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%.

I 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...

From 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

That 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

1: https://en.m.wikipedia.org/wiki/John_Macadam

Thanks for the correction! I can no longer edit my comment but you are right.

And there's a name that became a French word there: macadam https://fr.m.wikipedia.org/wiki/Macadam_(route). Thank you both parent and GP for the history/etymology lesson.

Interesting! I often wondered while sitting on a runway, if tarmac was so good, why don't we get any? It's just at airports!

Strangely, you don't sit on Tarmac, even on a runway. Airport ramps, aprons and most busier runways are paved with high strength concrete.

And 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.

Can 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)?

Broadly 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

37C 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).

Radiation 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).

Anyone out there trying to figure out white tarmac/asphalt?

Probably not the wisest idea to have a highly reflective material as the landing surface for planes humans are flying.

Yeah, but it might be useful for the roughly 4 million miles of road in the US.

Actually saw it mentioned that albedo isn't directly connected to colour (and in particular since it's thermal radiation it'll be towards the IR side of the spectrum) there might be alternatives to "nuke flash white" paint colour.

It's only directly related in the visible spectrum, which is a slice that contains around 50% of the spectrum that the sun throws out, by energy. I would imagine the actual color would be something near orange.

> I would imagine the actual color would be something near orange.

Which happens to be close to the color of adobe. I'm yet again starting to think that native peoples were onto something.

You can make runways out of concrete.

But then you have to close the runway for extended periods for repair. The beauty of tarmac is that it cures quickly.

Most runways, at least at busier airports, are already concrete.

I'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.

I think only in moderate climates. Concrete tends to crack when it gets really cold and then warm.

We've got concrete runways here at MSP (Minneapolis/St. Paul). A lot of the highways are concrete as well.

Probably absorbing heat quickly is good for cold climates, to keep warm on cold days. It might prevent ice buildup on days near freezing.

Surely there must be a way to cover tarmac and asphalt with a specially developed white paint to raise albedo to prevent melting.

How much would putting mirrors on the earth affect climate change? How large a mirror would you need to make a difference?

The most cost effective mirrors to cover a large fraction of the Earth and reflect back the sun's heat are... the polar ice caps. Which we are melting because of climate change.

This is one of the main reasons people are so worried about it being "too late" to fix climate change. There are positive feedback loops like this where a little increased heat causes even more rise in temperature.

Practically, 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_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), 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.

Some 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.

Yes, 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.)

Not necessarily true. It depends on the object's optical properties.

I well remember working on my parents' car in Phoenix summers. The chromed wrenches on our driveway surface got to instant-blister temperatures. Tools with black surfaces were uncomfortable to pick up, but were usable.

The chromed tools had a lower absorption of sunlight than the black tools, but their IR radiation coefficient was even lower. So equilibrium temperature (at which radiated energy = incident energy) was much higher for the chromed tools.

I was always careful to put my tools in the shade under the car :-)

(Used to live in Glendale.)

No need to be an asshole about it. Some folks probably don't know the relative darkness of asphalt and tarmac.

But 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.

Snow can have an albedo lower than 50% - and non-white sand can have an albedo higher than that.

It's not as simple as color.

In particular, there are dark things that have an albedo higher than asphalt's 0.05...

Dark will get hotter in sunlight but can be cooler at night if the sky is unobstructed.

> Dark will get hotter in sunlight but can be cooler at night if the sky is unobstructed.

What would the sky be obstructed by? Clouds? How could that affect how cool tarmac on the ground gets at night?

Yes, clouds.

With a clear sky, you're radiating (based on your own temperature) out to the blackness of space, which is radiating back at 4 Kelvin. Which is to say, it's contributing basically nothing. You're just giving up heat to the cosmos.

With a cloudy sky, you're radiating (based on your own temperature) out to the bottoms of the clouds, which are reflecting your radiation back to someone else on the ground, and their own heat is reflecting back to you. And such. The cloud looks white in the daytime because it's very reflective, so it acts like a mylar blanket to hold the heat in.

Physics is strange:

> At night, when cloud coverage increases, downwelling infrared radiation also increases. So, clouds act like space heaters, emitting energy towards the ground. Blankets only limit the transfer of heat energy away from your skin(convection).

Huh, that's very interesting and non-intuitive for a non-physicists like me. Thanks for the explanation!

This is what the greenhouse effect is, literally. At "thermal IR" wavelengths the sky is a blanket. Cloudy sky is even more of a blanket.

