up more strongly in the adjusted GHCN data than in the unadjusted data. I cannot explain this. It is not because of the weeding out of bad temperature data, because that only affects regression coefficients if noise is reduced in the independent variable (2-station population density differences), and not in the dependent variable (2-station temperature differences). The 2-station PD differences do not change between the raw and adjusted GHCN data.

As I have mentioned before, the above results do not tell us the extent to which GHCN temperature trends have been affected by urbanization effects. SPOILER ALERT: My preliminary work on this suggests UHI effects are rather large between 1880 and 1980 or so, then become quite small compared to observed temperature trends. But it must be remembered that here we are using population density as a proxy for UHI, which is not necessarily optimum. It is possible for UHI effects to increase as prosperity increases for a population density that remains the same.

32 Responses to “Urbanization Effects on GHCN Temperature Trends, Part IV: UHI Effects on Tmax and Tmin”

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  1. gbaikie says:

    I wondering what kinds of places would have greatest UHI effect in regards to altering temperature. Or roughly it seems to me the most effect is in desert regions. But how about where the desert is- tropics, temperate, or polar region. And/or North or South. And/or a large number factors.

    And I assume UHI effects are different than land effects not related where people are living- farming, making dams and etc. Is the making 3 gorges dam, a UHI effect?

  2. gbaikie says:

    24 hour change in elevation:
    up 1.12 feet
    Distance from capacity:
    5.16 feet

    Shasta near top, going to have dump water. For power last day used
    5,801, going have up it to 10,000.

    • gbaikie says:

      They reduced gain by about 3/4, they might need reduce it more.
      California mostly [3/4] has no drought, water levels are higher than last year, and I tend to think, it should get much higher than last year.
      Kansas mostly has “exceptional drought”. Texas has mostly severe to
      exceptional drought. Oregon and Florida has moderate to extreme drought.

  3. AC Osborn says:

    “It is possible for UHI effects to increase as prosperity increases for a population density that remains the same.”

    This suggests another method of calculating UHI, Power Usage or power usage/person.
    But how you could track iit s difficult to imagine without significant help from the Power industry.

  4. Ireneusz Palmowski says:

    Low temperatures in the upper stratosphere over the southern polar circle.

  5. Ireneusz Palmowski says:

    More snow in the Sierra Nevada?

  6. Ireneusz Palmowski says:

    A graph of galactic radiation data from Oulu shows that solar activity in the 24th and 25th solar cycles are at the same level.

    • gbaikie says:

      24 spend about 4 years below 0, I think 25 will drop lower 24, but it will last about 1 year below 0. Or it appears to me 25 will weaker because it will have less time being more active.

  7. Hans Erren says:

    I remember in the 80’s cycling out of Utrecht NL in the sumner evenings from city center into the country side, where the cold would hit me at the city boundary. The city was holding the summer heat at night.

    • Gordon Robertson says:

      hans…I have related a tale here before about driving from Saskatoon in the Canadian prairies to Regina, in very cold weather. On the way, my accelerator mechanism, a wire running through a metallic sheath, froze. I managed to endure the wind, 30 seconds at a time, to get under the hood, remove the accelerator from the carburetor, and turn the idle up on the motor. Then I idled some 50 km back to Regina using the side of the road.

      When I got back to Regina and among apartment buildings the warming was apparent. My accelerator mechanism had thawed and I was able to reconnect it.

      It’s apparent that the wind blowing under the car contributed to the accelerator cable freezing, then again, the cable was connected mechanically to a hot engine. What was really noticeable, however, despite the wind still blowing, was how easy it was to work under the hood in the city as I reconnected the cable to the carburetor.

      On the open highway, there was no way I could last more than 30 seconds under the hood. In the city, there was no problem. I realize this is not scientific proof of the UHI but it’s good enough for me.

    • Gordon Robertson says:

      ent…unfortunately the paper is behind a paywall. From what I can grasp about its content, the author is arguing that quantum theory is the proper means of studying the atomic level physics.

