2743: Hand Dryers

Explain xkcd: It's 'cause you're dumb.
Jump to: navigation, search
Hand Dryers
I know hand dryers have their problems, but I think for fun we should keep egging Dyson on and see if we can get them to make one where the airflow breaks the speed of sound.
Title text: I know hand dryers have their problems, but I think for fun we should keep egging Dyson on and see if we can get them to make one where the airflow breaks the speed of sound.


A hand dryer is an electrical device which uses air flow, typically of hot air, to dry the user's hands after they have just washed them. In the 30 or so seconds it takes to dry the hands, the user may feel as though the air coming from the hand dryer isn't actually warm, hence seeming like they "take forever to heat up."

Randall is pointing out that evaporation is an endothermic process, which is why sweating cools you down and therefore why regular fans, which actually increase the air temperature through friction, can cause a cooling effect. With hand dryers, even though the air is warm, the cooling effect makes it feel cold. The user will be able to feel the heat only after their hands are dry.

Randall has procured a small airplane, accompanied by a banner with a message explaining this phenomenon. He elaborates in the caption that he's spent dozens of years angry at the engineers of these hand dryers, as he was under the comic's erroneous impression that the air from the dryers was not actually warm. In an act of justice for hand dryer engineers everywhere, he now considers it his personal mission to explain to the public why this is actually a misconception. And indeed, it seems to be working - a person on the ground has already been enlightened by Randall.

In the title text, the speed of sound is the speed of a sound wave in a given medium, usually air. Breaking the sound barrier is often touted as a significant achievement for powered aircraft (this was first safely achieved in the 1940s, and became significantly 'easier' with the development of the jet engine). Here, Randall thinks it would be a good idea to try and get the Dyson company (a technology company known for making high-tech and expensive air-moving devices) to design a hand dryer whose airflow would exceed the sound barrier. Dyson's marketing often turns on the very high speed of the airflow produced by their motors, often many hundreds of kilometres per hour, indeed approaching the speed of sound. However, there are side effects of supersonic airflow, including sonic booms, which would make it impractical for purposes of drying hands in an enclosed area.

This comic is not the only one to involve people flying banner planes to inform people on technologically related things: see also 1965: Background Apps.


[An airplane tows a banner. In the distance, there are three small clouds and three birds]
[On the banner is written:] It seems like hand dryers take forever to heat up, but that's because the evaporation cools your skin, so the hot air feels cold until the water is gone.
Voice coming from the bottom of the panel: Ohhh!
[Caption below the panel:] I spent decades mistakenly annoyed at hand dryer engineers, so now I'm on a mission to save others from the same fate.

comment.png add a comment! ⋅ comment.png add a topic (use sparingly)! ⋅ Icons-mini-action refresh blue.gif refresh comments!


I have just tried this in 5 different restaurant restrooms. With dry hands, the air coming out is initially cold, even if you restart right after a cycle has finished. I can't speak for every restroom, but the ones I tried seem to invalidate the comic. SDSpivey (talk) 01:32, 7 March 2023 (UTC)

The mouseover text is trolling, since that would be impossible. 16:23, 27 February 2023 (UTC)

Indeed, Randall wrote about that in How To 2. Nitpicking (talk) 03:49, 28 February 2023 (UTC)

The Pratt & Whitney J58 is capable of producing exhaust velocities exceeding that of Mach 2 at ground level. It would be possible (though extremely inadvisable) to dry one's hands in the exhaust, at least for the brief period where one still has hands. 16:44, 27 February 2023 (UTC) J. Kupec

There are hand-sized supersonic blowers used to clean and dry train tracks. https://www.ge.com/news/reports/this-software-guided-supersonic-air-blower-sweeps They are very dangerous to exposed skin even several feet away. 06:52, 28 February 2023 (UTC)
You are supposed to dry your hands with them, not your feet. 08:50, 28 February 2023 (UTC)
Feet away, hands away, any body part you like really Boatster (talk) 13:46, 1 March 2023 (UTC)
Finally time to put those damned toes in the ground! 16:31, 1 March 2023 (UTC) 16:41, 1 March 2023 (UTC)

With a low enough vacuum in the surrounding area, a supersonic hand dryer should be able to apply drying without enough energy dissipation to damage the skin. 17:27, 27 February 2023 (UTC)

As far as I understand it, the low velocity dryers heat the air, the high velocity ones don't, but rely on the air being compressed and air speed is of the essence. The other problem with the idea of very high speed is that 'stuff' could penetrate the skin (there is a type of needle-less vaccination gun on that principle).RIIW - Ponder it (talk) 19:31, 27 February 2023 (UTC)

That matches my memory, the first ones I remember were fairly low airspeed and had a data tag "1500 watts" for the heating element. Has anyone tried one of these with *dry* hands, to see how long the element takes to get hot? I don't think they heat up instantly. They certainly get hot--motorcycling on cold days I've pointed the nozzle inside my clothing to warm up at a rest stop. 19:40, 27 February 2023 (UTC)

I don't think this effect fully explains observations. For example, the airflow feels warmer sooner when someone has used the dryer just before you. P1h3r1e3d13 (talk) 21:10, 27 February 2023 (UTC)

