Differential reflectivity (ZDR) is the ratio, in decibels, of the power a radar receives back in the horizontal polarization to the power it receives back in the vertical polarization, and it tells a forecaster something about the average shape and orientation of the targets inside the beam.
What ZDR is measuring
A dual-polarization radar transmits pulses in two orthogonal polarizations at essentially the same time. When both pulses hit a target, the returned power in each channel depends on how the target looks from that angle. A large raindrop is flattened by drag as it falls, so it reflects more horizontally polarized energy than vertical, and it returns a positive ZDR. A small drop is nearly spherical and returns a ZDR near zero. A tumbling, roughly round hailstone also returns a ZDR near zero on average because no shape orientation dominates.
The convention is decibels of ratio. Positive values mean horizontally elongated targets on average, values near zero mean spherical or randomly tumbling targets, and negative values indicate vertically elongated targets, which is unusual outside of highly aligned ice crystals or biological scatterers oriented tail-down.
How forecasters read it
The most common operational read of ZDR pairs it with reflectivity. A reflectivity core of 55 dBZ with ZDR near zero suggests a hail-dominated core, because the tumbling stones flatten the ratio even though the reflectivity is high. That same 55 dBZ core with ZDR of 3 or 4 dB suggests very large drops or a rain-dominated core, because the flattened drops preserve the horizontal excess.
ZDR also flags the ZDR column, a narrow plume of enhanced ZDR extending above the environmental freezing level. That column marks where a strong updraft is lofting big raindrops before they can freeze, and it is one of the most useful dual-pol clues that an updraft is intensifying. ZDR arcs along a supercell forward flank indicate size sorting by storm-relative winds and can help confirm supercell organization.
Important limits
ZDR is a bulk property. A single value collapses the shape distribution of thousands of targets into one number, so it cannot resolve a stone from a drop directly. It is also sensitive to calibration bias, and small offsets in the antenna hardware can shift operational ZDR by a few tenths of a dB, which matters when the interpretation thresholds are only a couple of dB apart.
Interpretation requires context from the other dual-pol variables. Correlation coefficient tells the reader whether the beam is filled with one kind of target or a mix, and specific differential phase tells the reader whether liquid water is present. ZDR read on its own can mislead in melting layers, in beam-filling non-meteorological returns, and at long ranges where the beam is wide and only partially filled.