OK, in a couple of prior postings we’d looked at some local airport and non-airport areas to see if airports were warmer. That was on a modestly sunny day in late summer / early fall ( October 2nd).
https://chiefio.wordpress.com/2010/10/02/asos-vs-sjc-vs-nearby-town/
and
https://chiefio.wordpress.com/2010/10/01/sjc-san-jose-international/
Then, we did, in fact, find airports were warmer. We also discovered that the ASOS automated airport temperature reporting gismos round temperatures UP in whole degrees of C so the ASOS tends to have a ‘jumpy’ graph that is typically higher than the “airport” at the same place (which looks to be a rounded degrees F data feed).
But this left open the question of “Why?”.
Was it the jets taking off?
The ground traffic and cars?
The instrumentation?
The tarmac and lack of ground cover?
Well, today in Silicon Valley it’s a wonderful mid-fall heavy overcast / high fog or low cloud kind of day with nearly zero wind. Exactly ZERO solar heating as of 8 am. But a load of airplanes will have taken off from the airport… A nice time to do a “Tarmac vs Jet Exhaust” examination.
So I’ve taken a look at our target sites.
Palo Alto Airport
Moffett Field Airport
Santa Clara
San Jose International Airport
Reid Hillview Airport
and some nearby areas.
The “punch line” is that everything is more or less about 48 F. The one exception is Reid Hillview which is at 45 F. Odd that. But it is over near the western hills and on cold days we get colder air sinking down those hills that could easily be giving them a slower start to the daily warming.
Santa Clara is 48.9 F and is in a location near cars. San Jose Airport is 48 F and MKSJC (the ASOS at SJC) is 48 F. Palo Alto is 48 F and Moffet is 49 F (but given that Santa Clara is 48.9 F that looks like it’s inside the rounding jitter). A quick scan of the entire list of nearby stations shows them ranging mostly from about 47.x F to 51 F (tossing the highest and lowest couple as outliers). So it looks to me like we’ve got pretty much 49 F +/- 1.x F over the entire south bay, and it’s not significantly higher at the airports than it is anywhere else under still air and heavy overcast. (Remember that this whole area is just one giant Urban Heat Island in a bowl of mountains… so that the whole heat island is about the same is not surprising).
But what about the prior readings? They showed excess heat?
Well, that was on a day with sunshine…
So my conclusion is a pretty simple one: A few thousand feet on a side rectangle of black tarmac and concrete gets hotter in the sun than does a tree lined sub-urban neighborhood with lawns and sprinklers. It’s all about Tarmac vs Trees and the tendency of plants to thermo-regulate via transpiration. In short, it’s the Hydrological Cycle vs Tarmac Desert.
I’ll be keeping an eye on this as the day unfolds. Watching for morning commute impacts near Santa Clara. If we get some significant change of sunshine it will be interesting to see if there is ‘separation’ of the stations. But for now it’s pretty clear that we have a modestly uniform UHI in The Bowl, but when the sun shines, the black dry parts get hotter than the green damp ones.
Want to cure “Global Warming”? Move the thermometer from the airport to the city park…
Some Graphs
Santa Clara. Away from the airport, but not so far for it to have different weather. And well inside the San Franciso Bay Area mountain ringed Urban Heat Island Bowl…
This is is a link to the Wunderground page for that site. The “history” button lets you get these graphs.
http://www.wunderground.com/cgi-bin/findweather/getForecast?query=95053&MR=1
KSANTA Graphs
Next we have SJC. That is the airport proper at San Jose International:
This is is a link to the Wunderground page for that site. The “history” button lets you get these graphs.
http://www.wunderground.com/cgi-bin/findweather/getForecast?query=sjc&wuSelect=WEATHER
SJC Graphs
Finally, we have the ASOS station.
