The Casper-Natrona County International Airport last month posted the largest number of people boarding planes in 28 years, the airport director said Tuesday.

But the reason for this good news eludes Glenn Januska, he said.

"We really don't know what we can attribute this to," Januska said.

In July, 10,047 passengers used the airport, up 8.7 percent from 9,244 a year ago, he said.

"The last time we had a higher passenger enplanement number in July was in 1987," Januska said.

"We have also only seen two 10,000-plus passenger months since 1988, last October and this July," he said.

June's numbers were up slightly from June 2014, and Januska wondered last month if the increase increase could be a result of a stabilizing economy, Sheridan losing its air service and more people driving from there and Riverton to fly from Casper, a growing local population, or the necessity of business travel compared to leisure travel which depends on disposable income.

But July's increase over a year ago and previous years was substantial, and Januska is trying to figure out why. "It's one that we're happy about, but what it means and can we attribute this to anything in particular? Are we going to see this going forward? Is this a blip?"

There were no major changes in the economy or regional events in July that could easily explain the changes, he said.

The increase could be the long-term effects of the Las Vegas-based Allegiant airline decision to offer year-round instead of seasonal service between Casper and Las Vegas, Januska said.

The Allegiant service may thwart the traditional sharp drop in passenger numbers in  August and September as vacations end and school begins, he said. "We won't have that seasonal stoppage."

This historic two-month increase doesn't make a trend, but it gives Januska and other airport officials something to think about.

"This is the kind of thing we're kind of scratching our heads on," he said. "It's better to scratch your heads when you're seeing an increase than trying to scratch your heads trying to figure out why you have a decrease."