I live near a grape growing are, and they have wind turbines to blow air on cool nights slightly above freezing because the radiation from the ground to to sky can be enough to frost-damage their crops.

So they turn them on the blow the stagnant warmer ambient air around and mitigate that.

I was told that light reflects heat, and heats more quickly, but also radiates badly (cools more slowly).

This is why heatsinks are often painted black.

It is also the reason for all the strange white/black colors on rockets.

> heats more quickly

Errata: “heats less quickly”.

The 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/

Yes, 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.

From 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.

If you're lucky there's a course you can overpay for.

They probably use a different ~~tarmac~~ material composition for the runway.

Luton is all asphalt. The long runway at Yuma is concrete and the shorter ones are part asphalt part concrete. And they likely chose the asphalt formulation appropriately given that they’re in the middle of a desert.

I would expect their asphalt being a lot harder and more heat resistant than the one that let my motorcycle fall multiple times in the past years because it melt during summer heat. I clearly wouldn't expect airplanes wheels to "sink" into asphalt like my bike stand, but they need a strong grip when braking during landings which I doubt a much softer surface would offer. Not an expert though, just speculating.

A softer surface would give more grip, not less. In fact it would give so much grip that it starts to adhere to the tires.

I’d assume rainy UK would have softer runway surfaces for wet-weather performance.

Unlike car tires where people may switch them between climates/seasons, aircraft tires need to be fairly universal.

You're right of course, what I wondered was if the asphalt could become so soft that it loses consistency; some of it surely adheres to the tires, but what if it would do that by detaching from the runway? However, I'm certainly biased for having seen asphalt so softened by summer heat that I could leave my tracks just by walking on it.

Have Yuma temperatures actually changed that much? A quick check suggests that the level of change is much smaller than that in the UK. Climate change has increased the extremes in the UK quite significantly in the last 40 years. It's a case that the changes in extremes are not accounted for in UK planning whereas Yuma is known to be consistently very hot.

I buy the 71 C ground temperature; ground temperatures can be 25-35 C hotter than the ambient air temperature, which is what gets recorded. This is also why rails buckle more than you'd expect: on a 40 C day, you're likely to see the rails heating up to 60-65 (as in, if you touch them, your finger will burn).

> ground temperatures can be 25-35 C hotter than the ambient air temperature, which is what gets recorded

Ground temperatures are almost entirely responsible for the majority of the thermal energy/temperature put into the air, down at the surface.

Also I'm sorry but the UK bloody loves a bit of weather melodrama. Climate change is a big problem but the specific effect of it on the UK in particular has honestly not been very bad at all yet. Tarmac has been melting in the heat here longer than I've been alive, which makes me suspect we use weak, crappy British tarmac. This was a little bit hotter than previous heatwaves, which is bad, but felt about the same and has also been really short - not sure what to make of that. However we scarcely get a weekend here without some "most (x) ever in (y)" fuss and haven't in 40 years which is really getting quite tiresome and making it harder to respect the real problems. I'll worry about it when I'm done drinking gin in the rain.

Not just the UK. It's been over 100º a lot of days here in Texas, and people from other places in the U.S. keep asking me how it feels, like it's a brand new and shocking experience. But from a perceptual perspective, it's normal. My perceiving brain doesn't do sophisticated statistics. It isn't a scientist. My brain's reaction to 100º+ weather is, well, this sucks but it's a familiar, age-old kind of suck. I spent time outdoors in 100+º weather every summer growing up, so I just don't make a gut connection between a string of hot days and my sense of fear and urgency about global warming.

> really getting quite tiresome and making it harder to respect the real problems

I agree that it trivializes and undermines the real problem of global warming. People have this false implicit assumption that if global warming is real it must be possible to immediately perceive it, and instead of challenging that, we keep trying to inject drama and danger into familiar experiences, and it just rings false. On a subconscious level, I suspect we all know it's silly. Also, what happens if it's not this hot next year? After spending 2022 essentially telling people that they can directly perceive global climate trends by walking outside, what will they think if the summer of 2023 is relatively cool?

I'm probably wrong, I'm not an expert in public opinion, it just feels false.

>most (x) ever in (y)

The hottest temperature ever recorded here isn't some niche thing like most rainy April for 5 years. Also maybe the fact that there's constantly new unusual weather should tell us something.

yeah lol not surprised. im in texas, not even as hot as arizona, and friends motorcycle kickstands have lowkey melted into ashpalt a few times.

