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The image below shows the slabbed area I can see from my work window

Outside my window

Whenever it rains some of the slabs stay dry however, it is never consistent which ones. Can anyone explain why this happens to satisfy my curiosity?

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closed as off-topic by Aaron Stevens, PM 2Ring, John Rennie, stafusa, Kyle Kanos Aug 29 at 11:28

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    $\begingroup$ Do you really mean that some slabs don't get wet at all, or that some slabs dry out more quickly (and more completely) than others after the rain has stopped? What evidence do you have that it is different slabs each time? $\endgroup$ – sammy gerbil Sep 11 '18 at 11:28
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    $\begingroup$ I think, there is a positive feedback loop: dry tiles warm up faster, and warmer tiles become dry faster. So, random temperature fluctuations are amplified to form random pattern of dry slabs. I have no evidence to back this assertion though. $\endgroup$ – FiatLux Sep 11 '18 at 11:35
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    $\begingroup$ they do not dry faster, they just don't seem to get wet $\endgroup$ – Kingsley-James Sep 11 '18 at 11:50
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    $\begingroup$ It is an intriguing phenomenon. However, I am sceptical about your claim that some slabs remain dry but that it is different slabs each time. Even if this issue is cleared up I am doubtful that a definitive explanation can be offered without further evidence from observations or experiment. Several plausible suggestions could be made, but I think there can be no conclusive answer based on the current evidence. $\endgroup$ – sammy gerbil Sep 11 '18 at 15:37
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    $\begingroup$ I'm voting to close this question as off-topic because based on the comments it seems like any answer would just be purely speculative. More information, experiments, etc. are needed to provide a sufficient answer to this question. $\endgroup$ – Aaron Stevens Aug 27 at 1:38
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If the OP had not mentioned that it is not consistently the same tiles that stay dry, I would have tended to agree with @anna v's conjecture about tiles being tilted. However, in view of this OP's remark, let me speculate that those tiles stay dry which have deeper seams around them, so water runs off from them more efficiently. The depth of the seams can change from time to times because pedestrians might transfer dirt with their feet from one place to another.

EDIT (08/26/2019): One more reason in favor of my speculation: in several places, there are narrow dry strips along seams.

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    $\begingroup$ If it was about the walking patterns of pedestrians then wouldn't you expect these areas to be connected somehow? $\endgroup$ – Aaron Stevens Aug 27 at 1:26
  • $\begingroup$ @AaronStevens : I don't see why. The picture does not seem to suggest that there are some stable traffic paths, so I would think dirt could be transferred from place to place pretty randomly. $\endgroup$ – akhmeteli Aug 27 at 1:30
  • $\begingroup$ That is what I am saying. I wouldn't expect people to walk around randomly. Typically areas like this have typical walking patterns. (I didn't down vote btw) $\endgroup$ – Aaron Stevens Aug 27 at 1:31
  • $\begingroup$ @AaronStevens : I respectfully disagree. We see "typical walking patterns" when we have snow, grass or soil, where people can gradually reshape the surface, and this does not happen with hard tiles. $\endgroup$ – akhmeteli Aug 27 at 1:37
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    $\begingroup$ Typically, such slabs are laid on sand, with no grouting between the slabs, so the tilt of each slab can change over time. But I agree with @Aaron that we need more information. $\endgroup$ – PM 2Ring Aug 27 at 5:20

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