On Friday night, starting around 0330 Z (7:30 p.m. PST, 8:30 p.m. MST) and continuing for at least 3 hours thereafter, lots of non-precipitation interference was detected by several weather radars scattered across the Great Basin region. These signatures easily overlaid each other on composite images that use data from the most recent scans of multiple radars at nearly the same time. Here’s an annotated, multi-radar composite reflectivity image, with radar locations labeled.

The locations marked on the map are for the radars themselves, not necessarily the towns they serve. For example, the Elko radar actually is west of Elko itself. True north is parallel to the Nevada-Utah border on this map projection.

Here are un-annotated images from 0320Z and from 0400Z.

Now, this sort of thing typically sets the tinfoil-hat brigade aflutter with all manner of feverish paranoia. Some background will help here, I hope.

All receivers of radio waves are subject to radio-frequency (RF) interference. This includes the national network of WSR-88D (Weather Surveillance Radar, 1988 Doppler) units, which transmit and listen at the 10-cm wavelength. Research has shown this to be a really useful wavelength for weather radars, to balance good detection of hydrometeors (rain, snow and hail) with lack of attenuation (signal loss) through the same rain, snow and hail.

Radar beams also expand with distance. They bracket a line between the antenna and the source of the interference. It’s nothing unusual at all! This actually happens on a daily basis, all across the U. S., upon a most routine pair of events: sunset and sunrise.

The sun transmits at all wavelengths, including 10 cm. Therefore, when the sun is positioned low in the sky, just before setting and just after rising, a sweep of the radar antenna is low enough to “see” the sun. The radar detection of the sun’s 10-cm transmissions shows up as expanding beam signals pointing directly toward sunrise or sunset. Here’s a great example from NWS Louisville.

Other common sources of RF interference at 10 cm also can cause such beam signatures. Really, any substantial source of 10-cm interference can, if crossed by the sweeping radar beam. The magnetron in your favorite microwave oven, and millions of others, transmits at 12 cm–not far from that of the radars (albeit at very high frequency suitable for cooking in close quarters).

Triangulating the source of simultaneous interference on multiple receivers is easy. The beam lines converge at the source of the 10-cm interference over southern Nevada, specifically the area above the USAF Nevada Test and Training Range. Clearly something there was causing enough RF interference to be detected by several radars, for quite some time on Friday evening.

What was it? Don’t ask me; I have no way to know, nor even a good guess. Countless electronic devices exist–or can be made–that transmit fairly strongly at 10 cm. Barring some official explanation, I guess the tinfoil-hat crazies will have to take over from there, imagining all kinds of conspiratorial bogeymen under the bed…or more geographically correctly, near Area 51.