Why The Ground Darkens With Rain

Q: Why is the ground darker after a rain?
Gyula, Kaposvar, Hungary

A: I looked out my window at sunlight slanting through piñons, and thought about rain. An object looks black, because it absorbs most light shining on it, leaving little to reflect to our eyes. So, the ground must look darker after a rain, because the ground absorbs more light when it's wet.

The question is, why should ground absorb more light, when wet rather than dry?

Dry dirt is composed of many fairly loose particles, surrounded by air. When light hits the dirt, and bops around through it, dirt particles scatter the light. Some light is absorbed, and some reflects back out. Light might, on the average, follow a path shown (ideally) by the top image in the simplified drawing.

Rain wets the ground by filling in the cracks and hollows among the particles with water. Water has optical properties close to those of dirt, writes physicist Craig Bohren in Clouds in a Glass of Beer. So, wet dirt looks somewhat like only dirt and no air. Because of this optical homogeneity, light follows a longer path through wet dirt than through dry dirt; moreover, the path goes deeper. The longer path (shown in the bottom image) increases the chances light will get absorbed. That's why wet ground absorbs more light.

Like any good scientist, I checked these ideas with an experiment. I couldn't wait for rain; that can take weeks in New Mexico. So, I filled a clean coffee pot with water, and poured the water on the ground. The effect was striking. As soon as the water touched the ground, the light dirt turned to a rich, deep almost chocolate brown. That made sense because the dry dirt reflected much light shining on it. The wet dirt darkened into brown as it reflected less light.

Craig Bohren describes another experiment in Chapter 15 of Clouds in a Glass of Beer, which you might like to do. This experiment shows how sand wetted with benzene looks even darker than sand wetted with water, because benzene's optical properties match dirt's better than water.

Further Reading:

Clouds in a glass of beer by Craig F. Bohren

Texture and color, University of Western Australia

Color and vision, The Physics Classroom

Why are things colored?, Webexhibits.com

(Answered May 28, 2007)