This is is a link to the Wunderground page for that site.
http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=MKSJC
MKSJC The ASOS station at SJC
A Chart of Temperatures Of The Moment
Between when I wrote the top part, and this moment, the ASOS has jumped from 48 F to 50 F, so one supposes that it ticked over a C boundary on the rounding…
ASOS_HFM SAN JOSE, CA, Santa Clara, CA 50.0 °F / 10 °C 43 °F / 6 °C 81% North at 0.0 mph / 0 km/h
30.15 in / 1020.9 hPa 0.00 in / 0 mm / hr 48 °F / 9 °C 55 ft 22 min 0 sec ago MADIS Website
APRSWXNET Santa Clara CA , Santa Clara, CA 47.0 °F / 8 °C 44 °F / 7 °C 90% ENE at 0.0 mph / 0 km/h
30.14 in / 1020.5 hPa 0.00 in / 0 mm / hr 47 °F / 8 °C 116 ft 7 min 24 sec ago MADIS Website
Santa Clara, CA 49.3 °F / 9 °C 46 °F / 8 °C 89% North at 0.0 mph / 0 km/h
30.10 in / 1019.2 hPa 0.00 in / 0 mm / hr 48 °F / 9 °C 12 ft 3 sec ago Rapid Fire
0.5 mi E of Homestead and Lawrence, Santa Clara, CA 50.2 °F / 10 °C 41 °F / 5 °C 70% North at 0.0 mph / 0.0 km/h / 0.0 m/s
30.00 in / 1015.8 hPa 0.00 in / 0 mm / hr 49 °F / 10 °C 140 ft 31 sec ago Normal Website
APRSWXNET Santa Clara CA , Santa Clara, CA 50.0 °F / 10 °C 46 °F / 8 °C 86% North at 0.0 mph / 0 km/h
30.12 in / 1019.9 hPa 0.00 in / 0 mm / hr – 137 ft 7 min 18 sec ago MADIS Website
Santa Clara – Lawrence & Monroe, Santa Clara, CA 49.6 °F / 9 °C 46 °F / 8 °C 88% NNW at 0.0 mph / 0 km/h
30.17 in / 1021.6 hPa 0.00 in / 0 mm / hr 49 °F / 10 °C 88 ft 1 sec ago Rapid Fire
Wooding and Spruance, San Jose, CA 48.7 °F / 9 °C 48 °F / 9 °C 98% WNW at 0.0 mph / 0.0 km/h / 0.0 m/s
29.83 in / 1010.0 hPa 0.00 in / 0 mm / hr 48 °F / 9 °C 0 ft 0 sec ago Rapid Fire
Starbush, Sunnyvale, CA 51.1 °F / 10 °C 38 °F / 3 °C 61% – at –
– – / hr – 75 ft 3 min 29 sec ago Normal
Birdland-Raynor, Sunnyvale, CA 49.1 °F / 9 °C 44 °F / 7 °C 82% NNE at 0.0 mph / 0.0 km/h / 0.0 m/s
30.11 in / 1019.5 hPa 0.00 in / 0 mm / hr 49 °F / 9 °C 180 ft 5 sec ago Rapid Fire
Saratoga Ave./Doyle Rd., San Jose, CA 49.7 °F / 9 °C 44 °F / 6 °C 80% North at 0.0 mph / 0.0 km/h / 0.0 m/s
30.14 in / 1020.5 hPa 0.00 in / 0 mm / hr 50 °F / 10 °C 190 ft 4 min 37 sec ago Normal
El Camino/Wolfe, Sunnyvale, CA 52.7 °F / 11.5 °C 45 °F / 7 °C 75% NE at 0.0 mph / 0.0 km/h / 0.0 m/s
29.94 in / 1013.8 hPa 0.00 in / 0 mm / hr – 90 ft 1 min 6 sec ago Normal Website
near eBay Headquarters, Campbell, CA 48.2 °F / 9 °C 47 °F / 8 °C 98% NNW at 0.0 mph / 0.0 km/h / 0.0 m/s
