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:
- The rain lasted only 2-3 minutes.
- The rain came in large drops, definitely not "misty".
- There was never enough rain for water to flow on the ground.
- Temperature was probably in the 80s °F the whole time.
- It had been on/off sunny previously.
- The picture was taken probably about 5 minutes after first hearing raindrops.
- 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.
- 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.