Explanation.

The reflectivity factor Z is defined as the integral shown above: it sums contributions from raindrops of all sizes.

The function N(D) = N₀ e^−ΛD is the drop size distribution — it describes how many drops of diameter D exist per unit volume per unit size interval. Small drops are common, while large drops are exponentially rarer.

Each drop’s radar return scales as D⁶. Because of this sixth‑power dependence, larger drops dominate the total reflectivity, even though they are fewer in number.

The solid curves show the quantity being integrated (N(D)·D⁶) for light and heavy rain. The dotted curves show the underlying drop distributions. Heavy rain contains relatively more large drops, which dramatically increases Z.