29.88 in / 1011.7 hPa 0.00 in / 0 mm / hr 48 °F / 9 °C 165 ft 3 sec ago Rapid Fire Website
Strawberry Gardens, Sunnyvale, CA 49.5 °F / 9 °C 44 °F / 7 °C 82% SSE at 0.0 mph / 0.0 km/h / 0.0 m/s
30.08 in / 1018.5 hPa 0.00 in / 0 mm / hr 49 °F / 10 °C 95 ft 5 min 37 sec ago Normal
Campbell Ave. and Foote St., Campbell, CA 50.0 °F / 10 °C 47 °F / 9 °C 90% North at 0.0 mph / 0 km/h
29.94 in / 1013.8 hPa 0.00 in / 0 mm / hr 50 °F / 10 °C 30 ft 0 sec ago Rapid Fire
Sunnyvale – Jasmine, Sunnyvale, CA 50.0 °F / 10 °C – 87% North at 0.0 mph / 0.0 km/h / 0.0 m/s
30.19 in / 1022.2 hPa 0.00 in / 0 mm / hr 50 °F / 10 °C 70 ft 3 min 47 sec ago Normal
Third and Campbell, Campbell, CA 49.1 °F / 9 °C 47 °F / 8 °C 93% SSW at 0.0 mph / 0.0 km/h / 0.0 m/s
0.00 in / 0.0 hPa 0.00 in / 0 mm / hr 49 °F / 9 °C 197 ft 1 sec ago Normal
APRSWXNET Sunnyvale CA , Sunnyvale, CA 50.0 °F / 10 °C 44 °F / 7 °C 80% NE at 0.0 mph / 0 km/h
30.17 in / 1021.6 hPa 0.00 in / 0 mm / hr – 127 ft 7 min 49 sec ago MADIS Website
Berryessa, San Jose, CA 48.0 °F / 8 °C 45 °F / 7 °C 92% South at 0.0 mph / 0.0 km/h / 0.0 m/s
30.13 in / 1020.2 hPa 0.00 in / 0 mm / hr 47 °F / 8 °C 131 ft 2 sec ago Normal Website
Comstock Ln and Park Wilshire Cr, San Jose, CA 47.3 °F / 8 °C 47 °F / 8 °C 98% ENE at 0.0 mph / 0.0 km/h / 0.0 m/s
30.06 in / 1017.8 hPa 0.00 in / 0 mm / hr 47 °F / 8 °C 188 ft 14 min 14 sec ago Normal
Purdue Drive, Saratoga, CA 48.9 °F / 9 °C 44 °F / 7 °C 86% West at 0.0 mph / 0.0 km/h / 0.0 m/s
30.12 in / 1019.9 hPa 0.00 in / 0 mm / hr 48 °F / 9 °C 242 ft 2 sec ago Rapid Fire
Near Saratoga and Cox, Saratoga, CA 52.2 °F / 11 °C 44 °F / 7 °C 79% SSE at 0.0 mph / 0.0 km/h / 0.0 m/s
30.14 in / 1020.5 hPa 0.00 in / 0 mm / hr – 165 ft 1 min 2 sec ago Normal
Near Westmont HS, San Jose, CA 49.0 °F / 9 °C 43 °F / 6 °C 83% WSW at 0.0 mph / 0 km/h
30.16 in / 1021.2 hPa – / hr 48 °F / 9 °C 260 ft 6 min 35 sec ago Normal
Homestead/Kennewick, Sunnyvale, CA 48.6 °F / 9 °C 46 °F / 8 °C 92% ENE at 0.0 mph / 0 km/h
30.24 in / 1023.9 hPa 0.00 in / 0 mm / hr 48 °F / 9 °C 225 ft 2 sec ago Rapid Fire
Rooftop – Larchmont Ave, Saratoga, CA 48.0 °F / 8 °C 44 °F / 7 °C 88% North at 2.0 mph / 3 km/h
29.78 in / 1008.4 hPa 0.00 in / 0 mm / hr 48 °F / 9 °C 301 ft 12 sec ago Normal
Graphs for Moffett, Palo Alto, and related
Well, as I type this, Palo Alto looks pretty grim. Like someone just turned the equipment on at 7 am when they showed up for work…
KPAO Palo Alto (light plane) airport
Moffett is a bit better and right next door.
