6 ms·
In case anyone is curious why tarmac gets so hot: > The rate at which an object can reflect solar radiation is called its albedo [source: Budikova]. The bigger
by onlyrealcuzzo 4y ago
In case anyone is curious why tarmac gets so hot:
> The rate at which an object can reflect solar radiation is called its albedo [source: Budikova]. The bigger the albedo something has, the better it reflects radiation. Traditional asphalt has a low albedo, which means it reflects radiation poorly and instead absorbs it.
Asphalt has an albedo of ~0.05 - which means it absorbs ~95% of solar radiation. Tarmac is ~0.1 - which means ~90%.
- drhodes 4y agoI didn't know there was a difference between the two! > Asphalt is a mixture of aggregates and bitumen that needs up to two days before it completely cures. Whereas, Tarmac is a combination of crushed stone and tar that cures quickly. https://tarmacdriveways.ie/key-differences-tarmac-vs-asphalt-driveways https://tarmacdriveways.ie/key-differences-tarmac-vs-asphalt...
- senkora 4y agoFrom your link I learned that tarmac is short for tarmacadam, named after John Macadam, who is also the name-origin for macadamia nuts. https://en.m.wikipedia.org/wiki/John_Macadam https://en.m.wikipedia.org/wiki/John_Macadam
- the_jeremy 4y agoThat is incorrect. John McAdam[0] has the name for tarmac, while John Macadam[1] has the name for macadamia nuts. 0: https://en.wikipedia.org/wiki/John_Loudon_McAdam https://en.wikipedia.org/wiki/John_Loudon_McAdam 1: https://en.m.wikipedia.org/wiki/John_Macadam https://en.m.wikipedia.org/wiki/John_Macadam
- senkora 4y agoThanks for the correction! I can no longer edit my comment but you are right.
- touisteur 4y agoAnd there's a name that became a French word there: macadam https://fr.m.wikipedia.org/wiki/Macadam_(route) https://fr.m.wikipedia.org/wiki/Macadam_(route). Thank you both parent and GP for the history/etymology lesson.
- JoeAltmaier 4y agoInteresting! I often wondered while sitting on a runway, if tarmac was so good, why don't we get any? It's just at airports!
- selectodude 4y agoStrangely, you don't sit on Tarmac, even on a runway. Airport ramps, aprons and most busier runways are paved with high strength concrete.
- Sharlin 4y agoYes, in other words: the darker a thing is, the darker it is. Is someone here actually suprised by the everyday phenomenon that dark objects get hotter than light-colored ones? (Edit: I apologize for the uncalled for smugness. But "albedo" is not some special physical quantity, it's literally a measure of how light/dark colored a material is. "Alba" is Latin for "white". Although granted, albedo is usually measured across the whole spectrum, not only the visible part.)
- powerhour 4y agoNo need to be an asshole about it. Some folks probably don't know the relative darkness of asphalt and tarmac.
- Sharlin 4y agoBut according to GP's numbers, tarmac is less dark than asphalt so should get less hot than asphalt. But those numbers are suspect and probably only refer to very fresh asphalt/tarmac; the albedo aka "lightness" of worn asphalt is something like 0.3.
- Swizec 4y agoThe difference between "not surprised" and "understand why" is vast. For example we (humanity) still don't understand _why_ gravity works despite not being surprised by its existence. And don't forget the 10,000 rule: https://xkcd.com/1053/ https://xkcd.com/1053/
- Sharlin 4y agoYes, but GP's quote was not an explanation, it was literally saying that "light objects reflect more light". Albedo literally means "whiteness" ie. how light something is.
- Swizec 4y agoFrom GP: > Traditional asphalt has a low albedo, which means it reflects radiation poorly and instead absorbs it Seems like a pretty good high level explanation of why that matters. Darker objects absorb energy instead of reflecting it. I'm sure there's more to it than that when you get into details.
- Scoundreller 4y agoAnd then you have other factors impacting a surface temperature including: rainfall (you could probably spray the runway and it will cool down and be dry quickly…) Length of day (we’re about 1 month past the summer solstice in the N. Hemisphere, and days are long in the northern latitudes) Shade (unlikely on runways, they’re in the sun all day) Wind (probably best anywhere on a runway unless it’s in a valley) Cloud cover during the night: can’t radiate to the universe where it’s about -273C if clouds are in the way.
- acallaha 4y agoCan you clarify how night cloud cover factors in? Do clouds really substantially reduce the radiative cooling capacity? Is that by functionally reflecting radiation back at the source (the earth)?
