Sidewalks - where worms commit suicide on wet days

I walk to and from work, a round trip of 6 km or 3 km each way. It isn't a long walk, but, enough time for a little exercise and lots of thinking before I get to the office. The walk home allows me to unwind. Since I live in an urban area, the sidewalks are paved the whole way. Everyday, I see weeds poking up through cracks, worms committing suicide on rainy days, and the changing colour between wet and dry concrete.

A paved sidewalk probably wears out my shoes and knees faster, but, do sidewalks make us safer? Apparently the U.S. Department of Transportation has studied this (I'm sure other nations have looked at this as well). They found that the presence of a sidewalk, along with the speed limit, reduced the likelihood of a vehicle hitting a pedestrian by 88.2 percent – as a pedestrian, that's a big difference.


On a rainy day sidewalks look darker than when it's dry. Concrete is a matte surface, which is not shiny at all. Light is reflected diffusely off a matte surface, scattering in all directions as shown in the diagram. Dry concrete looks rather featureless and the same from all angles.

When the concrete is wet, a thin coating of water forms a smooth and glossy layer on top. A portion of the incoming light is reflected by the water layer, meaning less light reaches the concrete. The concrete now looks darker because less light reaches it to be absorbed. As an added feature, multiple reflections within this thin water layer highlights surface features in the wet concrete that can't be seen when it's dry. When the weather is frosty, a near invisible slippery film of black ice can form, which looks just like wet pavement and is a result of the same optical tricks.

Frost heave can create cracks in the concrete as can roots of nearby trees. I suspect small earthquakes could also form cracks. Once there is a crack, plants move in and take advantage of this new growing space. Ultimately they widen the cracks and more plants move in, creating a cycle that can destroy a sidewalk. Where I live dandelions and chamomile seem to thrive in these cracky environments.

So, why do so many worms commit suicide on sidewalks? When it rains, the worms' underground home fills with water. Since worms breathe through their skins, to avoid drowning they come up to the surface. Once in the air they can breath again. If they wander about and end up on the sidewalk they may not find their way back into the ground again, ultimately drying up when the sun comes out or forming a robin's lunch. Good for the robin I guess.