Radiation Heat radiation (as opposed to particle radiation) is the transfer of internal energy in the form of electromagnetic waves. For most bodies on the Earth, this radiation lies in the infrared region of the electromagnetic spectrum.
One of the first to recognize that heat radiation is related to light was the English astronomer William Herschel, who noticed in 1800 that if a thermometer was moved from one end of a prism produced spectrum to the other, the highest temperatures would register below the red band, where no light was visible. Because of this position, this form of radiation is called infrared (infra being the Latin word for below or within). Sometimes this kind of radiation is called “heat waves” but this is a misnomer. Recall that heat is the transfer of internal energy from one region to another. As all forms of electromagnetic radiation transfer internal energy, they could all be called “heat waves”.
Hot objects are “brighter” than cold objects. Dark objects are lose and gain heat faster than light objects.
The Stefan-Boltzmann law relates the heat flow rate emitted or absorbed from an object to its temperature (and surface area and darkness). It was empirically derived by the Austrian physicist Joseph Stefan in 1879 and theoretically derived by the Austrian physicist Ludwig Boltzmann in 1884. It is now derived mathematically from Planck’s law.
Φ = εσA(T4 − T04)
|Φ =||(phi) net heat flow rate [W] emitted (+) or absorbed (−)|
|ε =||(epsilon) emissivity, a dimensionless (unitless) measure of a material’s effective ability to emit or absorb thermal radiation from its surface; ranges from 0 (none) to 1 (maximal)|
|σ =||(sigma) Stefan constant, 5.670 × 10−8 W/m2K4|
|A =||surface area [m2] of the object emitting or absorbing thermal radiation|
|T =||absolute temperature [K] of the object emitting or absorbing thermal radiation|
|T0 =||absolute temperature [K] of the environment|
Connect Stefan-Boltzmann law to Planck’s law.
|σ =||2π5(1.38× 10−23 J/K)4|
|15(6.63 × 10−34 Js)3(3.00 × 108 m/s)2|
|σ =||5.67 × 10−8 W/m2K4|
Dark colors absorb more radiant energy than do light colors. The burns on this woman’s skin mimic the pattern on her blouse. She was exposed to a monstrous dose of electromagnetic radiation from a nuclear blast. (Source: NARA)
Disconnected thoughts that aren’t quotes.
- blackbody radiation, cavity radiator, the sun is a blackbody
- For humans, the emissivity in the infrared region is independent of the color of the skin and is very nearly equal to 1, indicating that the skin is almost a perfect absorber and emitter of radiation at this wavelength. If we could see in the deep infrared emitted by the body we would all be nearly black. Under normal conditions, about half our energy loss is through radiation, even if the surrounding environment is not much lower than body temperature.
- Thermos Flask, invented by James Dewar, Scotland. Coating the inside of an evacuated double walled, glass bottle with a thin layer of silver reduced the heat loss by radiation by a factor of 13. Dewar commissioned a German glass blower to make some, who discovered that milk for his baby stayed warm in the flask overnight. He took the idea of the “Thermos Flasche” to a manufacturer.
- A glass cake pan will require 20% less baking time than a shiny surfaced pan.
wien’s displacement law
Warm objects are infrared, warmer objects are red hot, even warmer objects are white hot, even more warmer objects are blue hot. Color and temperature are related.
Wien’s displacement law relates the peak wavelength of the thermal radiation emitted by an object to its absolute temperature. First derived by the German physicist Wilhelm Wien in 1893 from a difficult thermodynamic argument that I will not pretend to understand; now derived mathematically from Planck’s law.
where x is the solution of…
|xex||− 5 = 0|
|ex − 1|
In shortened form…
|λmax =||(lambda max) the peak wavelength [mm] of the emitted thermal radiation|
|b =||Wien’s displacement constant, 2.898 mmK|
|T =||the absolute surface temperature [K] of the object emitting thermal radiation|
A more complete explanation of this law can be found in the section on Planck’s law.
|blackbody color by temperature|
|radiant energy source|
|2.73||cosmic background radiation|
|500||household oven at its hottest|
|660||minimum temperature for incandescence|
|770||dull red heat|
|1,400||glowing coals, electric stove, electric toaster|
|2,800||incandescent light bulb, 75 W|
|2,900||incandescent light bulb, 100 W|
|3,000||incandescent light bulb, 200 W|
|3,100||sunrise or sunset (effective)|
|3,200||professional studio lights|
|3,600||one hour after sunrise or one hour before sunset (effective)|
|4,000||two hours after sunrise or two hours before sunset (effective)|
|5,500||direct midday sunlight|
|7,000||overcast sky (effective)|
|incipient red heat||500–550||770–820|
|dark red heat||650–750||0920–1020|
|bright red heat||850–950||1120–1220|
|yellowish red heat||1050–1150||1320–1420|
|incipient white heat||1250–1350||1520–1620|
|T (K)||class||λmax (nm)||color name||examples|
|6000||G||480||yellow||Sun, Alpha Centauri|
- The total global energy consumption of all the humans on the planet is about 1.4 × 1013 W or about one ten-thousandth the total energy from the sun incident on the Earth. The energy use per area in US metropolitan areas is roughly 2% of the incident solar energy.
- 3.827 × 1026 W total solar luminosity
1368 W/m2 solar constant (energy perpendicular to direction of propagation)
0.30 albedo (latin albus, white), surface 0.04, atmosphere 0.26
342 W/m2 effective solar constant (averaged over time and surface)
- Arrhenius 1896 estimates CO2 warming
- Keeling 1960 shows CO2 is increasing
- Manabe 1967 develop–radiative convective model, first GCM
- Hansen 1988 GCMs indicate the signal of anthropogenic global climate warming would soon emerge from natural variability
- Ice cores
- Mann 1998 Hockey stick graph
The basic effect…
Global temperature and atmospheric carbon dioxide trends match. The very long graph made popular by Al Gore in An Inconvenient Truth.
Plot one against the other. The relation is approximately linear. Al Gore never did this one.
Naturally occurring greenhouse gases whose concentrations are increasing due to human activities
- CO2 from burning forests and fossil fuels
- CH4 from rice paddies, cattle, termites (whose population is thought to have increased due to global deforestation), oil fields, and pipeline leaks
- N2O of agricultural origin
Other naturally occurring greenhouse gases of lesser concern.
- Water is also a greenhouse gas, but its concentration in the atmosphere is affected by temperature and is not directly affected by human activities.
- Ozone is also a greenhouse gas but its greenhouse effects are not easily quantified
Greenhouse gases that do not occur naturally.
- CFCs from from discarded or leaky refrigerators and air conditioners
Chlorofluorocarbons (CFCs) do not exist naturally, but were invented in the 1930s by researchers at General Motors looking to replace the toxic and corrosive refrigerants in use at the tine: ammonia and sulfur dioxide. CFCs are also implicated in stratospheric ozone loss (the so called “hole” in the ozone layer).
- HFCs, hydrofluorocarbons
- HCFCs, hydrocholofluorocarbons
- PFCs, perfluorocarbons
Indirect greenhouse gases
- carbon monoxide
key infrared absorption bands in the atmosphere correspond to H2O, CO2, O3
(CO2 = 1)
|SF5CF3||see note below*||19,000||1,000||?||0.59|
Temperatures are rising across the globe.