The question pretty much sums it up. How TF is the asphalt 20-40° hotter than the air when it’s super hot?
In: 531
dark materials absorb more light (and therefor heat) than ligher materials, so dark concrete or tarmac suck up a ton of heat from the sun, its one of the reasons that old wood buildings seem to stay so much cooler in the summer than modern concrete buildings
Lots of great explanations above but also, it’s not just asphalt. Plenty of other things get hotter than air, like basically anything metallic. Enough to burn you. Even things like sand.
There are 3 methods of heat transfer: conduction, convection and radiation. Conduction means something hot touching the pavement and getting it hot so that doesn’t happen with pavement. Convection is the hot air moving across the pavement warming it up and that’s why pavement is still pretty warm in the shade. Radiation is the biggest factor here where the direct sunlight transfers to the pavement and gets it much hotter.
Heat can travel through conduction, convection and radiation. Conduction is the transfer of heat through an object (you touch a hot pan and you feel the heat), convection is through a liquid (you can “see” the conduction in a pot of water being heated) and radiation is through electromagnetic waves (think of the outdoor heat lamps you see at restaurants).
Heat is just energy and it will naturally move from high energy to low energy. Everything has energy, without energy it just wouldn’t exist (temperature is measure in Kelvin with 0k (-273c or -459f being 0 energy).
There is nothing between the Sun and Earth so the heat is not moving by conduction or convection. It must be moving through radiation. The radiation is energy traveling as an electromagnetic wave until it hits something, in this case it’s hitting the Earth. This energy hits the surface of the Earth and the surface gains energy (heat). This then transfers to the air near the surface of the Earth by conduction (because there are physical particles in the air). This now heated air can then move via convection, and new cooler air takes its place.
As it’s the Earth heating the air by conduction and the hot air moving away by convection the air is not as hot as whatever is in direct view of the sun.
Additionally air temperature is measured in the shade so the thermometer is not measuring the energy from the sun directly.
Energy moves Sun > Earth > Air
Heat transfers in 3 ways:
Conduction — Substance touching substance. Heat transfers from the hotter substance to the cooler substance depending on the difference in temperature and heat transfer properties of the substances.
Convection — Substance touching liquid/gas substance. Heat transfers to/from the liquid/gas, but, because it’s a liquid/gas and hot liquid/gases rise while cooler liquid/gases fall, it causes a flow where the liquid/gas rises/sinks and is replaced by more liquid/gas which then itself gets heated/cooled — and the cycle continues.
Radiation — Substance absorbing/releasing electromagnetic waves. Does not require a medium (can happen in a vacuum) and travels at the speed of light.
The heat transfer you’re thinking about is the first 2. Both of these have bounded limit of temperature change. If Substance A is hotter than Substance B, there’s no way for Substance A to cool down to below Substance B purely from contact and there’s no way for Substance B to get hotter than Substance A purely from contact.
However, no such limitation exists for heat transferred by radiation. If you hit a substance with heat radiation, and if you had a theoretical substance that didn’t give off any heat radiation of its own and had no way to dissipate that heat via conduction/convection, then the temperature of the substance would rise indefinitely.
This is part of the reason why a 60-70 degree day might feel like 80-90 in the sun. The 60-70 degrees reported by weather agencies is taken away from solar radiation (ie from a shaded area). But when you’re outside, you’re feeling the 60-70 degree ambient temperature PLUS whatever heat radiation your body is absorbing from the sun.
This is literally 6th grade curriculum that is repeated several times throughout grade school…
Think: What are the three methods heat can be transferred?
1. Conduction
2. Convection
3. Radiation
The air heats it via convection. The sun heats it via radiation with UV rays which takes it well past the temperature of the air. It’s not a closed system so it doesn’t matter if the air is cooler; it’s being heated faster than the air can cool it. Asphalt is an insulator in this context so this delta in temp can persist easily.
I figure it’s a function of density? Air is not dense allowing light to pass through bending it ever so slightly. Concrete is super dense and stops all of the light. That translates into the heat.
My best eli5 answer:
The sun adds energy to the concrete. The concrete loses energy to its surroundings. If the concrete loses less energy than it gets from the sun, it heats up.
It also heats more than other surrounding objects because the biggest factor of how much energy something gets from the sun is how dark it is. Black absorbs the most energy and tarmac is dark almost black.
There are two characteristics of a material or a body regarding heat.
The first is its sensible heat. Basically, the amount of heat a body needs to heat 1º. The bigger the sensible heat, the more energy is needed to heat that material. Generally speaking, stuff with lots of water has high sensible heat while stuff that are metallic/oxides in nature have low sensible heat. It’s easy to see this with a block of concrete compared to a water pool: the first needs far less energy to be heated than the water.
The second is the thermal radiation. Basically, how much energy is lost across a period of time for a body. This is proportional to the temperature of a body: i.e: the hotter a body is, the more energy it emits through radiation.
So with the two paragraphs above, its somewhat easy to understand what happens. Stuff full of water take too much energy to increase their temperature, making them at a somewhat low temperature and thus radiating little heat while dry stuff like concrete/asphalt heat up throughout the day by absorbing solar radiation, massively increasing their temperature up to a equilibrium of heat absorbed = heat radiated and only really cooling off at night.
Around 95% of the energy received by Earth from the Sun is in the infrared or ultraviolet spectrum. These spectrums of light do not pass through objects the way high energy radiation like X-rays do. This means when an atom is in the path of this radiation, the energy is reflected, deflected, or absorbed.
The darker the object is, the more of this energy is absorbed, and transformed into kinetic energy of the atoms, which means they end up excited, and release the energy in lower energy infrared radiation (heat), or into neighboring molecules as a transfer of kinetic energy.
The denser the object is the more likely an atom will be in the way, and therefore absorb the energy. Once again, this is transformed into kinetic energy, which is released as infrared, or into neighboring molecules as a transfer of kinetic energy.
Molecular kinetic energy is just another word for temperature, so if an object absorbs radiation and is in contact with another object, some of this energy is transferred kinetically, which is why it hurts more to touch a hot object then leave your hand just above it. In fact, if there were no molecules in between you and the hot object (like in a vacuum), the object would not be able to transfer energy directly to you, and must do it via radiation. Typically infrared. Which is how a thermos with a vacuum or double pane glass insulates you. This process requires a much higher difference in energy levels to transfer energy.
Most of the heat in the air actually is released by solid objects. This is because air molecules are not dark, and they are very diffused. So the odds are most of the radiation will just pass through the air and not interact. Some molecules are more efficient at absorbing infrared (think greenhouse gasses, the most efficient of which is water). As an aside, water is so good as a greenhouse gas, it makes cloudy days colder and cloudy nights hotter.
So, in reality, the air is only as hot as it is because it is absorbing some of the energy from the asphalt (mostly as a transfer of kinetic energy) so in order for the asphalt to heat up the air it must be considerably hotter.
Methods of heat transfer in order of efficiency, highest to lowest:
Conduction: from one solid object to another.
Convection: from a solid object to a fluid (gasses are a type of fluid).
Radiation: from one molecule to another without a molecule path between.
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