KNUQ Moffett Field NAS
Interesting to me that the “nearby” automated non-ASOS also is a bit ‘jumpy’ in the graph. More ’rounding up’ issues to find? Hmmm?…
MD2458 APRSWXnet Santa Clara
I could not resist adding KCASUNNY4 just because of the name… Strawberry Gardens:
KCASUNNY4 Strawberry Gardens Sunnyvale
Then, a bit more up in the hills and a rich area with lots of lawn and trees, Los Altos:
KCALOSAL7 Los Altos
In Conclusion
Not enough to make a definitive “call” on a conclusion, but where this points me is to the idea that we have a few things interacting.
1) It’s the Tarmac. Sure, some planes taking off can add a touch more heat for a moment. If it happens at just the right time in just the right place for the wind to take it to the thermometer, might even add a touch for a day. Worth further study… BUT, the “big lump” is the acres of tarmac that don’t go away, even when planes do not fly. So AGW True Believers hollering about how few daily takeoffs and landings there are in the New Zealand airports have missed the point…
Furthermore, to the extent Svensmark is right, decreased cloud cover from cosmic rays during “warm times” will have an amplified impact at Airport Thermometers. Right at the time we moved most of the thermometers to Airports. (Can you say “crappy instrumentation decisions?… I knew you could…)
2) It’s the Vegetation. To the extent that we’ve gone to other recording sites and put them over gravel, stone, cement, tarmac, asphalt, bare dirt; and cut back the bushes, shrubs, trees, etc. what we’re finding is that those surfaces have ‘warmed’ relative to the natural flora that was there in 1800… Basically, ‘grooming’ the recording stations landscape is an error term. Having it near a paved access road is also an error term. Yet that is what we do to make things “neat”.
3) And it’s the sunshine interacting with those two things. See Svensmark…
4) The “new” equipment does ‘odd things’ to the graphs with step functions, rounding up, all sorts of “minor” oddities. Even in equipment other than the ASOS (see the APRS WX graph…) Since we’re supposed to panic over 1/100 C, how about we use the equipment that reports a smooth temperature graph without that rounding induced step function? It also means we need to audit exactly what equipment is used in each location, and how that has changed over time, and characterize the impact that the shift to automation has had on the end result. You simply can NOT change the thermometer that much in a calorimeter and not have something bad happen to your results. Go ask a chemist…
5) It is also pretty clear to me that anyone attempting to measure the ‘Airport Heat Island’ at their nearby airport simply MUST take into consideration the amount of sunshine they get. Testing at Phoenix will give a far different result than testing at London in the fog. Any study that found “no difference” and did not report the degree of sunshine during the test is in need of an overhaul. Still air and sun ought to give the strongest AHI while moving air and no sun ought to give the least (and rain / snow will likely dampen it as well ;-). Find a place where those conditions both happen and you can characterize the magnitude of the impact.
Bottom Line: Airports are a lousy place to measure “climate change” and will give “bogus high” temperatures AND “bogus high” trends over time. Yet that is where we now measure over 90% of our temperatures.
[Sidebar On Trends: The reason you will get bogus high TRENDS is two fold. The first, and simplest, is that airports grow over time. SJC was a grass field not all that long ago, in climate terms. It was a small ‘prop plane’ place in the 1960s and added some modest commercial traffic in the ’70s. But now it’s a giant wall to wall tarmac place with 2 international terminals. That is a common history for most airports.
The second is more subtle. You will find the AGW apologists constantly waving about “The Anomaly” as a talisman. And it is somewhat true that if you did anomalies as a single instrument compared only to itself (as it ought to be) you would remove a lot of error and bias. BUT (and it’s a very very big “But”…) that is not what they do.
