Why does metal feel colder than wood?

Your hand makes a lousy thermometer. Grab the metal railing on the porch steps, then the wooden post right next to it. The railing bites. The post feels almost friendly. They’ve spent the whole night side by side in the same air, so they’re at the same temperature, and your skin swears otherwise. So why does metal feel colder than wood when a thermometer says they match? The clearest answer was written by a guy from Philadelphia more than 260 years ago, the same guy whose face is on the hundred-dollar bill. He nailed this one. A couple of pages later, in the same letter, he got another one badly wrong, and that part is worth sticking around for.

Franklin’s desk lock

On April 14, 1757, Benjamin Franklin was in New York and hadn’t heard from his friend Dr. John Lining of Charleston, South Carolina, in a long while. So he wrote first, and filled the letter with what he’d been chewing on about heat.

Halfway through, he drops this. His desk and the lock on his desk sit in the same air, so they have the same degree of heat or cold. Yet if he lays his hand on the wood and then on the metal, the metal feels much the coldest. Then comes the line that settles it: not that it really is colder, but that, being a better conductor, it takes the heat out of his skin more readily than the wood does.

That’s the correct explanation. Physics departments teach the same idea in fancier words. Franklin talked about “fire” where we’d say heat, because in his day heat was thought of as a kind of invisible fluid that soaked into things and leaked out of them. The vocabulary smells of powdered wigs. The reasoning holds up fine.

He also ran the test on himself. He’d rest one hand partly on the lock and partly on the wood, leave it there a while, then feel that hand with the other one. The patch that had been on the lock was clearly colder to the touch. His palm hadn’t been fooled. It really had lost more heat on that side.

Why does metal feel colder than wood at the same temperature

Anything that hangs around in a room long enough ends up at room temperature. Heat always moves from warmer to cooler until everything evens out. A spoon, a chair and a paperback that spent the night in a 68°F living room are all at 68°F. Metal doesn’t make cold, and wood doesn’t store up warmth.

You’re the one who isn’t at room temperature. Your skin runs a good deal warmer than an ordinary room, so the moment you touch anything cooler, heat starts flowing out of your hand and into the object. Up to that point, metal and wood behave the same. The difference is speed.

Every material lets heat through at its own pace, and that pace has a name: thermal conductivity. The gaps between materials are enormous. The American Wood Council, the technical body of the U.S. wood industry, puts structural softwood lumber at 0.7 to 1.0 in its usual units (Btu·in per hour, square foot and degree Fahrenheit). Steel comes in at 310. Aluminum at 1,500. So aluminum moves heat somewhere around 1,500 to 2,000 times faster than a two-by-four, and steel more than 300 times faster. In a table published in 2008 by the Mexican physicist Ernesto Marín in a physics-education journal, copper conducts heat 2,500 times better than a hardwood.

Metals are so good at this for the same reason they carry electricity. They’re full of loose electrons that don’t belong to any single atom and roam around the whole piece. The same electrons that carry current down a wire also haul heat from one end to the other, and they do it fast. Wood has no free electrons to speak of, so heat has to elbow its way through, molecule by molecule.

So when you grab that railing, the heat from your palm pours into the metal and spreads through the whole thing almost instantly. The railing keeps pulling, and your skin keeps losing. When you grab the wooden post, the thin layer of wood right under your hand warms up a little, and the heat gets stuck there with nowhere to go. That warm layer works like a cushion between your skin and the rest of the post.

The first second on your skin

Conductivity explains most of it. To work out what you actually feel in the first instant of contact, though, you need one more property with an ugly name: thermal effusivity. It describes how readily the surface of a material swallows a sudden gulp of heat. It depends on how well the material conducts, how dense it is, and how much heat it takes to warm it up. High effusivity means the material sucks heat out of you the second you touch it. Low effusivity means its surface warms up right away and stops absorbing.

When two things at different temperatures touch, the spot where they meet settles somewhere in between. That in-between temperature lands closer to whichever material has the higher effusivity. Marín uses this in his paper to show students how much physics you can learn just by touching things, and the math is simple enough to run with his own numbers.

His model puts skin at 98.6°F (37°C) and the objects at 68°F (20°C). Touch copper, and the skin in contact drops to about 69°F, practically the temperature of the metal. Copper’s effusivity is more than thirty times higher than skin’s, and it drags your skin down to its level. Touch a hardwood, and your skin stays at almost 91°F. Wood has a lower effusivity than skin itself, so the skin gets to call the shots.

Two objects at 68°F. One leaves your skin at 69, the other at 91. No wonder your brain reaches two different verdicts. What it gets is the temperature of its own skin, and on the copper that skin really has gone cold.

That’s an idealized model. It assumes perfect contact and ignores rough surfaces, oily fingers and how long you hold on. In real life the numbers wander a bit. The size of the gap doesn’t.

