The question pretty much sums it up. How TF is the asphalt 20-40° hotter than the air when it’s super hot?
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Concrete (and more especially asphalt because of its darker color) absorbs light and heat from the sun, unlike the air which lets radiation pass through it.
Heat can only transfer to the surrounding air so quickly, and on really hot days the sun just outpaces it. Concrete can store a lot of heat, and it gets hot from the sun faster than the air around it can wick the heat away.
The first thing to understand is that it isn’t the air that’s heating the concrete, it’s the sun.
Sunlight does heat the air, but for the most part sunlight just passes through the air and hits the ground
Concrete is just very efficient at absorbing heat compared to air, so when it gets baked by the sun all day it can get up to very high temperatures.
A lot of the ambient air temperature actually comes from radiant heat coming from the ground interacting with the air, not from the sun heating the air. This is in part why it gets colder at higher altitudes despite the sun hitting that air first. There’s less radiant heat from the ground heating the air up there, that and the air is thinner.
When nightfall hits it isn’t the air temperature that keeps the planet warm, it’s the radiant heat coming off everything else. The ground, buildings, trees, etc..
The concrete and the air are both being warmed by radiation from the sun.
When sun radiation hits a solid dark object like asphalt, the light is absorbed by the surface and converted into heat (heat is just how fast an object’s atoms are wiggling. The incoming light knocks into them and they wiggle more).
Air, being transparent, lets a lot of this radiation pass through rather than absorbing it and converting it to heat. So the air doesn’t warm up as much.
The ground surface is heated by the sun rather than by the air. In fact, the air is heated primarily by the ground rather than by the sun itself. In the absence of air, the ground would actually be considerably hotter.
Air is mostly transparent to infrared (heat) light from the sun. Asphalt is highly absorbant of it. Absorbing the energy from the light increases the temperature of the material. It also reradiates it after absorbing it which is why you feel heat coming off it even if your not touching it.
Imagine opening the oven door and getting hit with a wave of heat, this heat doesn’t make your face instantly get up to hundreds of degrees. Being hit with a high temperature for a short duration will increase the temperature but it won’t completely finish hearing it.
Now imagine sticking a plate into the oven for several minutes, this will cause the plate to reach hundreds of degrees in the end.
A similar effect is happening to the asphalt, the sun is really hot, but hitting your skin for short durations will not cause it to get that hot because there are many things your body does to cool itself down. The asphalt however, does not. So it will keep absorbing more and more of the heat if it is in the sun for long periods of time.
P.s. dark things absorb more heat from the sun than light things. This is why asphalt(black) is typically much hotter than concrete(grey/white)
Are you thinking the asphalt is hot because the air is hot? It’s actually the opposite, the air is hot because the asphalt is hot. In fact a chunk of asphalt in space orbiting the earth would still get very hot.
Asphalt is really good at absorbing energy from the sun, and therefore really bad at reflecting that energy.
Also, asphalt is only pretty good at radiating its heat energy away, so it can’t get rid of heat easily.
The combination of great absorber, fair emitter means that asphalt can get pretty hot in sunlight.
The sun doesn’t heat the air, it heats the ground. The concrete/asphalt is getting heated by the sun, and that, in turn, heats the air around it.
That’s why being in sunlight feels warmer than shade. The sunlight itself is heating you, even though the air around you is the same temperature in both cases.
When heat from the ground enters the air, it rises away, allowing the cool air from elsewhere to take its place, and the air heats much more evenly than the ground, and therefore doesn’t get heated as much.
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