First off, they change the hardware. So we now have automated things that do something different from what an old Liquid In Glass thermometer in a Stevenson screen did. An ‘anomaly’ between them is part temperature change and part instrument change. Oops… is that another Error Term I see? Yes, it is…
Then there are changes in process over time as well, so the temperatures “now” are differently handled from those “then”, so now you have a “Handling Charge” in your Error Terms as well.
Finally, they do “grid / box” anomalies in codes like GIStemp. Not “instrument to itself” anomalies. This is like comparing my 1967 VW to my present Mercedes and saying cars are going much faster now. An average of 4 thermometers in cow fields in 1850 in Santa Clara is simply NOT the same as an average of the thermometers at Moffett and SJC today. Making an “anomaly” out of those two averages simply tries to hide the stupidity of ignoring that they are measuring quite different places with quite different instruments and methods. Sweeping your Error Terms under the “grid / box” anomaly rug does not make them go away… ]
Musings from the Chiefio 
Isn’t is obvious that air over any paved area is going to be warmer as reflected sunlight takes a “second bite from the apple” instead of going in to the great heat sink of planet Earth? Water vapor is responsible for 99.9% of all atmospheric heating.
If you want to ruin the economy over a trace gas, by definition insignificant, and give the Elected Ruling Class more power than anything since the Magna Carta then be my guest. I have two patents pending on systems using CO2 that will make me very rich you very poor.
You too can speak “Conservative.” See: http://adrianvance.blogspot.com “Two Minute Conservative,” daily 300 word, two minute hot topic intros for broadcasters, opinion page editors and dinner table illuminators. Also on Kindle as “The Two Minute Conservative.” Read, speak and drive liberal progressives nuts.
Makes sense to me.
E.M.
Another story:
In 1984, I had reason to question the temperatures which could occur within post-tensioned 7 1/2 inch concrete slabs topped with one inch of asphalt. These slabs were in 30 foot square bays, and the ones with the temperature concerns were the top level of a parking garage – the one exposed to the sun.
So we bought a dozen thermocouples, a meter to read them with, a drill, and an accurate depth gage. We then drilled holes upward from the bottom of the slab such that the thermocouples would be lodged at various depths in the concrete starting at 1/4 inch from the surface.
Once setup, we returned before dawn the following morning and started reading the temperatures. We were astonished to find that the probe at 1/4 inch from the top of the slab, 1 1/4 inch +/- from the surface of the asphalt topping reached 162F for a brief period maybe 10 minutes after the sun first hit it – this in Chicago in August. It then dropped to 135F where it stayed for the rest of the morning. We came back two more days because we couldn’t believe it, but got the same result.
Our interest was in an effect we suspected this uneven heat was having on the slab and since we had our answer, didn’t look into the other aspects of this odd discovery.
If any structural engineer is interested in why were doing this, leave a note.
Obviously the heat went somewhere. We thought it was radiated out of the slab and off the topping into the local environment.
One might wonder, as I can see E.M. wondering, what the effects of a hot asphalt topping maybe 8 to 14 inches thick might be on the neighborhood.
The asphalt had formed a “heat tromb” and would hold it to dissipate into the earth under it and the air overhead would be cooler as a result. Ordinary white concrete is about 80% reflective where fresh snow is 99% reflective. The idea of painting everything white is exactly the wrong thing to do if you want to reduce air temperature. The new Science Czar, John Holdren, is a former Berkeley physics prof where ever class begins with a 15 minute “green” indoctrination. We are living in a new Dark Age.
You too can speak “Conservative.” See: http://adrianvance.blogspot.com “Two Minute Conservative,” daily 300 word, two minute hot topic intros for broadcasters, opinion page editors and dinner table illuminators. Also on Kindle as “The Two Minute Conservative.” Read, speak and drive liberal progressives nuts.
Well, the sun has come out, the overcast has cleared, and we’re getting a nice noonish spike in air temps.
