Here’s The Science Behind How Cars Stayed Cool Before ‘Real’ Air Conditioning

Swampcooler Top

No, that is not a rocket launcher strapped to the side of a car. It’s officially known as an evaporative cooling device, which was a popular accessory for drivers prior to the standardization of air conditioning in vehicles. Evaporative coolers are nothing new; ancient Persians used the trick to keep temperatures down in buildings long before lowriders were outfitting them to their sparkly sleds. While the usage has varied over time, the basic thermodynamic principles that make it work have not. Let’s talk about it.

Shown below is a patent diagram first filed in 1938. The mechanisms depicted in this “air conditioning attachment for automobiles” drawing are fairly simple, with the primary user-control being the drawstring meant to moisten a cylindrical air “filter” or “sponge.” A spring wound around the central axis of the device returns the filter to place once the user has pulled the string. There’s also a reservoir, which one tops off with a hose every once in a while; between pulling that string, filling up the reservoir, and making sure the filter doesn’t get moldy, that’s all there really is to this old-school air conditioner, which would later become known as a “swamp cooler.”

[Editor’s Note: Robert Peterson is a young engineer who will occasionally be writing technical stories for the Autopian (he has a full-time engineering job). Everyone please give him a nice welcome. -DT]

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Patent Images for KOOLAIR Swamp Cooler

Here’s a look at the guts, courtesy of a swamp cooler repair service on Etsy:

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Image: Etsy
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Image: Etsy

The unit hangs off the top of the side window glass, which allows for universal mounting, but poses some concerns regarding safety and stability. But hey, it’s from the 1940’s when mom-arm seat belts were a thing, so who cares right? 

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But now for the nitty gritty. Here’s your crash course on heat transfer in case you’re a little rusty. There are three modes of heat transfer: conduction, convection, and radiation. In the case of swamp coolers, convection is where it’s at. Convection here is the transfer of heat energy from a dynamic fluid (hot desert air) to a solid (the damp cloth cabin filter). Heat flux (q’’ or “energy on the move”) is dependent upon a heat transfer coefficient (h) and the temperature difference between the passing fluid (hot air) and solid body (damp cloth). Maximizing the temperature difference is an easy way to optimize the rate of heat transfer.

Here’s the heat transfer equation: q”=h(TSurfaceTFluid)

Okay, so you might be thinking; Air will transfer its heat to the colder water that you poured in via your hose, but once the water reaches the same temperature as the air, the cooler will stop working. However, these coolers have a cheat code: evaporative cooling, which supercharges the whole process. You’re right to think that energy moves from hot bodies to cold bodies (though “hot” and “cold” are always relative). Swamp coolers funnel hot dry air over a damp cloth, and energy from the hot air is indeed absorbed by the water particles in the cloth, which elevates the temperature of the water, but while you might think the cooler might become ineffective once the water gets warm, interesting stuff happens when it enters a liquid-vapor state.


Reply to @zanehuff1989 I wanted this for yearrrs before I got one for my truck. Enjoy ???? #1950gmc #retrotok #GetTheLook #vintagevibes

♬ original sound – Travis


According to the Wiley 9th Edition Fundamentals of Engineering Thermodynamics, “When the system is at the saturated liquid state… additional heat transfer at fixed pressure results in the formation of vapor without any change in temperature but with a significant increase in specific volume” (Moran et. al, pg. 63). This trend can be observed in steam tables as shown below. Energy (technically “entropy” in kJ/(kgK)) is still being absorbed from the air even though the temperature is stagnating within the liquid-vapor state. 

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Wiley 9th Edition Fundamentals of Engineering Thermodynamics

Now in English please: All of this heat transfer results in air that is now cooler in temperature and more saturated with humidity, as creating steam sucks an immense amount of energy out of hot air. Environments with low humidity typically experience the most dramatic effects of this process, as there is more opportunity to saturate the air with water.

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image: eBay (vdubrepro)

It’s the very same phenomenon that occurs when we sweat. It takes energy/heat to evaporate the liquid droplets, and that additional energy/heat comes from your body, helping you feel cooler. Generally, this leaves you more comfortable, but maybe a little smelly, too. 