"though this was first achieved many decades ago, in the 1950s" Yeager broke the sound barrier in level flight on Oct. 14, 1947, and planes had been doing it in dives for years. Cser (talk) 21:29, 27 February 2023 (UTC)

Without reading your comment, I further changed the (as it was for me) "1940s" version of the statement to include the original "inadvertent" barrier-breaking (of prop-planes in almost always irrecoverable dives, without control surfaces that would work well in supersonic/transonic airflows) and included the developments made, which these days are somewhat more trivial than having to sit on a rocket that is released from a high-altitude bomber's wing, and fight to keep it flying straight and level. (We even had a supersonic airliner, for several decades!) There's a lot of interesting history to this, but not really the place to say it all. 01:45, 28 February 2023 (UTC)

Should we make an "airplane banner" category? 02:31, 28 February 2023 (UTC)

I was thinking the same thing, but I'm here about Covid (below). Bismuthfoot (talk) 04:16, 28 February 2023 (UTC)
I agree to that. If we can find at least three others. Can see two are mentioned below. Are there more than those two and this one? --Kynde (talk) 08:14, 28 February 2023 (UTC)
Previous usages of planes with banners: Background apps and Airplane Message, both of which's banners bore information and the first one commented on the cheapness of the banners. Maybe mention them in the explanation and/or add a category about them? Xkcdjerry (talk) 08:05, 28 February 2023 (UTC)
Found one more 2463: Astrophotography. I will make the category --Kynde (talk) 08:17, 28 February 2023 (UTC)
Done. Please add more if you can find them: Category:Airplane banner --Kynde (talk) 08:25, 28 February 2023 (UTC)

Hand dryers were disabled in the early days of Covid in 2020 before hand transmission was ruled out as significant. I still feel awkward using one in a bathroom with others. I'm old and still mask when indoors publicly more than briefly. In 2023, I submit that you risk appearing hypocritical with a mask and a hand air dryer. Thus, I saw this XKCD as a reminder that hand air dryers had nothing to do with Covid. Still, there seems to be a bunch of fuss about the dryers. Apparently, some use mechanical air force (jet air) more than warm air for drying, from respectable gavi.org and wired.org in 2021 (https://www.gavi.org/vaccineswork/we-know-hand-dryers-can-circulate-germs-through-air-why-are-they-still-used and https://www.wired.com/story/wash-your-hands-but-beware-the-electric-hand-dryer/). I'm just rambling here; I'm not ready to do any editing. Bismuthfoot (talk) 04:16, 28 February 2023 (UTC)

The alternative to dryers was often a stack of paper towels. Always a 'contact threat', in my eyes (I never really like using a shop's communal hand-sanitiser bottles, on entry, for that very reason; I just didn't touch anything, that I wasn't already taking off the shelves to take with me, if I could help it), although thankfully that wasn't a great a problem as it was initially feared.
Ditto, the precaution of taping off every other seat (or two out of every three, etc), in order to prevent congregation of people in public seating areas. This forced every new arrival to always choose from the more limited number of pre-touched seats, rather than just advising people to randomly choose to sit only in any non-neighboured seats and so diluting the hypothetical risk.
(I still mask up for entry to supermarkets/shops/indoor places of similar kinds. I would for buses/trains, if I used them. I'm otherwise still mostly "bubbling" with close family, or only going to places where we're mostly/all going, together or separate, but that probably equally applied before 'things changed' as well.) 05:20, 28 February 2023 (UTC)

On the '30 or so seconds it takes for user's hands to be dried', at least for the Airblade style ones, the cycle is much shorter, they turn off after 12s. It just *feels* that long. There is one at the place where I do rock climbing (where you want your hands to be very dry) and I heard people complaining that they'd prefer paper towels (for dryness reasons and because that would be faster, not because the Airblades spew everything everywhere). So I started to actively take mental notes of the efficacy of each drying-mode. My conclusion: Yes, with towels you get the palm somewhat dry very quickly. However, forget about any water left between your fingers. If you want all around dry skin on your hands, the Airblade is just better and faster. If you just want dry enough hands to proceed in your normal day, towels are sufficient, convenient and silent. 09:13, 28 February 2023 (UTC)

Personally I find that it takes at least three cycles on the Airblade to actually get my hands dry (the between the fingers bit being a particular trouble spot), so it's still 30 or so seconds, just with the additional annoyance of having to pause a couple of times in the middle to wait for it to reset. 10:40, 28 February 2023 (UTC)
If towels and a dryer are both available, I use a hybrid method. I first use one paper towel. (If the bulk of the water isn't completely removed I'll wring out the towel and remove more.) Then I use the dryer to complete the water removal. These Are Not The Comments You Are Looking For (talk) 03:24, 8 March 2023 (UTC)

Does anyone think that the title text is a reference to the fact that hand dryers that blow cold air call themselves "hypersonic"? Barmar (talk) 19:34, 28 February 2023 (UTC)

I very much find this rationale to be plucked out of nothing at all. But I don't want to knee-jerk undo it, so... Can the editor (no User Talk page), or anyone else, explain its veracity a bit better..? 19:13, 2 March 2023 (UTC)

Yes, it's a mess. But too late now to simply undo it, so I'll do a bigger cleanup. Jkshapiro (talk) 15:54, 7 January 2024 (UTC)