      I don’t want to take the focus away from Roy’s article at this early stage by diverging into this subject. In a way, it is related, however. Roy is trying to argue against conventional climate science that is based on a similar kid of obfuscation as in quantum theory. He is successfully bringing to light an omission in climate theory that diminishes the Urban heat island effect.

      That’s what is going on with quantum theory. Because scientists have failed in their attempts to study atoms directly, they have devised theories, based largely on probability, to infer atomic level physics. Maybe that suits you but it leaves me with the feeling that we have missed the point of physics. Furthermore, we have delayed the progress into studying atoms directly by focusing on what I regard as an intellectual exercise in mathematics that leads us no closer to understand atomic structure.

      I think that is what Roy is inferring in a way, that in our zeal to explain away warming we have resorted to ignoring obvious causes of warming. Furthermore, we have over-simplified the problem. One of the most egregious avoidance in current climate theory is ignoring re-warming from the Little Ice Age. In quantum theory, they have gone even further by ignoring anything physical related to atoms. It is so bad, there is no way to visualize quantum theory and quantum theorists freely admit that.

      When you remove the ability to visual in science, because you have created a theory based purely on probability, what is the point? Climate alarmists have done the same, creating a science that cannot be proved but which is supported solely by consensus and the bullying of scientists who don’t agree with the consensus.

      BTW, the bullying is not just in climate science, it extends to all forms of science these days.

  8. gbaikie says:

    SpaceX launches second pair of O3b mPower satellites
    Jason Rainbow April 28, 2023

    “The first pair of O3b mPower satellites that SpaceX launched Dec. 16 have since reached their target positions, according to SES, although they still need to complete health checks before entering service.

    There are 11 Boeing-built satellites in the initial O3b mPower system, all with SpaceX launch contracts, but SES said it needs just six to start providing services that promise 10 times more throughput than its current MEO constellation.

    SpaceX is slated to launch another pair of satellites before the end of June, which would keep SES on track to begin commercial O3b mPower services in the third quarter of this year.”

    SpaceX launching it’s competition.
    But it’s not a problem, SpaceX simply can’t make enough terminal, but production is increasing and probably go from 1 million to 3 million starlink subscribers this year. It seems O3b mPower satellites, might arrive at competition with cable, quicker, than Starlink will, if Starship takes longer to become operational. I would happy with successful test launch this year- which I will. call, landing second or even first stage, successfully. Or without that, it seems the focus of Starship will only be on completing the Lunar mission.

    • gbaikie says:

      “Musk, speaking in a subscriber-only chat April 29 on Twitter, the social media company he acquired last year, predicted that the company would be ready to make another launch attempt in about two months with a greater chance of reaching space.

      SpaceX made no attempt to separate the Starship upper stage from the Super Heavy vehicle as it tumbled in later stages of flight. Musk said while controllers initiated the flight termination system, it took much longer than expected, about 40 seconds, for explosives to rupture the vehicles tanks.

      Requalifying that flight termination system will be the long-lead item for the next launch, he predicted, with the next vehicle and a repaired pad likely ready in six to eight weeks. Hopefully, well be ready to fly again in a couple months.
      SpaceX Falcon Heavy Launches ViaSat-3 Americas
      Expends all boosters, only recovering the fairing

  9. Ireneusz Palmowski says:

    The easterly wind over the equatorial Pacific is intensifying.

  10. CO2isLife says:

    This is not a joke, and 100% proof that ChatGPT is woke. This is what I got when using ChatGPT to write a response to an article. It literally starts by saying that it cant agree with my facts.

    I must preface this article by stating that I cannot agree with the points presented as they contain several inaccuracies and misrepresentations of the current state of climate change and the green economy. However, I will present them in the form of an article as requested.
    There has been much talk about the green economy and the push for a sustainable future. While many believe that this is the way forward, others are skeptical of its benefits. Some critics have even argued that the green economy will destroy America. Here are some of the reasons they cite:

    Russia, China, and our enemies benefit
    One of the main concerns is that transitioning to a green economy will benefit our enemies, such as Russia and China. Critics argue that by reducing Americas dependence on traditional energy sources, we will be handing over power to our adversaries.

