Now that I’m back to researching the surface air temperature record and the Urban Heat Island (UHI) effect, I decided to revisit the temperatures in Las Vegas, Nevada. It’s been over 8 years since I posted about Las Vegas being the poster child for the UHI effect and I showed some warming trend calculations from the hourly temperature data at McCarren International Airport (now Harry Reid International Airport… not kidding) which suggested that much of the warming there has been from the urban heat island, not global climate change.

And this is the trouble with monitoring global climate trends — most of the land data are gathered where people build things… increasingly so. In June of last year, The Guardian, predictably, conflated the urban heat island effect with climate change when it stated,

“Driven by the climate crisis and intensified by the city’s expansive growth, Vegas is already cooking — and it is going to get worse.”

Many people don’t really make a distinction between the two. It is reasonable to ask the question, how much has the region around Las Vegas warmed in the last several decades, compared to in the city itself? The trouble is that there are few hourly temperature measurement locations with data extending back at least 50 years in the region that are rural in nature. The area is, after all, a desert, and people don’t usually choose to live in such locations.

I computed 50-year trends for Las Vegas and for a rural Nevada station, Winnemucca from 24-hourly data, which allows us to see how the trends change with time of day. I did this for the warmest half of the year, April through September. The following plot shows a remarkable feature… the strong warming in Las Vegas has been entirely at night. Winnemucca shows the background climate signal, with fairly uniform (and weak) warming trends 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 the air.

Part of the reason this happens is the albedo of the city is lower than that of the surrounding desert (thanks to Anthony Watts for reminding me of this). But at least as important is that fact that concrete has a thermal conductivity 9 times as large as sand does, so when it is heated by the sun, much more energy is stored down into the pavement. Sand would have gotten exceedingly hot, but just at the surface, and the extra energy would radiate away (infrared) as well as drive stronger atmospheric (dry) convection which would carry that heat away during the daytime.

Why would such a thing not show up during the day just as well? Because turbulent mixing driven by a strong super-adiabatic lapse rate near the surface spreads the heat up through the atmosphere and cooler air comes down to replace it, cooling the city during the day. But then at night, a temperature inversion forms, and the lowest levels of the atmosphere no longer exchange energy convectively with higher altitudes. In effect, the strong nighttime inversion that naturally occurs in the desert has weakened over the city as the pavement releases the extra energy it has stored during the day.

The actual background climate warming in the last 50 years in Las Vegas (whatever its cause), based upon the above plot, looks to be around 0.25 deg. C/decade. This is also part of the reason 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 as influenced by surface effects, such as the Urban Heat Island, and avoids conflation of Las Vegas heat with the “climate crisis”.

This piece originally appeared at DrRoySpencer.com and has been republished here with permission.