Weather radar is a powerful tool for peering inside a thunderstorm, but interpreting what it sees is a subtle science. The radar works by sending out microwave pulses and then measuring the energy that reflects back from precipitation particles. Identifying hail is one of the most challenging tasks for a severe weather forecaster, as it requires distinguishing ice from extremely heavy rain. This identification relies on analyzing specific radar signatures that reveal the properties of the particles themselves.
Identifying Hail by Extreme Reflectivity Values
The most immediate indicator of intense precipitation is the reflectivity measurement, expressed in units of decibels of Z, or dBZ. Reflectivity measures how much energy is returned to the radar, which is strongly influenced by the size and number of objects in the air. Larger particles reflect significantly more energy, meaning a few large hailstones produce a much higher dBZ value than a cloud full of small raindrops.
This is why hail cores are often highlighted on radar images with the brightest colors, such as magenta or white. When a storm core exhibits reflectivity values of 55 dBZ or higher, it strongly suggests that hail is present. Values climbing above 60 dBZ are nearly certain indicators of hail, as rain alone rarely produces such an intense return signal.
However, relying solely on high dBZ values can be misleading. Extremely heavy rainfall can sometimes mimic the appearance of small hail, as a large volume of very big raindrops can approach the reflectivity values associated with hail. For a definitive confirmation, meteorologists must look beyond simple reflectivity to more advanced measurements that reveal the particle’s physical characteristics.
Using Dual-Polarization Data to Confirm Hail
The limitation of traditional radar, which only sends out horizontal pulses, led to the development of dual-polarization technology. Dual-pol radar transmits and receives energy in both horizontal and vertical orientations, allowing forecasters to determine the size, shape, and uniformity of the precipitation particles. This provides a much clearer distinction between hail and rain.
One key measurement is Differential Reflectivity (\(Z_{DR}\)), which compares the horizontal power return to the vertical power return. Raindrops flatten as they fall, reflecting more energy horizontally and resulting in high \(Z_{DR}\) values. Hailstones are more spherical or tumble, meaning the horizontal and vertical returns are nearly equal, resulting in low or near-zero \(Z_{DR}\) values.
The second crucial dual-pol product is the Correlation Coefficient (\(\rho_{hv}\)), which measures how uniform the targets are within the radar’s sampled volume. Uniform precipitation, such as pure rain, produces a high \(\rho_{hv}\) value, typically close to 1.0. A low \(\rho_{hv}\) value suggests a mix of non-uniform particles, such as hail mixed with raindrops or air, often creating a “hole” signature within the hail core.
When meteorologists observe high reflectivity, low \(Z_{DR}\) (indicating spherical particles), and low \(\rho_{hv}\) (indicating mixed particle types), they have a strong confirmation of a hail-producing storm. This combination allows for greater accuracy in distinguishing hail from intense rainfall.
Recognizing the Three-Body Scatter Spike
The Three-Body Scatter Spike (TBSS), sometimes called a “hail spike,” is a visual artifact that strongly indicates very large hail. This signature appears as a distinct, elongated extension of weak reflectivity protruding outward from the main storm core, always away from the radar site. No precipitation is actually falling in the area of the spike itself.
The TBSS is created by a three-step process involving the radar beam. First, energy hits a dense core of large hailstones. Second, that energy is scattered downward to the ground below the storm. Third, the energy reflects off the ground, travels back up to the storm core, and returns to the radar receiver.
Because this path is longer than the direct return, the radar interprets the returning energy as coming from a location farther away, creating the downrange “spike” feature. The presence of a clear TBSS is a nearly certain indicator that the storm contains hailstones often exceeding one to two inches in diameter.