Hail Spikes on Doppler Radar Imagery

Hail spike is the name given to an elongated area of reflectivity values on Doppler radar imagery, such as on the annotated image from July 17, 2010 at 1830Z (below). Rather than displaying an area where precipitation is occurring, this elongation is an anomaly resulting from hail within the thunderstorm’s updraft.

Base Reflectivity radar image from Buffalo, NY NWS on July 17, 2010 at 1830Z. (larger image)
Cross section of base reflectivity from Buffalo, NY NWS on July 17, 2010 at 1830Z showing the storm's updraft and imbedded hail. (larger image)

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. As suggested by the animation below, only a small portion of the energy transmitted by a radar station 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 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 ones at the top of the page) that provide a visual representation of the data.

Animation of a Doppler radar station.

Hail is a very effective backscatterer of radar pulses which contributes to the creation of hail spikes. As shown on the schematic below, the radar pulse hits the hail in the storm’s updraft and a portion of this energy is deflected towards the ground (the green dashed line). Some of this deflected energy is subsequently backscattered upwards where it once again encounters the hail core which deflects it back towards the radar site.

Schematic of a hail core in a thunderstorm updraft and the resulting radar hail spike.

An energy pulse that is subjected to all of this backscattering takes longer to return to the radar site than a pulse that is backscattered directly from the hail core back to the radar site. This increased travel time is interpreted by the radar site’s algorithms as increased distance between the radar site and the object that backscattered the energy. The erroneous extra distance, roughly twice the distance from the hail core to the ground, appears as a flare of radar reflectivity extending beyond the storm (the shaded area to the upper right of the hail core in the schematic above).