Interesting that Palo Alto and Moffett are showing wind and the rest not so much… Leaves out my window are slightly rustling but tree tops are still. I’d call it minor to trivial gusts and nothing more inland. Wonder what the deal is at Moffett… Nearer the bay? More traffic taking off? ;-)
At any rate, everyone is running about 60 F to 61 F right now. Guess it takes more than 1/2 hour for the tarmac to get hot…
By comparison, Phoenix Arizona (Coronado historical district) is running 72 F while Phoenix Sky Harbor Airport is 80 F indicated. Wow! 8 F. (In fairness, the rest of the Phoenix area looks a mess. Some low, some as high as the airport. Scottsdale airport is 78 F while Glendale airport is 79F. APRS WXnet Phoenix is 79F and ASOS is 81 F. Then we have 16th Street (one presumes downtown tarmac land) at 82.4 F. While Tempe Lakes reports 72 F… (Southern suburbs and one presumes near water… East Tempe is 78.3 F.
All of which leads me to believe that what we have here in our surface measuring system is a large black solar collector with a thermometer in it, in cities downtown and near airports; while places further away and out in the country, or near water, are much much cooler in the sun.
Will be fun to watch the rest of the afternoon unfold…
BTW, I grew up in central valley California experiencing first hand (or first foot..) the hot asphalt as we would try to walk barefoot to the community pool. You got really really good at walking the white paint stripes and at dashing from plant to plant… I’d guess it was about 150 F on the black stuff, and about 90 F on the plants… Yeah, it makes a difference.
Of relevance here is a post of mine from July that looked at ‘green roof weather stations’, where several different types are compared with asphalt and one ‘white’ roof. http://diggingintheclay.wordpress.com/2010/07/17/thermal-absorption-a-black-and-white-and-green-issue/
Figure 4. Differences between sites and roof types on a sunny and overcast day
Golly, just 2pm ish and already SJC and the ASOS MKSJC are pulling a degree or two ahead of the others.
Things that make you go “Hmmmm…..”
OK, the graph above I was able to make work with this text:
img src=”http://diggingintheclay.files.wordpress.com/2010/07/max-f.png” width=”442″ height=”250″ alt=”Put Caption Here” /
inside angle brackets…
Verity’s article is well worth a read. A question I’d wondered about but never got to testing. What is the impact of surface type on a measuring station over it. Compares a black tar typical roof with one that is made of plants “green” and one painted white. Finds that surface color makes a big difference… yet we ignore surface colors in our present measuring stations…
Just to document it… The cloud cover is returning with about 50%+ wispy to thick ‘cirrus’ I think. Filling in fairly fast as the sun angle lowers in the afternoon.
I suppose I ought not to be surprised at how fast the sun went from ‘overhead’ to ‘near the horizon’ but it’s just a few hours… and with it the ‘noon’ clearing is turning back to ‘overcast’.
It will be interesting to see how fast the 1-2 F extra at SJC and Moffett return to ‘just like nearby’… (Aren’t live graphs fun ;-)
There are updating links for the three ‘green roof’ weather stations:
http://128.59.122.78:82/command=RTMC&screen=Temperature%20QBG
http://128.59.122.78/command=RTMC&screen=Temperature%20CU
http://128.59.122.78:81/command=RTMC&screen=Temperature%20FS
The other thing I found interesting was how the green roofs responded depending on the depth of the undersurface.
j ferguson
If any structural engineer is interested in why were doing this, leave a note.
=======================================
I’m not a structural engineer (but I play one on tv), and was quite curious about the project.
I will trade you the story why I grew corn in my closet with a black light.
mrpkw,
You gotta be familiar with ACI 318, or the thing wouldn’t make sense. The short take is that the slab was designed with the assumption that the temperature would be constant with depth. Due to the sudden onset of heat at sunup, for a brief period the top of the deck would reach 162F while the bottom was ambient, in this case maybe 75F. Usually design can handle this differential but due to a misconstruction of an allowance in the 1981 edition of ACI 318, something was done which removed the tolerance for the temperature delta causing cracks on the bottom at 1/4 span.
