Don’t rain on my parade – but don’t wind gust on it either! Instead of signs of rain, people are looking cautiously at the wind vane to see if the Macy’s Thanksgiving Day Parade balloons will get to go. But it turns out these balloons actually could have faced cancelation months ago – because the same thing that makes these balloons float is an element called helium of which vast supply having we don’t!
These balloons may seem massive in size (most are 5-6 stories high, ~60 feet long & 30 feet wide), but, thanks to the lightness of the helium they’re filled with, they’re less “mass”-ive than the air they displace, so buoyancy makes them float! And a team of handlers keeps them from floating away! But that’s quite a task – especially when it’s windy.
Snoopy didn’t get to go for a walk until 1968, but the first Thanksgiving Day Parade balloons got their start in the 1927 parade (the parade itself started in 1924) to replace zoo animals that were scaring children. But turns out the balloons can be scary too…
The balloons almost didn’t get to be walked today because of high winds. Officials have a right to be concerned – accidents have happened, one of the most serious being an incident in 1997 with a Cat in the Hat balloon ran into a lamppost causing injuries including putting a woman into an almost month-long coma. This led to stricter rules about when balloons can & can’t fly.
If you add up the weight of the helium inside the balloon and the weight of the balloon material, these guys are actually pretty heavy (~450 pounds). But this weight is actually lighter than the weight of the air that they displace. So, as Archimedes would tell you, it will float.
Basically, the molecules of gas in air are constantly moving around and they’re gonna bang into whatever’s in their path. But since the air molecules are moving in all directions, stuff’s gonna get banged into from all directions, so all those little pushes from the sides will cancel out.
But gravity is NOT in all directions – it’s just down – so in order for something not to fall down it has to be able to “push up” And the “push up” force is called the buoyancy force and it comes about because the molecular packing of molecules is closer at lower depths, so there are more molecules available to “push up” from below than there are to “push down” from above.
The reason the molecules are closer packed as you go deeper is because on the whole there are a lot more molecules above that are experiencing the downward pull of gravity (like a whole atmosphere’s worth). BUT because those molecules are further apart, there are fewer molecules actually banging into an object from above than from below at any given time. And this upward banging gives a lifting force – which may or may not be enough to overcome the “sinking force” of gravity.
Archimedes discovered that this lifting force, buoyancy, is equal to the weight of the displaced fluid (this fluid can be a gas like air or a liquid like the water a pool – the same forces that make balloons float or sink make you float or sink!), NOT the weight of the object. The net upward force will be equal to the difference in weight between the air that was there and the thing that’s now there. So if your object is lighter than the fluid you displaced, it’ll float. If it’s heavier, it’ll sink.
So if you want something to float, you want to find a way to displace the air that is there with something really light so that it doesn’t succumb to the downward pull of gravity (which is proportional to mass). But that light thing also has to be able to hold the shape of its container (our balloon casing) so it doesn’t just get crushed by all the outside air particles.
Thanks to something called the ideal gas law, (much more about it here: http://bit.ly/idealgaslaws we know that the same # of gas molecules (any gas molecule) will exert the same pressure. So we could fill a balloon with any gas and it would push back just as hard. Even if the gas molecules themselves are lighter.
