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
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.
Latest Answers