World War II, Radar, Astronomy and
Sun Spikes on Doppler Radar


Although a time of extraordinary upheaval and untold human suffering, World War II prompted a wealth of scientific discoveries and technological development. Originally designed prior to World War II, improvements in the design and utilization of radar is one example. RADAR, an acronym for Radio Detection and Ranging, was developed to detect aircraft and ships while they were beyond visual range. It wasn't long before military radar operators discovered this novel resource was able to detect precipitation.


Although a distraction from radar's primary wartime purpose, meteorologists quickly seized upon its forecasting benefits and over the past sixty-five years radar technology has dramatically improved. Radar imagery, a staple of television weather forecasts, has become ubiquitous. Television stations even tout the superiority of their radar stations as a marketing ploy to woo viewers.


Imagery from NWS Jet Stream

Before connecting World II, radar, astronomy, and the radar anomaly known as "sun spikes", a short primer on radar is in order. Although not all radar stations operate on the same wavelength, most function by transmitting pulses of energy within the microwave section of the electromagnetic spectrum. Early radar sites operated on a much longer wavelength.


As shown by the animation to the right, only a small portion of the emitted energy is backscattered by objects towards the station's antenna. In addition to rain, snow and hail, radar pulses can be backscattered by non-meteorological objects such as airplanes, birds and even insect swarms. By analyzing the intensity of this returned energy (measured in dbZ) and the elapsed time from transmission of the pulse to its return, computer programs are able to create imagery (such as the one below) that provide a visual representation of the data. Additional information on Doppler radar can be gleaned from the National Weather Service's educational site.


Base reflectivity radar image from the NWS Cleveland site on January 13, 2008 at 2135Z.


The implementation of Doppler radar, so named because of its ability to detect the doppler effect, allows meteorologists to measure movement of objects relative to the station. This technological development provides meteorologists with the data needed to assess the overall movement of a storm, and the rotation associated with mesocyclones and tornadoes. There is no doubt -- Doppler radar has saved many lives.


Because radar imagery results from the interpretation of electromagnetic energy received by the station's antenna, there are circumstances which result in unusual images referred to as radar anomalies. One anomaly are sun spikes, such as the narrow ray that appears in the lower left portion of the base reflectivity radar image from 2208Z on November 10, 2007 (below).


Doppler radar image from 2208Z (5:08pm local time) on November 10, 2007 courtesy of the NCDC archive. Click here to view the image full size.


As mentioned earlier, radar stations emit energy within the microwave portion of the electromagnetic spectrum, with National Weather Service (NWS) sites currently operating at a wavelength of +-10 cm. It turns out that the Sun emits energy across the entire electromagnetic spectrum. At sunrise and sunset when the Sun is near the horizon, radar stations are susceptible to picking up and interpreting the microwave energy as precipitation. The sample image was produced at 5:08pm local time (2208Z) and the spike is clearly oriented in a direction that is consistent with the setting Sun in mid-November. It doesn't require an expert in radar interpretation to discern the difference between the anomaly and the widespread precipitation on the image from January 13, 2008.


Serendipity - an aptitude or good fortune in making unexpected and desirable discoveries.


So what is the common denominator of sun spikes, World War II and astronomy? The answer is schoolteacher J.S. Hey (1909-2000), a member of Great Britain's Army Operational Research Group, who was assigned to troubleshoot technological problems with British radar during World War II. In February 1942, radar operators were regularly plagued with a screen that was entirely illuminated with "targets", making it impossible to identify German aircraft. Fearing that the Germans had invented a sophisticated jamming technology, the Army asked for assistance in overcoming a very troubling development that further imperiled England's survival.


Hey observed that the "jamming" occurred just around sunrise and sunset and that the Sun was experiencing a peak in its eleven year sunspot cycle. He correctly deduced that the Sun was emitting electromagnetic energy and the "jamming" was little more than the radar station picking up the emissions and interpreting them as airborne targets. The army radar operators were simply the first observers of what we currently refer to as sun spikes. The Sun as a source of radio emissions was a remarkable, and unanticipated, discovery.


Hey's wartime pioneering efforts were far from over. In 1944, while responsible for designing radar technology to detect V-2 rockets, Hey discovered that comets were also a source of radio signals. Upon the conclusion of the war, Hey requisitioned a few surplus radar stations and pointed towards the heavens, assuring his status as a pioneer in radio astronomy. He certainly lived a serendipitous life.





© 2005-2008 Mark A. Thornton