They’ll burn your butt off — but why are they hotter than the rest of the playground?
When you were younger, did you play outside on a playground, perhaps in the middle of summer with the hot sun directly overhead? Do you remember wincing in pain when you went, unthinking, to go down the metal slide and found that it was burning hot to the touch?
Ouch, I still remember that pain! But it also seems a bit strange, when I consider the rest of the playground. All of it was exposed to direct sun, all in the same air.
- Why was the slide so much hotter than the rest of the playground?
- Is the metal more absorbent of heat than the other playground construction materials, like plastic?
- Does metal truly get hotter when it’s in the sun, or is this some sort of illusion?
No, this knowledge isn’t going to save the world, but it’s an interesting question, and if you have kids, they may well ask you this very question someday (assuming that we can still breathe the air outside and will still go to playgrounds in the future).
So let’s slide into some new knowledge!
Heat stored, versus heat transferred
A layman, looking at the metal slide, might assume that the slide is hotter because it has more energy. “Think of the materials like batteries,” they might say. “They absorb heat from the sun until they can’t hold any more. The metal can hold more heat than the plastic, so it’s hotter.”
It’s a reasonable proposal, but it’s also incorrect.
When we talk about the amount of energy stored in a material, we’re talking about a feature called specific heat. This is a value that is different for each material. Here’s a few:
- Wood: 1.76
- Water: 4.184
- Aluminum: 0.89
- Steel: 0.466
- Mercury: 0.14
- Plastic: 1.67
All of these are measured in J/g°C, or “how many joules of energy are needed to heat 1 gram of this substance to be 1 degree Celsius warmer.” The higher the value, the more heat that substance can store.
Note that plastic has a specific heat that’s more than 3x higher than steel. It takes much more energy to warm up plastic than to warm up steel, and plastic holds far more energy than steel does.
So, if anything, our plastic playground pieces store more heat, and have more energy that they would transfer over to you when you touch them.
But this flies in the face of our observations; the plastic feels cooler, not hotter!
This is where another trait of various materials, called thermal conductivity. Thermal conductivity is a measure of how fast a material can transfer heat from one area to another. This varies with temperature, but we can look at the average thermal conductivity of a few different materials:
- Wood: 0.15
- Water: 0.5918
- Aluminum: 237
- Steel: 43
- Mercury: 8.3
- Plastic (high density polyethylene, the stuff slides are made of): 0.5
Now we’re getting somewhere! Steel has a much higher value for thermal conductivity than plastic, meaning that it much more rapidly transfers heat. When you touch the metal slide, this higher thermal conductivity means that it transfers the heat much more rapidly to your skin or clothes, making it “feel” hotter.
A metal slide feels hotter to the touch because it can rapidly transfer that heat to your skin, much faster than plastic.
What can we do about it?
Yes, yes, this is all well and good, you may be saying. But my children want to play on the playground in summer, and they come running to me crying because the slide burned them! As a parent, what am I to do?
Well, we can’t change the thermal conductivity of the steel slide itself. But we can intervene!
- We can put a barrier between our skin and the metal, acting as an insulator! You could wear long-sleeved clothing, like long pants, to act as a barrier. White clothing, while prone to staining, will also keep you cooler in the direct sunlight.
- If you don’t want to dress in long sleeved clothing, you could bring a different barrier object. Some companies sell plastic pads that you can sit on when you go down a slide, or drape over a hot swing to provide a cooler sitting surface.
- Finally, you could cool the surface by sending the heat somewhere else! Spraying water onto a metal slide will quickly cool it off; there isn’t a ton of heat stored in the metal, and it will quickly transfer to warm the water, leaving a cooler surface behind.
So wearing the right clothing, bringing a mat or pad to sit on, and making use of a conveniently nearby hose or bucket of water are all good methods to avoid getting burned by a hot slide.
In summary: metal conducts heat, from the sun to you
Metal gets hot in the sun. It actually doesn’t hold more heat energy than other objects, but it has high thermal conductivity, a quality that makes it able to rapidly transfer that heat to anything that comes in contact with it — including our skin.
When we touch hot metal, the heat energy quickly transfers to us, leading to us getting burned. The best intervention is to either put an insulating barrier between us and the metal, or to put something on the metal that has a high specific heat, such as water, which can absorb the heat and remove it from the metal.
Metal slides can pose a danger to us in the summer, with their risk of burns, but they are also very durable and more easily recycled than plastic components. They’re not likely to fully disappear from playgrounds, so keep your cool around them!
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