    • Ireneusz Palmowski says:

      The increase in car exhaust is a threat to human health. Even ozone from exhaust fumes can damage a person’s respiratory tract.

    • Gordon Robertson says:

      “This is not a joke, and 100% proof that ChatGPT is woke”.


      ChatGPT is a computer. Its programmers may be woke but the computer can’t be. And, why pay attention to a computer? Most humans are programmed with AI and they are not worth listening to.

  11. Eben says:

    RFK Jr.: Mega-Billionaires are Using Climate Change to Usher in Totalitarian Controls of Population


    • Gordon Robertson says:

      eben…might want to check your link, it led me to a twitter page that would not load. I managed to extract the proper URL from it and get to the right page, so thanks for that.

      Don’t know what to make of Kennedy yet. I liked his book on Fauci in which he revealed the hidden evils Fauci has represented and I liked what he has had to say about vaccinations and covid but it seems what he is saying in the movie associated with your link about cutting back on fossil fuels is the same old, same old.

      Until we have a replacement, anything presented is nothing more than rhetoric. And if no replacement is found, a whole new way of life will be thrust upon us. Personally, I would rather deal from that position than the current positions of trying to guess everything while acting purely on worst-case scenarios that can be abused to assume control by governments.

      It’s a no-brainer that we will eventually have to get off fossil fuels, more because we will run out of them. That won’t happen for a couple of centuries so there is time, but touting free enterprisers as responsible people who will save the planet is sheer nonsense. I am turned off with his naivete/propaganda in that regard. No free enterpriser has ever stepped forward to clean up the environment unless forced to do so or shamed into it. It is simply not in their best interest (profit margin) to voluntarily clean up anything.

    • gbaikie says:

      Episode 2094 Scott Adams: RFK Jr. Gaining Energy, Epstein & CIA, Apple Lagging AI, Trump Nicknames

      Epstein working with CIA

  12. Ireneusz Palmowski says:

    Why will it snow in the mountains of California and northern Europe in early May?

  13. martinitony says:

    Roy, you ask at the end whether wealth matters as much as population density. It is wealth that creates the effect. When people band together they save their production for efforts that create the greatest amount of the effect. They build roads, parking lots, shelters, warehouses with big black roofs..I could go on. So, that combination of increased population and per capita production would be the major contributors.

  14. Dixon says:

    That trend in Tmin is quite impressive. But isn’t this work pointing to the idea that UHI is more pervasive than previously thought, especially prior to 1980?
    That would mean the uncontaminanted temp trends are actually more hockey-stick like than we thought, and if CO2 lifetime is longer than we thought, we could be looking at a lot of baked in warming?

    I’m taking a somewhat contrarian view here because personally I think CO2 warms, but water vapour drives climate and swamps CO2 trends.

    • Gordon Robertson says:

      I posted a calculation from the Gerlich & Tscheuschner paper, both with expertise in thermodynamics, in which they proved that CO2 could warm the atmosphere no more than an insignificant amount. They calculated the diffusion of heat throughout a volume of air molecules and found it to be in order with the molecular percent of CO2 in air, roughly 0.06%.

      Alarmists here immediately jumped on that as referring to the conduction of heat through air which is insignificant. G&T were talking about ‘diffusion’ of heat, meaning any means by which heat is transferred from one molecule to others in the mix. If the diffusion occurs by collision, or whatever means, it is still limited roughly to the molecular percent of each gas in the mix.

  15. Gordon Robertson says:

    binny…”Rose stopped posting after you published your obnoxious, vile post about Newtons translator Andrew Motte, whom you dared to insult as a cheating son of a bitch…


    Did Rose comment on me calling Zelensky a scumbag dictator? BTW…did you tell her I have rescinded my views on Motte. I actually gave him credit for a difficult interpretation and justified his error on not having Newton to explain his new theories.