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On a warm and partly cloudy day this summer, I was sitting in my office and suddenly heard rain on the roof. I went outside to shut my car windows. By the time I did that and was walking back into the building, the rain had stopped. Such short showers from a small single passing cloud are not unusual here in New England.

The interesting part is that on the way back I noticed a strange pattern of wetness on a concrete area against the building. I went back to my office, grabbed the office point-and-shoot camera, and took this picture:

This was looking from the top of a staircase down onto the concrete area. This area is around 10x10 meters, maybe a bit less.

The question is, why does the concrete look dryer around the cracks? The effect became less evident in the minute or two it took me to go back to my office, grab the camera, come back out, and take this picture. But, you can still see light-colored streaks in the pattern of darkness due to wetness. It may not be evident in all cases in the picture, but these streaks all followed cracks in the concrete. Why?

Other facts that may be relevant:

  1. The rain lasted only 2-3 minutes.

  2. The rain came in large drops, definitely not "misty".

  3. There was never enough rain for water to flow on the ground.

  4. Temperature was probably in the 80s °F the whole time.

  5. It had been on/off sunny previously.

  6. The picture was taken probably about 5 minutes after first hearing raindrops.

  7. The concrete was already noticably drier from when I first saw it until the picture was taken. The darker wet areas had also diffused out more.

  8. The whole area has a gentle slope towards the drain in the upper right corner of the picture. Note that this phenomenon seems to be independent of the orientation of the crack.

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    $\begingroup$ My guess would be that's because the soil in the cracks has been warmed up relative to the concrete itself because of its darker color. Then when some small amount of rain water appeared on the surface, it evaporated first from the warmest parts, which are around the cracks. Also part of the water may have been absorbed by the soil, while concrete doesn't absorb it too much, thus the layer of water to evaporate is thicker far from the cracks. $\endgroup$
    – Ruslan
    Dec 24, 2014 at 16:36
  • $\begingroup$ I bet part of the effect is from cracks having more surface area for water to spread over which aids in evaporation. $\endgroup$
    – Brandon Enright
    Dec 24, 2014 at 17:42
  • $\begingroup$ @Brandon: But the effect can be seen some distance from the crack, a few inches in some cases. Also note that the entire effect happend in about 3 minutes, maybe 5 by the time the picture was taken. $\endgroup$
    – Olin Lathrop
    Dec 24, 2014 at 19:43
  • $\begingroup$ It would be good if you could collect more careful data next time this happens - ideally with an IR camera (can buy those as an add-on for cells phones for relatively little money). Whether this relates to temperature before the rain fall starts, or porosity of the concrete, is hard to determine. My bet is on temperature. Intriguing! $\endgroup$
    – Floris
    Dec 25, 2014 at 1:28
  • $\begingroup$ @Floris: It's winter here now. I think this is a summer phenomenon, so it'll be some months before I can collect more data. Short rain squalls with heavy drops are not that unusual, but they come without warning, and for best results it seems I need to photograph it within 2-3 minutes. I'll try to be vigillant next summer, but it's not certain I'll catch anything good. $\endgroup$
    – Olin Lathrop
    Dec 25, 2014 at 15:12

5 Answers 5

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Just a guess: The air get warmer inside the cracks and this (warmer plus moving lower pressure air) evaporates the water on the surface near the cracks. The reason why the air gets hotter inside the cracks is that the concrete is still warmer than the outside air, and the cracks have a larger surface to volume ratio.

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I think it's because the surface water runs off into the cracks rather than soaking into the concrete. If it had rained for a longer period of time I suspect that this wouldn't happen, but there's only one way to find out! :)

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    $\begingroup$ No, this is definitely not it. See my point #3 in the question. There was never enough water on the ground at any time to flow. Also, selective dryness due to water flowing into cracks looks quite different. The dryness is only on the downhill side of each crack. Note the dry areas in the picture surround the crack on both sides about equally and regardless of crack orientation relative to slope. $\endgroup$
    – Olin Lathrop
    Dec 24, 2014 at 16:49
  • $\begingroup$ what about the fact that the concrete is porous and has a larger surface area where the crack is so it doesn't soak in as deep, its the same amount of water over a larger surface area so it will dry quicker aswell $\endgroup$
    – Daz Hawley
    Dec 24, 2014 at 16:58
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The surface tension of the aqueous film draws the water toward the center of the sidewalk panel. That is why water will puddle instead of drying uniformly.

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Yet another theory - Consider that the cracks allow water to penetrate the surface of the concrete and that the sub-surface structure may have a higher porosity than the finished surface, allowing water to penetrate. Perhaps the concrete contains components that can dissolve and be carried away by water. So the cracks provide a pathway for the water to easily penetrate. The water dissolves components in the more porous concrete accessible by the cracks, and lastly those parts of the concrete that are depleted in this dissolvable component tend to appear lighter in color.

A hypothesis easily testable in the laboratory using spectographic methods.

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The cracks are dry because this is a finished piece of concrete. As concrete sets, A finisher will apply pressure to the surface of the pad, pushing down any exposed stone and filling in any voids.

Concrete, as it sets, releases CO2 gas. The viscosity of the setting mix is such that the bubbles get trapped, causing voids, This is actually desirable, It is a mechanism that makes the composite's weight to strength what it is.

The cracks have exposed some of these voids, allowing fast flow of the moisture to enter the porous body of the concrete, avoiding the finished surface, Thus, two factors are at work, Migration and more surface area.

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