- GCA10 4y agoBroadly yes. It's why deserts have such extreme temperature fluctuations between day and night. No clouds are part of the reason, as well as very little humidity to serve the same heat-retaining/heat-reflecting role. On my first time in the Nevada desert I was astonished to find that I needed a jacket at dawn to stay warm, after all kinds of daytime heat. Looking at Elko NV's weather this week, it's got highs of 99F/37C and overnight lows of 61F/16C
- swores 4y ago37C high earlier today and 16C low later tonight are coincidentally the exact numbers for my city in the UK. Which isn't of course anything like a desert, though the whole country is in the middle of a heatwave (and last night didn't drop below 20c).
- Scoundreller 4y agoRadiation is a function of the temperature delta between two points. The sky around where clouds are might be around -20C, while outer space is at -273C or so, so you get much more radiative heat loss in the latter state (clear sky). (This is also why you’ll see gardeners try to cover their plants when there’s a “frost warning” even though the temperatures won’t be going below freezing). And from the opposite perspective, the sun is really far away but incredibly hot, so it transfers heat to us, and more as you get closer (and less as you get further away).
- lamontcg 4y agoAnyone out there trying to figure out white tarmac/asphalt?
- deleted 4y ago[deleted]
- anamexis 4y agoYou can make runways out of concrete.
- jedberg 4y agoBut then you have to close the runway for extended periods for repair. The beauty of tarmac is that it cures quickly.
- selectodude 4y agoMost runways, at least at busier airports, are already concrete.
- bpye 4y agoI'm not sure that's true? Maybe it depends on the location but LHR is asphalt, CDG is asphalt, AMS is asphalt, YYZ is asphalt... Only when you look at hotter climates do concrete runways appear common.
- jedberg 4y agoI think only in moderate climates. Concrete tends to crack when it gets really cold and then warm.
- anamexis 4y agoWe've got concrete runways here at MSP (Minneapolis/St. Paul). A lot of the highways are concrete as well.
- 4y ago
- andy_ppp 4y agoHow much would putting mirrors on the earth affect climate change? How large a mirror would you need to make a difference?
- elil17 4y agoPractically, high albedo materials (essentially they reflect a lot of light but aren't necessarily mirrors because they do scatter the direction of that light) can reduce building's HVAC energy consumption. They can also effect city level microclimate (mitigating the heat island effect). It takes a lot of high albedo area to effect the overall climate. For example, melting ice caps do decrease the albedo of parts of the Earth, accelerating climate change. Large scale increases to Earth's albedo in the form of space mirrors were seriously considered as options for global warming mitigation as late as the Obama administration. However, the risks of unintended modifications to the environment (e.g. crop failures, unpredictable weather) are generally considered too great. At present, space mirrors, with an estimated lifetime cost of $5 trillion for 50 years of cooling, could be a decent "last resort" option compared to the possibility of spending hundreds of trillions upfront to permanently solve the problem with carbon capture (although cost estimates vary widely based on the amount of CO2 that needs to be captured). Other options include stratospheric aerosol injection (https://en.wikipedia.org/wiki/Stratospheric_aerosol_injection https://en.wikipedia.org/wiki/Stratospheric_aerosol_injectio...) which could minimize the effects of climate change for as little as $55 billion/year, and marine cloud brightening (https://en.wikipedia.org/wiki/Marine_cloud_brightening https://en.wikipedia.org/wiki/Marine_cloud_brightening), which could cost as little as $5 billion/year. Possible side effects might include ozone depletion and acid rain for aerosols. Marine cloud brightening could cause weather changes. So yes, you could manipulate albedo but it will always come with side effects and costs.
- aeroman 4y agoSome of the side effects are common to all kinds of solar radiation management (SRM) - geoengineering that makes the Earth more reflective in some way or another. Greenhouse gases absorb infrared radiation emitted by the Earth and heat the atmosphere as well as the surface. SRM geoengineering just changes how much energy the Earth absorbs from the sun (which has the strongest effect at the surface). If GHGs increase, but you use SRM to keep the surface temperature fixed, you still end up heating the atmosphere. This means in a geoengineered world, you have less rain. A reduction in rain might be more important to some countries than others - not everyone would want the same amount of geoengineering.
- Maursault 4y agoSurely there must be a way to cover tarmac and asphalt with a specially developed white paint to raise albedo to prevent melting.