[Editor’s Note:

1965 Amc Ambassador Detail Of Vent Window
Image credit: CZmarlin/Wiki/Public Domain

The headline of this article is “Here’s The Science Behind How Cars Stayed Cool Before Air Conditioning,” and while the “Swamp Cooler” is the main topic of discussion, the truth is, the oldest, most basic way to keep a car cool is via old-school “AC” that doesn’t stand for “air conditioning” but rather “ambient cooling.” This, of course, is just a joke, just like “four-forty air conditioning,” which represents four windows cranked down, and the vehicle driving 40 mph. Either roll down the windows, or — on older cars — pop open the awesome quarter windows:

This keeps you cool in much the same way that a fan does. Your body gives away heat to try to evaporate sweat off the skin, but that high-humidity, evaporated sweat sitting right there against your skin makes evaporating more sweat difficult, as the air can only take so much vapor. So rolling down your windows or cracking your quarter window displaces that humid air, allowing your body to evaporate its sweat, and give off heat.

So rolling down the windows and the “Swamp Cooler” are working on the same principle, leveraging Heat of Vaporization to remove heat. As Robert mentioned, Heat of Vaporization, (Hv or Hvap) is “the amount of heat needed to turn 1g of a liquid into a vapor, without a rise in the temperature of the liquid,” per Sciencedirect. Think of it as energy required to break the bonds between water molecules. Anyway, let’s get back to the cooler. -DT]

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Image of Thermador Coolers from Nacifan  of H.A.M.B. message board. (sale listing)

Once the air from the Swamp Cooler has been “conditioned,” the occupants can enjoy a pleasant, cool summer breeze rather than Death Valley’s finest dragon breath. And to boot, the entire process takes zero electricity or fossil fuel. 

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As neat as all of this is, there are plenty of reasons we no longer see swamp coolers. For one, the units are large and crudely secured to the side of the car. They represent a giant aerodynamic anchor on the side of your vehicle — not ideal for fuel economy, driver visibility, or neighborhood mailboxes. Evaporative coolers can cool air up to 30 °F in ideal conditions, but the effect is diminished in more humid environments. The particular Thermador unit featured in this article requires the passenger to pull a drawstring to dip the filter into the water pool to re-moisten the cloth as it dries out. This renders the device useless if nobody happens to be sitting shotgun. I guess this is what friends are for? The car also has to be moving for it to work. But, ultimately, the standardization of AC in cars made this party trick invention all but obsolete. 

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Image of Thermador Coolers from Nacifan  of H.A.M.B. message board. (sale listing)
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Image of Thermador Coolers from Nacifan  of H.A.M.B. message board. (sale listing)
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Image of Thermador Coolers from Nacifan  of H.A.M.B. message board. (sale listing)

In the words of Stephen King, “Sooner or later, everything old is new again.” As the world contends with growing populations and the ever present climate crisis, some neat sustainable tricks from our ancestors may be worth revisiting. 

(A massive thanks to the Lane Motor Museum in Nashville, TN for their hospitality and pictures)


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70 Responses

  1. Hi there, welcome aboard! I hope you have many more articles like this – I had no idea that this thing ever existed!
    On a related note, seeing the word “filter” used to describe the innards of this thing, I remembered of the actual filters that we had back where I come from. You see, the tap water was not safe for use (or we just didn’t trust it for generations), so we bought those contraptions that are just vessels made of clay, partitioned in two halves. The filtering element sat in the upper half, dripping towards the bottom. They had the nice side effect of keeping the water not only safe, but cool and fresh, because the porosity of both vessels allowed the same effect to take place. So, same result, inverted method/goals 🙂

  2. While fuel economy was mentioned as a concern later, seems a bit disingenuous to say these take no electricity or fossil fuel.

    And beyond just cooling buildings, the Persians (and Indians, if memory serves) used these principles to make ice in regions that seldom ever saw temperatures below freezing hundreds of years before refrigerants were thought of.

    1. Rob actually mentioned those ancient refrigerators! I cut it from the article because I thought it took the reader too far away from the way these particular coolers work, but I’m glad you mentioned it!

    2. Yes! I agree that cutting down on fuel economy results in the use of electricity and fuel. The evaporative effect is ‘free’ energy transfer but it’s application does require other energy input in this case. And I did come across the some ancient Persian examples which could honestly get its own article. 🙂

  3. “Evaporative coolers can cool air up to 30 °F in ideal conditions, but the effect is diminished in more humid environments.”

