Urbanization raises valley temperatures

Wednesday, March 31, 1999 | 11:05 a.m.

As black asphalt roads crisscross Las Vegas to new neighborhoods, the pavement has helped create an urban island with higher temperatures and more humidity.

In the past five years, the robust growth that has added more rooftops and blacktops to the landscape has created a virtual sponge to trap heat in the Las Vegas Valley. With more concrete and pavement, the valley releases daytime heat more slowly overnight, boosting nighttime temperatures as well.

"I have no doubt we have had enough urbanization to warm the valley," said Ron McQueen, meteorologist for the National Weather Service, pointing to weather records from the 1990s.

The summer of '94 brought the highest daytime temperatures in the valley's history, McQueen said. Daytime highs reached 110 to 116 degrees for most of July and August.

The second-hottest summer occurred in 1996, but it differed from 1994 because the higher temperatures were at night. McQueen said nightly temperatures were 7 to 10 degrees above the normal 70s.

"It's a little scary how a relatively little city has changed our weather," McQueen said of the valley's more than 1.2 million residents.

Las Vegans not only are feeling the heat more, they are also paying more to keep their houses cooler in summer.

And even figuring in the phenomenal growth in the valley, Nevada Power is seeing the effect on power use.

"We have broken the record for using the most electricity in July and August for the past five years, when it is hottest," spokesman Tom Henley said.

Urban warming is not a new trend, but it is one that is just being documented by scientists.

NASA scientists studied Atlanta and discovered that when suburbs expanded and replaced 350,000 acres of forest from 1973 to 1998, the Georgia city grew hotter by 8 to 10 degrees.

The 1996 Las Vegas heating pattern mirrors that of Phoenix in the 1970s and 1980s, McQueen said.

The Arizona city nearly doubled in population during the two decades and its nighttime temperatures increased 8 to 9 degrees. Phoenix, with more than 2 million people, has continued to sprawl into the desert in all directions.

Communities growing in valleys such as Las Vegas and Phoenix replace the natural buff color of desert sand with black asphalt highways and high-rise buildings that collect heat from the sun all day, then release it slowly all night. Desert sand, on the other hand, reflects heat out of a valley.

Contrary to popular belief, McQueen said, those who switch their green lawns to desert landscaping may not be making their yards hotter. Light-colored rock acts like the desert soil, reflecting sunlight.

The urban corridor with the greatest development, from Henderson through the center of Las Vegas to North Las Vegas, traps the most heat, he noted.

Heat is not the only weather change Las Vegas is seeing. It's getting more humid, too.

And it's not just because of the number of pools.

Overwatering of Las Vegas lawns raises the humidity above 30 percent on many mornings.

"We've known about urban heat islands for many years," McQueen said.

By 1996, when Las Vegas Valley temperatures strayed above the average summertime readings for the second time, the National Weather Service began to speculate that Southern Nevada growth was affecting its weather, he said.

Although summer 1998 came closer to normal temperatures, McQueen said the higher temperatures of this decade may be setting a new pattern.

The only solution to reversing the rising heat is halting growth.

"I haven't heard anybody advocate that," McQueen said.