The lightest you can get is hydrogen – it only has a single proton – but hydrogen can react with oxygen & explode. So those balloons would really really be dangerous! Your next option is helium. It’s the second-lightest element, with only 2 protons. And, being a “noble gas” it has a complete valence shell (its outer electron housing district is full) which makes it happy & non-reactive. more on valence electrons here: http://bit.ly/ridiculousredox
But, because helium floats, it floats away – so it’s rare on Earth, only making up about 0.0005% of our atmosphere. It’s largely found in natural gas deposits, where it’s spit out during the radioactive decay of other elements like uranium & thorium. In alpha decay, which typically happens in really big, heavy nuclei, an atom gives off an alpha particle 2 protons & 2 neutrons (basically it gives off a helium nucleus), which can pick up a couple electrons to become elemental helium, He. But this process builds up He stock over years and years, not “on demand”
And it’s not just balloons at risk – helium’s main uses aren’t balloons – the helium typical party balloons are filled with isn’t the really good stuff – that’s saved for more important things. It has a really really low boiling point – the lowest boiling point of any element. This means you can get a really really cold liquid you can use to get other things really really cold – like the superconducting magnets used for MRIs & NMR. It’s also used for other things like LCD screens. And space stuff
Due to both political and resource-y reasons, there’s a helium shortage problem that’s driving the cost of helium up and driving some scientists to speak out. Here’s a really great podcast episode about it from NPR’s Short Wave https://n.pr/35IMpU1
This isn’t the first year a helium shortage has been a potential holdup. From 1942 to 1944 the shortages of helium & rubber (used to be used but isn’t anymore) were so bad (because of WWII) that the whole parade was canceled). And in 1958, they didn’t have enough helium to use, so they filled the balloons with air to plump them up and then used cranes to hoist them up onto trucks cuz, being filled with just air they couldn’t float.
But the main reason Snoopy almost missed his walk today was wind. We saw how you have the buoyancy force to combat the gravity force, so you’re good to go in the up/down direction – but what about side to side? Before we said that the air hitting from the sides cancels out – but this is only true if the air molecules really are moving about randomly. And wind gusts aren’t random… They’re basically like having a ton of molecules crashing into one side of the balloon and this can push it off-course.
The handlers with their ropes are there to help pull it back on track, but that’s no easy task. The balloon’s bigness means it can displace a lot of air and, since volume grows faster than surface area (think of the peanut butter to chocolate ratio of a bit-size Reeses peanut butter cup versus one of those king-size ones) you can increase the amount of air you displace without having to add a lot of balloon skin weight. But the bigness also means that there’s a lot of places for the wind to hit it and push it and unbalance that nice balance.
Walking Snoopy takes a lot of dog walkers (about 90 people with the title “balloon handler” are tasked with keeping a leash on these balloons (holding the guide ropes) and keeping them floating on course). These walker teams include a leader, pilot, captain, & 2 drivers – and, while the people are doing most of the work, the balloon is also tied to 2 800-pound utility vehicles.
You need all this leashing because the balloons have more lift force than they need to keep the balloon afloat. In the pics I walk you through how we calculate this using the ideal gas law and molar masses, but it turns out that we have enough lift force to lift ~345kg, but our balloon only weighs ~200kg, so it would float away if it weren’t on a leash – leashES.
The shape of the balloon influences where the pressure will push and how the balloon will move, so engineers specializing in aerodynamics folks get involved early on – a team gets together and draws sketches, then make little clay models, and exact-to-scale replicas. They actually stick their tiny exact-to-scale models in water to see how much water gets displaced and thus how much helium they’ll need.
And finally they make the real thing. And they make that real thing out of fabric coated with a lightweight, flexible, plastic material called polyurethane (they used to be made of rubber). And they have to be really sure there aren’t any holes that’ll make them lose all those mols! And the seams have to withstand a lot of pressure, so they’re heat-sealed.
In 1971, bad weather kept them from even being inflated. This year, the balloons were inflated, but it was unknown for a while whether they’d get to be walked. The balloons are actually inflated the day before the parade (takes ~90 minutes) – but not all the way. They get a little more the morning of the parade, but still not all the way, because as the sun comes out, the temperature rises – temperature is a measure of the average energy that molecules have. And the more energy molecules have, the more they can move around – so the air molecules move further away from one another and the balloon inflates a bit more (volume increases) even though you don’t add more helium (Charles’ Law).
Superman’s six-pack gets some help from the sun too. Kinda like how food photographers have tricks for getting food to look fab, balloon photographers (or balloon makers who want balloon photographers to get their balloon’s “good side”) can set their balloons in the sunlight’s path to get that perfect tautness.
This year there were 56 balloons including 16 giant character balloons – including SNOOPY! – back after being replaced by Charlie Brown for a few years… I really love Snoopy… He has a knack for cheering me up, so I have a lot of stuffed Snoopy’s to snuggle with during hard times. Maybe I should make a lab coat cape for one of them…