# Why is the pavement wet?

Saw this the other day. So I took this picture. There was another nearly identical wet patch 2 spaces away.

Why is the pavement wet?

Note: This is a straight forward physics puzzle. This is a ordinary parking lot with ordinary / usual interactions.

Note: Matthew Jensen's answer was quite reasonable, however, in this case, no car was present in that parking spot during or after the rain.

• Welcome to puzzling! You might want to add some more hints that lead to a (single) correct solution. As it stands now, the question has multiple "correct" answers and might be closed as too broad. Commented Oct 2, 2020 at 13:04
• It was Sponge Bob's car. Commented Oct 2, 2020 at 13:14
• Is it confirmed as wet? Looks like sticky plastic pieces. Commented Oct 4, 2020 at 1:59

@MatthewJensen has provided a very reasonable possible explanation. The OP has however indicated that the car left the parking spot before the rain. This prompts the consideration of an alternative mechanism:

A car has been parked there for some time in the hot sun. The surrounding asphalt in direct sunlight has heated up, while the asphalt beneath the car has remained relatively cool. The car has then left the parking spot, and there has subsequently been a small drizzle. This has evaporated quickly from the surrounding hot asphalt, but is in the process of evaporating more slowly from the cold asphalt where the car was located, and some water still remains there.

In fact, if we constrain ourselves to inside-the-box thinking in a car-in-the-rain situation and approach this as a physics problem, these two timelines (or some combination of them) seem to be the only ones possible:

Assume that the rectangular shape is due to a car, and that all the water has come from rain falling vertically (ignoring a multitude of possible lateral-thinking mechanisms including street-cleaners), and can only leave by evaporation, and that the only significant heat source is the sun. Then the total amount of water that has fallen on the surrounding area over any time period must be greater than or equal to the amount that has fallen on the area under the car(s) over that period (more than one car may have used this spot). Thus there can be less water on the surrounding area at some time only if there has been sufficiently greater evaporation on the surrounding area, i.e. the temperature of the car-area has been sufficiently lower than that of the surrounding area. This must be due to the car-area receiving sufficiently less sun than the surrounding area, and this by assumption means that a car has been parked on this spot at some point shading the ground beneath it.

Relative to any period of rainfall then, a period in which a car was parked there must be either before, during, or after the rainfall. After corresponds to @MatthewJensen's solution (the car shades the wet asphalt from the sun while the surrounding asphalt dries), before corresponds to my solution above (the car shades the dry asphalt, so that the surrounding asphalt dries more quickly after a short rain), while the car being parked during the rainfall does not add anything new to the solution (as during these times the surrounding asphalt receives at least as much water).

The true answer thus could be a combination of these two mechanisms (there may be multiple drizzles and multiple cars), and these seem to be the only two physical explanations with the assumptions we have detailed. Of course, this does not exclude unusual lawn sprinklers or sneaky OPs with watering cans...

Without such tight constraints on possible solutions, the open-ended explanations for a rectangular wet area on asphalt are essentially infinite. These include:

The 'wet patch' is actually grey paint.

After a period of rain, a magic carpet was parked above the asphalt, and this area was in its shadow.

There is currently a square cloud above the parking lot, and this is its shadow.

A long wide roll of wet carpet was left on the pavement.

An inflatable pool was sitting in the parking lot.

The two square wet spots were the footprints of an enormous square-footed robot that just walked through the lake.

Although these ideas perhaps highlight the problems with open-ended real-world puzzles like this, I admit that the rain-fell-on-the-cold-asphalt-where-a-car-had-been explanation seems the most likely to me

• The first one..
– John
Commented Oct 2, 2020 at 19:38
• @John Are you sure? I was rooting for the giant robot.
– Anon
Commented Oct 4, 2020 at 0:45
• A much more thorough answer than mine! I appreciate the credit to my answer :) Commented Oct 4, 2020 at 19:45
• I dunno, Aladdin always parks so neatly. Commented Oct 5, 2020 at 10:31

I believe it's because

It rained. While still raining, a car parked there. While the car was still there, the rain cleared up and the sun came out. Then the car left.
The sun could not dry the rain under the car.

• This is not what happened. But it's a reasonable explanation. The car had left before the rain....
– John
Commented Oct 2, 2020 at 12:13
• @John - You need to put this in the question plus any other preconditions. Commented Oct 2, 2020 at 12:41

Here's my explanation

The OP, John, wanted to give us a puzzle. He went out on a dry day with a watering can and deliberately wet the area to provide the above photograph.

• You my friend got a ditto from me.
– hola
Commented Oct 3, 2020 at 9:04

Another idea:

It is an unusually humid day, and an unusually cold car parked here for a time before driving away; the pattern resulted from an unusually high volume of condensation forming on the undercarriage and falling/splashing onto the pavement.

• I wish my cart was that coo! Commented Oct 3, 2020 at 0:59

I would say

two street cleaning vehicles parked there after they did their job. Since they are wider than a "normal" car, they needed to park two spaces apart from each other.

The wet patches resulted from the excess water dripping down from the underside. That is why the pattern looks so "spotted". There are no wet tracks though, because the sun dryed them up already.

I think this is because

it's a parking for a carwash. The other cars look perfectly clean as well. After the owner payed inside he/she drives off, but the tires are already dry from driving to the parking spot. Standing still causes the car to drip water that remains after the car has left.

Someone parked a couple of airboat trailers there shortly after taking them out of the water so they were still soaking wet. (Unlike most boats, these have roughly the same footprint as a car, so that matches up). Before the picture was taken, the (now dry) airboat owners hooked the trailers back up and drove off with them.

One more rain-related possibility:

tl;dr - Car with a wet undercarriage dripped onto the pavement.

A car drove through a wet area (broken water main, hydrant, the aforementioned street cleaners, etc., or of course rain) to the point where the undercarriage was sufficiently wet, but then entered a drier area (thus water from non-rain sources being more likely, as this would have had to be a fairly short distance after the water). It drove on the drier area and the pictured parking lot for the tires and wheel wells to be fairly dry, but the undercarriage to still be wet. It parked and remained there for long enough that the water on the undercarriage dripped down a lot, and then left the parking lot. The much less wet tire tracks dried before the OP took the picture.

If this was posted in Britain, it would be a trick question: Pavement implies paving stones, and the only paved zone there is the drainage, which collects the rain. Roads haven't been paved since roman times? Sidewalks are often made of pavement. Roads are made of tarmac :) The cars appear to be European, which is confusing, because Europeans use the logical etymology, i.e. pavé is a slab. There are no slabs/pavements visible in the photo other than the curb.

If here was no car before or after the rain, and there was rain and it's not dew, I'd suggest that a car was parked there for a long time which allowed a lot of dust and organic residue to gather under it while the other zones were scoured by rain. The dust has stayed for a few weeks, enough so that it makes a difference in water or dew retention on cold mornings or when it rains.

Perhaps

There are water sprinklers on the lawn that spray water towards the car park. All car spaces contained cars, except those two car spaces. Naturally those two spaces got wet and now have a wet patch. The water has not dried out yet.

• But the wet spot is the size and shape of a car, not the size and shape of an empty parking space. This doesn't explain why it would be dry a foot inside the lines on both sides. Commented Oct 2, 2020 at 13:26
• @NuclearWang Very big cars. Some say the biggest. Commented Oct 5, 2020 at 9:57

There was a car and a sprinkler sprayed the car.

There was a tent

Someone knew that there would be a person that would take a picture of this so he put a giant marble block and dumped water over it, let it evaporate, then left no trace of manipulation