After an hourlong run and finishing my leftover tostada salad from The Little Chihuahua this afternoon, I noticed out the kitchen window a helicopter flying fairly low over the Pacific Heights area of San Francisco. I didn’t think much of it till I heard it again, and again.

A twin-engine Bell 412 helicopter, operated by the Remote Sensing Laboratory Aerial Measuring System from Nellis Air Force Base, heads west over San Francisco Monday, February 1, at 3:37 p.m.

Usually if there is news, the helicopter isn’t flying back and forth, so I thought perhaps it was searching for something. I did a quick Twitter search for “san francisco helicopter” and found the answer in the replies to the top search result, a Tweet from @missmobtown.

As a top-floor dweller, the low-flying helicopter over #sanfrancisco is freaking me out a tad. Anyone know what it's for?

— Catherine G. (@missmobtown) February 1, 2016

Oh, apparently embedding a tweet doesn’t include the replies. Here’s a screenshot instead.

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Following that link to a National Nuclear Security Administration press release, I learned the helicopter is apparently measuring background radiation. No mention of Super Bowl 50, which some other twitterers postulated was involved. (I’m not sure why it would be since the game is apparently being held in some lesser city.)

(SAN JOSE and SAN FRANCISCO, California) – A helicopter may be seen flying at low altitudes over portions of the San Francisco Bay Area from January 29 through February 6, 2016. The purpose of the flyovers is to measure naturally occurring background radiation.

Officials from the National Nuclear Security Administration (NNSA) announced that the radiation assessment will cover a collection of areas spanning approximately 22 square miles. A twin-engine Bell 412 helicopter, operated by the Remote Sensing Laboratory Aerial Measuring System from Nellis Air Force Base, will be equipped with radiation sensing technology. The helicopter will fly in a grid pattern over the areas at 150 feet (or higher) above the ground surface at a speed of approximately 80 miles per hour. Flyovers will occur only during daylight hours and are estimated to take about three hours to complete per area.

The measurement of naturally occurring radiation to establish baseline levels is a normal part of security and emergency preparedness. NNSA is making the public aware of the upcoming flights so that citizens who see the low-flying aircraft are not alarmed.

In case anyone is a helicopter enthusiast or something, I took a bunch of photos over four passes near my house. Enjoy. :-P

A twin-engine Bell 412 helicopter, operated by the Remote Sensing Laboratory Aerial Measuring System from Nellis Air Force Base, heads west over San Francisco Monday, February 1, at 3:25 p.m.

A twin-engine Bell 412 helicopter, operated by the Remote Sensing Laboratory Aerial Measuring System from Nellis Air Force Base, heads west over San Francisco Monday, February 1, at 3:37 p.m.

A twin-engine Bell 412 helicopter, operated by the Remote Sensing Laboratory Aerial Measuring System from Nellis Air Force Base, heads west over San Francisco Monday, February 1, at 3:37 p.m.

A twin-engine Bell 412 helicopter, operated by the Remote Sensing Laboratory Aerial Measuring System from Nellis Air Force Base, heads west over San Francisco Monday, February 1, at 3:37 p.m.

Helicopters over San Francisco. I'm not sure if these are related to the radiation measurements, as they are too far for me to identify.

Helicopters over San Francisco. I'm not sure if these are related to the radiation measurements, as they are too far for me to identify.

A twin-engine Bell 412 helicopter, operated by the Remote Sensing Laboratory Aerial Measuring System from Nellis Air Force Base, heads west over San Francisco Monday, February 1, at 3:37 p.m.

A twin-engine Bell 412 helicopter, operated by the Remote Sensing Laboratory Aerial Measuring System from Nellis Air Force Base, heads east over San Francisco Monday, February 1, at 3:31 p.m.

Vera waits patiently in the cold wind for me to decide if I am done photographing mysterious helicopters.

Helicopters over San Francisco. I'm not sure if these are related to the radiation measurements, as they are too far for me to identify.

A twin-engine Bell 412 helicopter, operated by the Remote Sensing Laboratory Aerial Measuring System from Nellis Air Force Base, heads east over San Francisco Monday, February 1, at 3:31 p.m.

Helicopters over San Francisco. I'm not sure if these are related to the radiation measurements, as they are too far for me to identify.