From Dr. Global Warming Blog. Spencer
by Roy W. Spencer, Ph. D.
This is the first of many posts on the urban heat island (UHI) effect on daily high temperatures. Previous UHI work has used GHCN monthly average station data “Tavg” (average daily maximum (Tmax) and minimum (Tmin) temperature. So, I moved from Tavg to Tmax (because it records very high temperatures), and daily values rather than monthly (although I will also include monthly results to give context).
This post is mostly a teaser. Finally, I will describe a new dimension to the work of UHI that has just begun.
2024 Poster Child for US Warming: Palm Springs, CA
I was guided by a Google search on US record high temperatures for 2024, and it seems that Palm Springs, California is the place to start.
With a name like “Palm Springs” this place sounds like a good place to lounge under palm trees and enjoy the cool, refreshing spring water that surrounds you. However, the location is mostly desert, with the original downtown spring spewing out 26 gallons per minute of hot water. “Palms” do exist… as “desert palms”, naturally growing in clusters where water from melting mountain snow passes through the cracks associated with the San Andreas fault.
Like all US metropolitan areas, population growth in Palm Springs over the past 100+ years has been rapid. Even in the last 50 years the population has almost doubled. The natural desert surface has been replaced by pavements and roofs, which reach higher temperatures than the original desert soil, and the “impervious” nature of the artificial surface (low air content) means that heat is conducted downwards, leading to long-term storage of excess . heat energy and, on average, a higher temperature. More on “opaque” surfaces later….
Palm Springs Airport Weather Observation Site
The Google Earth image below shows the current location of the ASOS (Automated Surface Observing System) site at Palm Springs Airport, which recorded a record high temperature of 124 degrees. F on July 5 this year.
What is rather interesting is that the ASOS meteorological instrument location guide prefers natural surfaces to be placed on, but since most of these weather stations are located at airports (and because they mainly support the needs of aviation weather, not the needs of climate monitoring), “natural” locations are usually. right next to runways, airplanes, and asphalt roads.
The next Google Earth image is zoomed in to show the greater Palm Springs area, with the ASOS site in the center (click the image to enlarge, then click to zoom in).
July Temperature Record and Urbanization
It only makes sense that people want to know the temperature where they live, and most of the US population lives in urban or suburban locations. However, the temperature they experience is probably higher than before people moved there and started building roads, buildings, and airports.
But what is wrong for those who follow the global warming narrative is that the high temperatures reported in these locations almost always cite climate change as the cause, but they have no way of knowing how much urbanization has contributed to record high temperatures. (Remember, even without global warming, high temperature records will continue to be broken as urbanization increases).
As mentioned above, on July 5, 2024 Palm Springs broke the all-time high temperature record, reaching 124 deg. F. There are 26 GHCN stations every other day within 40 miles of Palm Springs, all with varying levels of urbanization, but even more importantly, at very different elevations. If we plot the July 5 high temperature reported at that station as a function of station elevation, we see that Palm Springs is an “outlier,” 5 degrees warmer than expected based on elevation-corrected temperatures (dashed regression). row):
Now, keep in mind that many (if not most) of the surrounding 26 stations have their own degree of urbanization, making them hotter than no sidewalks and roofs. So, 5 deg. F excess likely underestimate how much urban warming contributed to Palm Springs record high temperatures. Palm Springs was incorporated in 1938, and has grown in population since World War II.
If you’re interested in seeing how the previous plot looks averaging all July temperatures, here it is:
For the month of July, Palm Springs averages 3 deg. F is warmer than the surrounding stations (after adjusting for elevation effects, and keeping in mind that most of the *other* stations likely have their own level of urbanization).
Clearly, Palm Springs has a spurious warming effect from the airport and surrounding urbanization that did not exist 100 years ago. But how much?
Impervious surface data as a surrogate for Urbanization
This blog post is a prelude to a new project we have started where we will compare daily (as well as monthly) temperatures to a relatively new USGS data set of annual impervious surface coverage from 1985 to 2023, based on Landsat data. I had previously experimented with a “Built Up” dataset based on Landsat data, but it turned out to be just buildings. The “impervious surfaces” dataset is the one that I believe will have the biggest direct physical connections that cause UHI warming: roads, parking lots, roofs, etc. I think this will produce more accurate results (even though it is only ~40 years long) than my population density work (which is, we hope, close to being accepted for publication).
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