The cold sensor has a name and a Nobel Prize

Everybody knows skin can feel cold. How it does that was only pinned down in 2002. That year, two research groups working separately, David Julius’s lab in San Francisco and Ardem Patapoutian’s in San Diego, published, in the same year, the discovery of a protein in nerve endings that switches on with cold. It’s called TRPM8, and both teams tracked it down using menthol.

That detail is a fun one. Menthol flips the very same switch, which is why a stick of mint gum leaves your mouth feeling chilly even though it’s at room temperature. Your brain gets the exact signal it would get if the skin had cooled down. In 2021, Julius and Patapoutian shared the Nobel Prize in Physiology or Medicine for their discoveries of receptors for temperature and touch.

So when the porch railing chills your palm, those sensors are what fires. They don’t care what the railing reads on a thermometer. They react when the skin around them drops in temperature, and they sound the alarm.

The pool and the patio

Effusivity also explains something everyone has felt in summer. An evening on the patio at 68°F is perfect. Jumping into a pool at 68°F knocks the wind out of you.

In Marín’s table, air has a laughably low effusivity, around seven hundred times lower than skin’s. Water’s is a bit higher than skin’s. Run the same math as with copper and air at 68°F leaves your skin at practically 98.6, because it can barely take any heat at all. Water at 68°F drops your skin straight to about 81°F. That’s why still air hardly chills you and an unheated pool has you shivering at the same reading as the thermometer on the patio.

Franklin was onto something similar in the same letter. He writes that damp, moist air makes a man feel colder than dry air that is actually colder, because moist air is better at picking up and carrying off the heat of the body.

The coin, the candle and the teapot

All of this works in reverse too. If the object is hotter than you, the good conductor pushes its heat into you faster and burns you sooner.

Franklin describes a test anyone in 1757 could run with what was in their pocket. Hold a silver dollar between the fingers of one hand and a piece of wood the same size in the other, and bring both to a candle flame at the same time. You’ll drop the dollar before the wood, because it carries the heat of the flame to your flesh much faster. The man on the hundred was burning his fingers on a dollar for science.

He draws a kitchen lesson from it that nobody questions anymore. If a silver teapot had a handle of the same metal, it would carry the heat from the water to your hand and be impossible to pick up. That’s why, he writes, metal teapots get wooden handles. He adds that a china or stoneware teapot can have a handle of the same stuff, because that kind of material conducts heat poorly. More than two and a half centuries later, the handles on skillets and saucepans still follow the same logic.

The letter has a stranger experiment too, with molten lead. Franklin lays a bar of lead and three pieces of wood of the same size around a square gap and pours melted lead into it. The lead hardens first along the side touching the lead bar, well before it sets against the three wooden sides. All four pieces started at the same temperature, but the metal was pulling heat out of the liquid lead much faster.

The ice cube that messes with your head

There’s a modern version of that experiment used in plenty of physics classrooms, and Britain’s Institute of Physics recommends it precisely to catch students off guard. You need an aluminum block, a plastic block and two ice cubes.

First you touch the blocks. The aluminum feels cold and the plastic feels warm. In the demonstration at the University of California, Santa Cruz, they check both with an infrared thermometer before starting, and both read 17.5°C, about 63.5°F. Then you set an ice cube on each block.

The cube on the aluminum melts way faster. That’s where heads start to spin: the block that felt cold is the one melting the ice. It makes perfect sense. The aluminum carries heat from the block into the cube just as efficiently as it was carrying heat out of your hand. The plastic, which felt nice and warm, hardly passes any heat along, so its cube just sits there.

If you want to try it at home, an uncoated aluminum pan and a plastic cutting board will do, as long as both have been sitting in the same room for a good while.

Where Franklin blew it

The 1757 letter has one last twist. After explaining the desk lock so neatly, Franklin asks why a mix of salt and snow gets colder than snow on its own. He’d tested it with his own hands: the mixture feels much colder to the touch and sends the mercury in a thermometer lower than either ingredient alone.

So he reaches for his favorite theory. He guesses the mixture conducts heat better than salt and snow separately and, like the lock compared with the wood, has a stronger pull on heat. For the desk lock, he’d hit the bull’s-eye. For the salt, he missed.

The mixture really is colder, and the thermometer proves it. Salt means water has to drop below freezing before it can turn to ice, so some of the snow melts into brine. Melting takes heat, and the ice grabs it wherever it can: from the mixture itself, from the bowl and from the fingers of whoever’s touching it. According to chemistry faculty at Frostburg State University in Maryland, a mix of ice, salt and brine settles at about −21°C, roughly −6°F, for as long as all three are present.

Franklin had both effects on his table, the lock and the salt, and they feel the same against your skin, so he gave them the same explanation. One was your sense of touch playing tricks. The other was the real thing, cold enough for a thermometer to see it too.