Giant Hail on Dual-Pol Radar: Hondo, Texas 2021

On April 28, 2021, a 6.4-inch hailstone fell near Hondo, Texas. At the time, it was the largest hailstone in Texas history. Let’s take a peek at the storm that produced it.

This is the perfect storm for learning how to diagnose hail with dual-pol radar. First, let’s look at what the different radar parameters tell us. Then we can see how to interpret them in the Hondo storm.

Reflectivity - how large and how many

Radars send out pulses that bounce off objects such as raindrops, birds, insects, and planes. The radar measures the power that returns and uses it to calculate reflectivity. Reflectivity depends on the number and size of the objects in the radar beam.

For raindrops that are small compared with the radar wavelength, reflectivity is related to the sum of their diameters raised to the sixth power. That means a larger drop contributes much more to the return than a smaller one. Very large hail can scatter radar energy differently, so that relationship becomes more complicated.

Diagram comparing the relative radar returns from 1 and 2 millimeter drops and from equal-size water and ice particles — For small particles, doubling a drop's diameter can produce 64 times the return. The water and ice comparison is an idealized example at the radar wavelength.

Polarization - horizontal and vertical waves

The pulses that radars send out are electromagnetic waves. Dual-pol radars transmit and receive both horizontal and vertical polarizations. Comparing the returns gives us information about the shape and variety of objects in the beam.

Diagram of a radar sending horizontal and vertical waves toward a raindrop — A dual-pol radar sends horizontal and vertical waves and measures the energy that returns.

Differential reflectivity - aspect ratio

Differential reflectivity (ZDR) compares the horizontal and vertical returns. It is expressed in decibels and gives us information about the shape of the objects scattering the radar pulses. Positive values mean the horizontal return is stronger, while negative values mean the vertical return is stronger. Large raindrops often have positive ZDR, while vertically oriented ice crystals can have negative values. Hail often has values near zero, though the result depends on the hail and what else is in the beam.

Diagram comparing the shapes and modeled differential reflectivity of 1, 3, and 5 millimeter raindrops — Larger raindrops become flatter as they fall and tend to have higher ZDR. The values shown are examples for individual drops.

Correlation coefficient - how similar are the returns

Correlation coefficient (CC) tells us how similarly the horizontal and vertical returns behave. Rain made up of similar drops generally has a high CC. A mixture of rain and hail can produce a lower value because the radar is seeing objects with a wider variety of shapes and sizes.

Illustrative horizontal and vertical return traces for rain and for a mixture of rain and hail, with example correlation coefficients of 0.99 and 0.88 — In these examples, the returns from rain change together more closely than the returns from a mixture of rain and hail. The traces and CC values are illustrative.

The Hondo storm on radar

The WeatherWall images below show the storm around 7:35 PM CDT, when the record hailstone fell.

The reflectivity image shows a core over 65 dBZ near Hondo. At the brightest point I checked, it reached 70.5 dBZ. That is a strong return and a good reason to take a closer look at the dual-pol products.

KEWX 0.5 degree reflectivity at 7:35 PM CDT on April 28, 2021, with a magenta 70 dBZ core northwest of Hondo, Texas — Reflectivity at 0.5 degrees, 7:35 PM CDT. The magenta core northwest of Hondo is above 65 dBZ. The beam center over the core is about 6,000 feet above the ground.

At the same point in the core, ZDR was -1.0 dB. That low value alongside the high reflectivity is one clue that we are looking at hail.

KEWX 0.5 degree differential reflectivity at 7:35 PM CDT on April 28, 2021, with a purple band of negative values over the hail core and extending west — Differential reflectivity for the same scan. Purple shows values below zero.

The correlation coefficient at that point was 0.83. That lower value gives us another clue, alongside the strong reflectivity and low ZDR, that the radar may be seeing a mixture of rain and hail.

KEWX 0.5 degree correlation coefficient at 7:35 PM CDT on April 28, 2021, with a band of low blue values west of Hondo — Correlation coefficient for the same scan. Blue shows areas where the horizontal and vertical returns are less closely correlated.

The blue band in the CC image shows decorrelation, where the horizontal and vertical returns behave less alike. A mixture of rain and hail can contribute to low CC, though other factors can also affect the measurement.

Together, these products show a storm capable of producing very large hail. A ground report is needed to measure the size of a hailstone.

Look higher into the storm

WeatherWall lets us choose a higher radar tilt, so we can see what the storm looked like above the lowest scan. At 4.0 degrees, the Hondo storm still had a 66.5 dBZ core near 29,000 feet above the ground.

KEWX 4.0 degree reflectivity at 7:36 PM CDT on April 28, 2021, with a magenta core above 65 dBZ over Hondo — Reflectivity at 4.0 degrees, 7:36 PM CDT. The beam center over the core is about 29,000 feet above the ground.

In WeatherWall, we can switch products and tilts while keeping the hail core in view. Linked panels also let us compare the products side by side.

Explore archived storms in WeatherWall or learn more about these products in the dual-pol radar guide.

How giant hail looks on dual-pol radar: the Hondo storm