Repost from Dr. Roy Spencer’s Global Warming Blog
November 10, 2022 by Roy W. Spencer, Ph. D.
Now that I’m back to studying the surface air temperature profile and the Urban Heat Island (UHI) effect, I decided to revisit the temperatures in Las Vegas, Nevada. It’s been more than 8 years since I posted about Las Vegas being the poster kid for the UHI effect and I came up with some warming trend calculations from hourly temperature data at McCarren International Airport (now Harry Reid International Airport… no kidding), which suggests that most of the warming has come from urban heat islands, not global climate change.
And here’s the problem with tracking global climate trends – most land data is collected where people build things… more and more. In June of last year, Guardianspredictably, has combined the urban heat island effect with climate change as it claims,
“Driven by the climate crisis and enhanced by the city’s increasingly aggressive growth, Vegas has been heating up – and it’s only going to get worse.”
Many people don’t really distinguish between the two. It makes sense to ask, how much has the area around Las Vegas warmed over the past few decades, compared to the city itself? The problem is that there are very few locations that measure hourly temperatures with data spanning at least 50 years in rural areas. The area is, after all, a desert and people don’t usually choose to live in such locations.
I calculated 50-year trends for Las Vegas and for a rural station in Nevada, Winnemucca from 24-hour data, allowing us to see how trends change over time of day. I did this during the warmest half of the year, from April to September. The following plot reveals a remarkable feature… the dramatic warming in Las Vegas that took place entirely at night. Winnemucca shows a background climate signal, with a fairly uniform (and weak) warming trend throughout the day. But the impervious surfaces in Vegas – buildings, concrete, asphalt – absorb more sunlight during the day than the surrounding desert, and then at night release that heat into atmosphere.
Part of the reason this is happening is because the background of the city is lower than the surrounding desert (thanks to Anthony Watts for reminding me of this). But at least, the important thing is that concrete has a coefficient of thermal conductivity 9 times greater than sand, so when it is heated by the sun, more energy is stored in the pavement. The sand will become extremely hot, but only at the surface, and the additional energy will radiate away (infrared) as well as promote stronger (dry) atmospheric convection that will carry that heat into the day. day.
Why wouldn’t such a thing appear during the day? Because turbulent mixing is driven by a powerful super adiabatic lapse rate near the surface that spreads heat up into the atmosphere and cooler air descends to displace it, cooling the city during the day. . But then at night, temperature inversions form and the lowest layers of the atmosphere no longer exchange convective energy with the higher altitudes. In fact, the dramatic nighttime reversal that occurred naturally in the desert weakened the city as the pavement released the extra energy it had stored up during the day.
The actual background climate has warmed over the last 50 years in Las Vegas (whatever its cause), based on the chart above, it seems to be around 0.25 degrees. C/decade. This is also part of why it is important to monitor global temperature trends with satellite measurements of the deep troposphere – it provides a more robust measurement that is not affected by the surface effects, such as the Urban Heat Island, and avoid the association of hot Las Vegas with the “climate crisis”.
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