The thing that made me write the comment here was the startling discovery that the top of the concrete would heat to 162F for 10 minutes or so at sunrise and then cool off and not get above 135F until early afternoon when it would get up toward 150F, but never again reach 162F. It isn’t a trombe effect, at least as I understand it, but must have something to do with some thermal conductivity issue with the concrete.
I presume you checked for cars parking over it at just after sunrise ;-)
Cement has strange chemistry. I wonder about things like water content and CO2 or other gas adsorption effects too..
Would be very interesting to investigate, had we world enough and time…
E.M.
It was our garage, and no cars were allowed in – easy at 5:45AM. I do think that there might be something to your concrete and moisture idea though. Maybe something like the concrete not initially conducting the heat away from the very hot asphalt topping at first but then the conduction coefficient rapidly increasing due to something like no more moisture to evaporate.
One of the things I worry a little about is the sort of local environments that the thermometers live in and how little the folks who accumulate the numbers seem to know about them.
When we could get this screwy phenomenon on something seemingly simple, how many other effects can there be that are not being recognized? Surely there must somewhere be a better study of thermometer siting issues than Jones’ China UHI “report.”
Perhaps the following could be a source of warming error caused by tarmac in cold winter climates.
Tarmac and the compacted fill under it allows ground heat to escape more efficiently as compared to sod, grass and snow cover. For example, the frost depth under a paved road might extend from 10 to 15 feet below the surface, where the frost depth on an adjacent field might be only 1 to 2 feet.
Don’t know whether or not this is significant…
Heat is transmitted to “tarmac” and “white concrete” by absorption of IR radiation.
Heat goes from “tarmac” into the air by conduction. Once the layer of air next to the surface becomes warmer, it is carried away from the surface of either paving material by convection. There is a lag time.
Tarmac and white concrete emit IR radiation, cooling either. Tarmac gets much warmer (hotter) than white concrete, thus holds more heat.
The history of using asphalt concrete (AC) (tarmac) as a paving material in the US is telling.
Before WWII, AC was not much used as a paving material. Paving material except for Portland cement concrete (PCC) was “oiled” aggregate, either road oil applied to the surface of a layer of aggregate, or road mixed. Most roads were graded dirt, or if more traveled, covered with a layer of gravel. There were some few roads paved with “macadam”, and they held up well if well drained, but as there were rocks as large as 3″ in diameter in the mix, sections were about 6″ deep minimum.
With the advent of WWII, Portland Cement became a strategic war material. Machines were developed to premix aggregate with asphalt (cold mixing it), place it on compacted base in a windrow, blade it to grade, and then compact it. The resulting pavement was not durable.
After WWII, development of asphalt concrete began in earnest, due to the need for paving roads at least expense. With the huge increase of automobile and truck traffic, asphalt (a byproduct of refining crude oil) supplies became huge and cheap. Hot mixing of asphalt and graded aggregate began to be the rule. In time, thin overlays of AC over PCC became feasible. PCC paved streets began to be overlaid with 2″ layers of AC, and as a section of AC 2″ thick would stand up to parking lot traffic, parking lots previously of gravel began being paved with AC. New streets were constructed of AC over a compacted crushed aggregate base
Asphaltic shingles also became common.
So, in short, tremendous areas were paved with asphalt concrete over time, after WWII, streets and parking lots which had mostly been gravel with some PCC paving,and asphaltic shingles replaced shake (wood) shingles.
In about 1972 (working for CalTrans) I roughly estimated that at least 2,300 square miles of the Los Angeles Air Basin were paved with asphalt concrete, not counting asphaltic roofing. I was being extremely conservative in that estimate.
The Heat Island Effect increase during the last 60 years has been grossly underestimated.
j ferguson
No, I have no knowledge of ACI 318 but I am over all curious about why/how slabs may heat in the context of how it may relate to AGW.
Thanks