For example, consider a field of large, falling raindrops. The drops tend to fall with an oblate orientation -- similar to a hamburger bun. The field of drops, as a whole, will have a larger cross-section of water in the horizontal compared to the vertical. A horizontally-polarized radar pulse will, therefore, be backscattered more in this field of drops than a verically-polarized pulse, resulting in more radar return for the horizontal pulse than the vertical pulse. In this case, P_h > P_v, so Z_DR > 0. Differential reflectivity values above 2 dB are commonly observed in rain.

Although hailstones are not necessarily spherical, studies have shown that they fall with a tumbling motion -- meaning a field of falling hailstones within the radar resolution volume will "appear" to consist of nearly spherical hydrometeors. Therefore, the value of Z_DR for hail is usually close to zero.

Some graupel and hail hydrometeors with a conical shape can fall with their major axes oriented in the vertical. In these cases, the Z_DR will be found to be negative.

Z_DR is reflectivity-weighted, meaning the shape of the more strongly reflective hydrometeors will contribute more to the Z_DR of a radar resolution volume than the more weakly reflective hydrometeors in the same volume.

For example, consider a resolution volume with a mixture of raindrops and hailstones. We know that, among other things, the reflectivity factor is a function of the average diameter of the hydrometeors in the volume to the 6th power, and the dielectric constant of the hydrometeors. Adding hailstones to a field of raindrops increases the average hydrometeor diameter, leading to a much higher reflectivity factor. Because in fall, appear nearly spherical, and because hailstones are much more reflective than raindrops, the reflectivity for horizontally-polarized radar pulses should be about the same as that for vertically-polarized pulses. This means Z_DR should be near zero, in spite of the presence of rain.