Retirees who flock to St. George for its warm winter weather contribute. So do visitors who enjoy Zion National Park and other outdoor recreation. New research shows that Washington County’s population swells by a third at peak times such as Labor Day weekend.

The Kem C. Gardner Institute at the University of Utah released the findings on Tuesday, saying officials always knew the temporary resident population there was large — but until now had only guessed about its size. Reliable numbers could help improve planning.

It estimates that the county’s temporary population peaked at 57,069 people in 2017, equal to about a third of the permanent population of 162,592.

About half of those temporary residents were seasonal, such as retirees, and the other half were overnight visitors.

So the report figures that on a busy weekend, “the total peak population is then 74 percent permanent residents and 26 percent temporary residents.”

It also found that about 20 percent of the county’s housing units are used by seasonal residents. Also, it said that temporary rentals, such as through Airbnb, have “been exponentially increasing since 2014,” including 1,779 such listings in December 2017.

The report notes that Washington County is now the fifth most populous in the state, behind Salt Lake, Utah, Weber and Davis counties on the Wasatch Front.

Washington County has seen a tenfold increase in its permanent population since 1970. The St. George Metropolitan Area was the nation’s fastest growing in the most recent U.S. Census Bureau data.

The report said the area’s proximity off of Interstate 15 between Salt Lake City and Las Vegas, regional economic growth and its unique recreational opportunities “strengthen its renown within the Western United States as both a place to live and visit.”

The report said the estimates offer a more comprehensive view and definition of the population that should help planning in a high-tourism area.