Our new apartment has laundry in the building, but in a room that can only be accessed by going outside and then in through a separate entrance. The result in winter is that on its way back, a hamper full of freshly dried clothes will acquire a layer of cold clothing around its warm cozy center. When I feel this cold clothing later, part of my brain says, “Oh, no! It didn’t get dry!”
Partly, that thought is from experience: you can tell whether clothes are ready to come out of the dryer by whether the heat has gone from evaporating the water to warming the clothes, so if they don’t feel warm yet it’s because there’s still moisture present. But the cold clothes also just feel damp, even though they’re not. That got me wondering: why do cold clothes feel damp? The answer has to do with how we experience temperature at all.
Anyone who has sat on a toilet seat in a cold house has probably experienced some part of this continuum of how warm the seat feels, based on what it’s made of:
- Porcelain: coldcoldcold
- Plastic: a bit chilly
- Wood: kind of warm. Also, eww.
The difference between these seats isn’t their actual temperature: they’re all approximately the same temperature as the ambient air. What differs is how easily they conduct heat away from your body, a property called thermal conductivity.
When two objects of different temperatures (your butt and the toilet seat) come into contact via some intermediate substance (the surface of the seat), the formula for the rate Q at which heat flows from the hotter object to the cooler one is
where is the temperature difference, K is the thermal conductivity, A is the surface area of contact, and L is the thickness of the material. The factors that I want to focus on are those first two: the rate of heat flow is proportional to the temperature difference, yes, but also to how thermally conductive the material is. My guess is that when we think we are sensing temperature, we’re actually sensing the rate at which heat is leaving our bodies, which depends equally on those two factors.
For example, here are some approximate thermal conductivities of common materials:
- Air: 0.03
- Wool: 0.07
- Wood: 0.1
- Cotton: 0.2
- Water: 0.6
- Concrete: 0.8
- Porcelain: 1.5
- Steel: 50
- Brass: 100
- Aluminum: 200
Since water is about three times more thermally conductive than cotton, if I feel some damp clothing that is 20 degrees cooler than my hand, it will register about the same temperature-feeling as dry clothing that is 60 degrees cooler than my hand. That’s why the winter-chilled dry clothes feel like they’re wet!
In fact, thermal conductivities tend to vary over a much wider range of orders of magnitude than most of the temperature differences we encounter, so you could even claim our touch sense of temperature is mostly a touch sense of thermal conductivity, and is more useful for distinguishing materials than temperatures.
(Incidentally, this is the same reason the rivets and zippers on freshly dried jeans feel so hot compared to the denim: they’re not actually any hotter in terms of temperature, but the high thermal conductivity of metal means they’re very efficient at transferring their heat into your skin!)
How about you — do you have any stories of your sense of temperature led astray by thermal conductivity? Do our other senses perceive something slightly different from what we think they do? I’d love to hear your thoughts in the comments!