I've been at Phoenix airport on a hot, but not crazy hot, day where the parking lot asphalt was tacky and stuck to your shoes.

Latitude of Yuma, US: 32.6° N

Latitude of London, UK: 51.5° N

To be clear they are talking about the air temperature rather than runway temperature and I've added the latitude so you can understand why it's a big deal that London has hit 40 degrees.

While we're picking random data points - Malillia (Sweden), at 57° N, set a record temperature of 38°C in 1947.

They are not random. Those are the two locations being discussed in the thread you replied to :)

Oh wow, that’s really interesting, was it just a freak event or part of a trend?

-> Hard to Say

That’s weird, some cursory searches show a definite upward trend for quite some time, i do find these weird sites where they consistently pick outliers in a dishonest manner. It really wouldn’t be worth engaging with them though

Trend. Check out the latest IPCC report for more info.

The Phoenix airport has to close down during peak temperatures on summer days because it exceeds the maximum operating temperature of the airplanes. Pretty cool.

Any reason that airports in super-hot places like that, don't run heat-transfer coils under their runways, to cool them down with a heat pump like a skating rink does? Runways aren't an order-of-magnitude larger in surface area than skating rinks, so it's not cost. And they wouldn't even need to turn on the heat pump for 3/4ths of the year. Do we just not know how to create heat-transfer coils that wouldn't break from the stress of planes landing "on" them?

Your comment fascinates me. A normal ice hockey rink is 1,800 square meters. The biggest indoor ice rink in the US is the Schwan Super Rink = 28,000 sqm which consists of 8 separate rinks.

Yuma's 4 runways total 543,000 sqm. At midday those runways are absorbing 543MW of energy from the Sun. So that might require 100-200KW of electricity to run the heat pumps. And then you still need somewhere to dump the heat.

In short, I think the reason they don't cool runways is due to the cost.

There's a ton of comments on HN like this. A quick brainstormed "why don't they just.." as if cooling a runway, building a subway, conducting space travel or doing anything else bound by physics is as trivial just because it is conceptually simple. Might as well ask why you can't ski in your backyard in London because after all it is trivial to add some pipes to your refrigerator.

Also I don't know of any skating rinks that operate outdoors in summer, or to use a more favourable example, something like lowering a surface temperature from 20C to -5C (an ice rink, indoors) vs lowering a surface temperature from 80C to 55C (an assumption for a runway, outdoors). Even Dubai, a major transport hub, much hotter than UK even this week, with cheapish energy costs, little public debate uses other mitigations such as runway materials and scheduling many takeoffs for 2-5AM (there are many costs due to high temperature besides runway degradation).

Then we wonder why when it's hot, it's really bloody hot at airports. Is that sum really right? that an airportsworth of tarmac is like a whole power station pumping its entire output into ground heat? because it looks right to me and it seems like the easy, sensible option is just to rip up the airport...

Isn't a large part of electricity production around making things hot to drive turbines?

Airplanes are very heavy so this may not be physically possible, but if you could run coolant pipes under runways, couldn't that captured heat be used in energy production elsewhere?

Yes. You could heat fluid and use it to generate electricity as you can assume the runway is hotter than the ambient air. But if you look at other solar thermal power stations [0] you can see that they generally use concentrators to get the working fluid very hot for more efficient operation. So a runway based system probably wouldn't be economical even if it were good reuse of space.

0 - https://en.wikipedia.org/wiki/List_of_solar_thermal_power_st...

I’d just douse them with water. It’s not like UK airports and pilots can’t handle wet runway landings.

Hm. An Olympic ice rink is 30mx60m, or 1800m^2. The "largest ice arena complex in the world" (https://en.wikipedia.org/wiki/Schwan_Super_Rink) is 28,000m^2 but has 8 Olympic ice rink, sort of.

Yuma International/MCAS has roughly 540,000m^2 of runway (plus taxiways).

I imagine skating rinks in temperate/hot places are indoors and well insulated. Your solution would work well for an indoor runway.

> Runways aren't an order-of-magnitude larger in surface area than skating rinks,

International hockey rinks are 30m by 60m. My local airport, which isn't that big as far as these things go, has two runways each of size 45m by 2200m. That's 55x larger per runway.

They either use concrete or have different asphalt blends that handle higher temperatures.