    In 1976, I moved to the San Joaquin Valley of California. At the time, many buildings had swamp coolers, but they did not seem to work well and were being replaced by air conditioners. Some of the natives told me that increasing amounts of agricultural irrigation had raised the humidity high enough that swamp coolers did not work as well as they had in earlier times.

    1. Yup they were really common here in the Phoenix area too, most of our friends’ houses had both, only using the AC when it got really hot. Not sure how will they work now, because the same thing here as in San Joaquin; lawns, golf courses etc., has driven up the humidity. In fact we now have mosquitos! Something our area was almost immune to as water never collected anywhere for more than an hour.

  4. I wonder if one couldn’t engineer something like that to mount sleekly over a sun-roof hatch on the roof, or add a retractable scoop into the hood with some bypass ducting to mate up with the internal vents.

    I know A/C and modern aerodynamics on a car are remakably efficient, but it’d still be fun to see.

    1. The Lamborghini Espada’s hood NACA ducts are actually for the vent intake. Something like that might work and they’re low drag, as well (if possibly not able to flow enough air for the case here).

  5. Please sir may I have some more?

    You managed to explain that with no mention of sling psychrometers or wet bulb thermometers? Of course I grew up in a house where there was a drawer full of them for some reason. Never wondered why until just now.

    What’s next, a treatise on those canvas bags filled with water people used to hang on the front of their car? Please?

    Note to self: Add “The Sling Psychrometers” to the list of unused band names.

  6. A friend had one of these on hisbathtub Nash in the 90s. It apparently worked OK but was more for show as part of his period accessories.
    The irony of the name “swamp cooler” is that hat they don’t work in swamps but are very effective in deserts.

  7. Welcome, Robert! Always glad to see new bylines here.

    I have no experience with the mobile ones, but the swamp cooler at my grandparents’ house in Grand Mesa worked incredibly well-and was much loved by Gramps as he serviced it himself ‘without having to mess around with gauges & hoses & stuff’. Much cheaper, too: no compressor to suck up amp-hours, just a fan. I understand some units can be fancy with nozzles & a pump, but still way simpler than AC.

    1. I just moved out of a house in Colorado that had a swamp cooler as the only cooling source… and it was WONDERFUL.
      The best part was that you could double-dip on the hardware – overnight, when everything cooled down outside, you ran it “fan-only” to displace all the warm air in the house for nice, cool outside air. Shut down the house for the rest of the morning until things got warm again and then re-fill the house with cold (and humidified!) air from the evaporative cooler. Cost hardly anything to operate, had nothing but superbly-generic parts that could be replaced from any hardware store on the planet with only basic tools… I’m pretty sure that almost all of it except the water pump was original 1977 components 40 years later until I had to replace the belt and pulleys.

      1. It’s hilarious to this New Englander that you see added humidity in the summer as a positive attribute. I mean I get it, but also when I started work yesterday it was a balmy 70°F and yet my shirt was soaked through before I even got my ladders up because my sweat just had nowhere to go. By the time it got up into the 90s, I looked like I had been for a swim in my work clothes.

      2. I forgot the humidification! That alone is a huge benefit: much below 40% humidity, our systems have much less resistance against viruses. Yep: cheap, easy, AND more healthy. Swamp coolers rule

  8. Sometime in the late 70’s, there was a big Packard reunion at the old manufacturing plant in Detroit. My father, a man who owned one, decided we should pack up the 5 person family and drive our 1953 Packard from Tampa to Detroit in July. It was hot, very hot. My mother does not like it hot.

    In an attempt to convince her, my Dad bought a used swamp cooler, just like the one shown and affixed it to the left side rear door – my assigned seat. At first I thought I had won the lottery since Dad assured us this was just like a real air conditioner. Well, it isn’t.

    Shortly after setting out in the Florida heat, it became clear that the reconditioned “Car Cooler” was misnamed. And it had a leak that resulted in a classic car version of water torture. I could no longer open my window because it was supporting the rocket so I was stuck in a hot zone being pelted in the eye with water droplets at an infuriatingly unpredictable frequency.

    A minor mutiny occurred before the return trip and the valuable antique car cooler was consigned to the hotel dumpster.

  9. A swamp box was all we had growing up in New Mexico. I didnt experience refrigerated air in my home until I moved to Phoenix when I was 18. Worked well except on the hottest days or if the humidity was up after a rain storm. Keep a window cracked in the rooms you want to cool and you are golden.

