@raphjd While the IDEA of that sign is very helpful, it is not that accurate. For instance.. it is not air temperature that is the issue, but whether or not the SUN is beating down on the asphalt (or concrete). For instance.. let's say it is 90 degrees F out.. That is certainly hot, but not deadly. If you stand on asphalt that is in the SHADE of a tree or otherwise not being blasted by the sun, it is not that hot.. but if it is being exposed to the sun, it will fry your bacon! Air temperature is not the critical factor. Also, if you stand on the grass - even if the grass is exposed to direct sunlight, it doesn't get that hot. And.. if something is wet.. you know it is not over 212 F.. because if it was, that water would evaporate. The temperature would not rise above 212 F until all the water was gone. Likewise, if you see fresh water in a lake.. you know that the water is at least 32 F. It cannot be colder than 32 F until the water is frozen to ice.
This is the problem when people know a little science...
It is certainly true that air temperature alone does not determine the temp of asphalt: BUT
if you want to get technical, it also depends heavily on the kind and color of the surface,
it also depends heavily on the angle of incidence of the sun: the sun hitting the asphalt at noon is very different from the same sun, at the same temperature, at 6pm. Similarly, tropical climes (where the sun is higher in the sky) will see hotter temps than those at more northern (or, in the Southern Hemisphere, southern) latitudes... and for the same reason: angle of incidence.
BUT: if you don't get lost in the numbers,
the CONCEPT is valid and the WARNING is poignant!
But, your other science facts are also off base:
water CAN be at temps above 100-C (212-F)... WELL ABOVE! Super-heated water is common, so long as it is stagnant (as it might well be - say, in a puddle on asphalt!) Read more here
water can also be cooled to well under 0-C (32-F), and again, in stagnant water this is quite common. Ask anyone who lives in a far-Northern climate: a still pond can be well under the freezing point ... throw a rock into such a pond and it will turn solid in a matter of seconds! (It's REALLY cool to watch!)