UPDATE: El Paso’s high Tuesday was 100 and the low was 60.

El Paso’s long, hot summer will finally break Tuesday night when a cold front sweeps through, sending temperatures plummeting more than 40 degrees in a matter of hours.

The National Weather Service is forecasting a high of 100 on Tuesday and an overnight low of 55. That 45-degree temperature swing seems extreme, but it actually was fairly common for El Paso a few decades ago.

The biggest same-day temperature swing since 1900 came on April 6, 1974, when the high was 90 and the low was 34, a spread of 56 degrees.

I looked at weather records back to 1900 to understand how often we see spreads of more than 40 degrees between high and low temperatures in any given day. An interesting pattern emerged, one that defies obvious explanation.

In the first decade of the 20th century, El Paso averaged 12 days a year where the high temperature was 40 degrees or more above the low temperature. That dropped to between one and seven days a year on average from the 1910s through the 1950s.

Then things got weird. 

El Paso averaged 17 days a year in the 1960s with highs and lows at least 40 degrees apart, then 44 such days on average annually in the 1970s and 38 in the 1980s.

In 1975, we had 81 days where 40 or more degrees separated the high and low — once every 4.5 days on average.

One-day extreme temperature swings generally occur when a cold front moves in during the afternoon or evening. That’s what is happening Tuesday.

But in 1979, El Paso had five consecutive days from Sept. 26-30 where the high temperature was 40 degrees or more above the low. Each day, the high was between 86 and 90 and the low between 45 and 50. Talk about a need for layers of clothes.

After the 1980s, those extreme temperature swings became less common. If Tuesday’s forecast holds, it will be only the second time this year that we’ve had a 40-plus degree same-day temperature swing.

Such extreme temperature swings most commonly occur between February and April and are virtually non-existent in July and August. They’re exceedingly rare in September. El Paso has only had 28 days in September since 1900 where the high temperature was at least 40 degrees above the low, and 19 of those were in the ‘70s. The last time we had a 40-plus degree temperature swing in a September day was in 1994.

As I said, there’s no obvious explanation for why we rarely had extreme temperature swings, then they became common, and then they became rare again. 

“This is really an amazing stat you have uncovered Bob. I find it super fascinating,” said Jason Laney, a meteorologist with the National Weather Service in Santa Teresa. But coming up with an explanation left him “dumbfounded.”

His colleague, Tim Brice, said some of the explanation may lie in geography, not meteorology.

“I know one factor that may have made a difference is that the official observing site moved from Downtown (down in the river valley), up to the International Airport in December 1942,” Brice said. “I’m pretty sure the airport was ‘out there.’ Big hunks of what we call the ‘Eastside’ were not even in the El Paso city limits until 1950. So that might explain some of the differences in the decades.”

He said a phenomenon known as the urban heat island effect — where extensive paving retains heat and keeps nights warmer — might explain why we haven’t seen so many extreme swings the past 30 years. 

“I know it has been my experience that in the last 20 to 30 years, build up around the airport has raised our minimum temperatures at night,” which could minimize the opportunity for extreme temperature swings, he said.

Finally, Brice said the weather service’s instrumentation may play a role in the decline of days with extreme temperature shifts.

“Our current system, ASOS, which came online in 1995, takes a five-minute average to get the official temperatures, where I know the older alcohol thermometers used to get the low temperatures would have been more of an instantaneous reading,” he said.

Those various elements would account for a small amount of the changes over the years. For a more complete explanation, we’re left with a simple observation. Sometimes, El Paso’s weather is just weird.

Robert Moore is the founder and CEO of El Paso Matters. He has been a journalist in the Texas Borderlands since 1986.