Specific differential phase (KDP) is the rate at which the phase between a radar's horizontally and vertically polarized pulses accumulates as they pass through precipitation, and it is used as a direct indicator of liquid water content along the beam.
The physics
When a radar pulse passes through a volume of flattened raindrops, the horizontal component of the wave slows slightly more than the vertical component because it interacts with more water along its polarization axis. The two components arrive back at the antenna with a phase difference. KDP is the range derivative of that phase difference, reported in degrees per kilometer.
Because KDP measures a phase shift rather than a returned power, it does not depend on how well the radar is calibrated and it is not strongly reduced by rain along the path between the radar and the target. That makes KDP an unusually clean measurement of the amount of oriented liquid water in the beam right now, at that gate.
How forecasters read it
KDP is read together with reflectivity and ZDR to fingerprint what is falling. A local KDP maximum above roughly 1 degree per kilometer marks heavy rain in a warm-season environment. KDP values climbing above 4 to 5 degrees per kilometer typically indicate very intense rain, often with melting hail mixed in that adds liquid water skins to falling stones.
The most operationally useful KDP signature in severe storms is the KDP foot or KDP core. A narrow plume of enhanced KDP extending down and out from the reflectivity core marks where the melting layer is dumping liquid water on the forward flank of the storm. Combined with a ZDR column overhead, that pattern often signals an intensifying updraft that can loft ice, melt it aloft, and rain it back through the low levels at high rates.
Where KDP misleads
KDP is derived from the range derivative of differential phase, so it is smoothed over several gates and cannot resolve fine-scale detail. In hail, in the tornado debris signature, and in the melting layer, the correlation coefficient drops and the phase measurement becomes noisy. Operational KDP fields mask out gates where the underlying phase estimate is not trustworthy, which is precisely where a naive read might expect the highest values.
KDP is also insensitive to dry ice. Snow and dry hail with little liquid skin produce almost no KDP signal, so a storm can be dropping large hail with a KDP field that looks quiet. Read KDP as a liquid water gauge, not as a rain-versus-hail decision variable on its own.