A new book by UFO experts Lisa Miller Costa and Cheryl Costa has compiled information drawn from the National UFO Reporting Center's (NUFORC) database to "present data and analysis for 100,000+ sightings of unidentified flying objects reported by individuals during the first 15 years of the 21st century." The book, titled UFO Sightings Desk Reference: United States of America 2001-2015, shows that over the span of 15 years California led the nation with over 16,000 reported sightings. 3,200 of the sightings took place in Los Angeles County.

The authors who have compiled the data show a strong correlation between warmer weather and increased sightings, so it is to be expected that sightings will increase going into spring and summer. According to NUFORC's database, California has had 90 reported sightings so far in 2017, with 30 of those reported since March 1st. The reason for this is not entirely clear, but some experts believe it is simply because more people are active outside during this time. This could also help to explain why warmer states tend to have more reported sightings.

In an interview with Tech Insider, the Mutual UFO Network's chief archivist Cassidy Nicholas said "[People] are noticing the weather, hiking, camping, doing outside activities. And there seems to be an increase in live events like concerts, fireworks, and other community gatherings."

California is followed by Florida, Texas, Washington, and Pennsylvania for the most UFO sightings. Overall, there have been over 120,000 reported sightings of UFOs in the USA between 2001-2015.