  10. Great article Robert! It’s great to find a fellow thermodynamics nerd … error enthusiast I mean!

    I’m now expecting a follow up on how to compute h using the Prandtl, Nusselt and Reynolds numbers and the Chilton-Colburn analogy to get the rate of evaporation!

    Until then, there’s still some science hidden behind these coolers!

  11. All very well and good, but I still wish I had a little swinging triangle on the front windows of my very well a/c’d Toyota. There was nothing like directing a nice little breeze where you wanted it. Crack the windows on the Toyota and you’re engulfed in noise.

  12. In 1957 my folks worked out a deal to move a company car from KC to LA, so they took my grandmother with them and took the train back.

    The ’57 Plymouth 4dr they drove was a “salesman’s special” which meant it was the barest of bare bones strippers, with no sound insulation, rubber floor mats only, no power steering or anything else, with (I think) the flathead 6 cylinder engine and 3 on the tree manual transmission. To make the trip possible for my grandmother with her heart condition they fixed one of these swamp coolers on the rear side window near her and headed out – in the middle of July – for the west coast.

    She did make the trip OK, tho I’m sure it was tough on her, but getting to see the old homestead where they used to live was a strong incentive. I expect my folks would have preferred to take a vacation trip like this without her, but it was a good thing that they did for her…..she died a few months after their return.

    I have a clear memory of the car – it seemed like the paint was already faded pale blue, tan mohair seats and the swamp cooler mounted to the read side window – as they left on the trip.

      1. Welcome Rob! Great article! As I told you when you visited, I am NOT an engineer, and Lane Motor Museum has so many engineering oddities that someone better equipped than I should explain the literal nuts and bolts of these cars.

  13. We had a swamp cooler for our house in the dry foothills of California. It was mounted on the roof and was fairly effective. A mistake many people make with them is that you must leave another window open for the air to escape. It only works when air is flowing.

    I wonder how many folks with these on their cars, leave the other windows closed. This essentially just dumps humid air into the car and would not be effective for very long. Is this discussed in any of the literature for those swamp coolers?

    In our house I was able to direct the cooler air down a flight of stairs, though a long room, into another room where the air had to make a 180 degree turn to go into my office, and then out through a window in the office. I had some box fans placed strategically to turn the cooler air around corners, but it did work.

    1. Based on other replies to the post it seems many in the past mistakenly took these devices to be ‘regular’ AC units and made this mistake. The advert David pulled for this article does mention cracking a window in fine print at the bottom.

  14. I have one of these Thermadore units. I use it sometimes on my Volvo. Living in the Las Vegas area, it actually works pretty well, with two caveats. It provides no cooling at all when you are stopped of course, and if you don’t use distilled water, it will leak down the side of your car and leave all kinds of mineral streaks etc, which can be rather difficult to wash off. I also use a window mounted electric swamp cooler to cool my house, my unit isn’t capable of keeping up when outside temps are above about 107, but cuts my electric bill by about $5.00 per day compared to using my central AC.

    1. Swamp coolers are great here, except during monsoon season. Is your whole house unit not big enough? If it’s dry mine can usually keep it at least at 80 even on the hottest of days.

  15. Very cool article! This is the nerdy stuff I look forward to at Autopian and hope to see more of it.

    I’ve been aware of swamp coolers, buy growing up and still living in Pennsylvania, I’ve never seen one in the wild.

    1. They’re not uncommon in houses in central Australia where it’s dry nearly all the time. The difficult part to understand is that unlike air-conditioning, they work best if the doors and windows are wide open. Many a tourist has reduced themselves to hot puddles of sweat by running the ‘air-conditioner’ on full with the doors and windows closed.

  16. I’ve never seen (or at least never noticed) these out in the wild, which shouldn’t be too surprising since they’re quite a bit before my time, though seeing as how they seem to be in-demand among collectors, I thought that there might be a chance that I’d encounter one at a car show.

    I learned all about swamp coolers recently through a video by the Youtuber: Technology Connections. He did a nice review of those little “personal AC units” that were popping up everywhere online. The devices were nothing more than small simple swamp coolers, and the video illustrated how they work (and how ineffective they were). (Incidentally, if you like the combination of technical content and irreverence presented at The Autopian, then Technology Connections may be a YouTube channel that you’d enjoy.)

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