Monday, April 08, 2013

Relative temperature or why do some things feel colder than others?

With snow falling it's time to turn up the heating; except wait a minute why do we need to do that? If you have thermostatically controlled heating it will switch off when it reaches a certain temperature regardless of what it is outside. Sure you can turn it up to make it heat up quicker; but once the heater switches off the temperature now should be the same as yesterday. So why is it colder?
Take a stainless steel and a wooden spoon and stick them in the freezer overnight. Then take them out - which is colder?

If you said the metal spoon - congratulations you're wrong.

If you said the wooden spoon  - congratulations you're wrong.

Given the length of time they've been in the freezer they should both be exactly the same temperature. The freezer extracts heat down to a specific amount. It's taken as much heat from both as it can so they should be both at the same temperature. But the metal spoon feels colder; why?

The short answer is because the temperature that is measured via thermometers and the temperature we experience aren't always the same thing.

Materials have differing conductive properties; that is how quickly they transfer heat.

Stainless steel has a conductivity of  around 16W/(m.K). Wood has a conductivity of 0.1 W/(m.K) (roughly it depends on the wood). This means the energy from your fingers is passed through the steel quickly, it's spread out, whereas the energy passing to the wood sits on the surface.

Keep both in your hands and over time the metal spoon will start to match the temperature of your fingers, but the wood will remain 'cool'. This time it's due to each materials "heat capacity" (or specific heat); that is how much energy is required to raise the temperature of a specific block of that material.

Stainless steel is 402 J/kg K that is it takes 402 J of energy to raise 1kg of metal by 1 degree Kelvin (which is the same amount as 1 Celsius). On the other hand Wood is about 1700 J/kg K.

So the energy flows through the metal quicker than for wood, feeling colder, but heats up quicker.

What's this got to do with why it feels colder today than yesterday? Well air has a conductivity of 0.024W (m.K) and a specific heat of 1005 J/kg K so it's a bit like the wooden spoon in that the heat sits on the 'surface' that's touching you. This is where wind chill comes into play; the air moves and energy is expended into the surrounding air again. So far so what. Well air isn't the only thing floating around us; there's water too.

Water has a conductivity of 0.58W (m.K) and a specific heat of 4186 J/kg K so it takes and spreads the heat around, but takes a really long time to heat up itself.

Add in that as the water is suspended in the air and thus can also move around; it can also require reheating.

So cold and dry feels warmer than cold and damp in the same way steel feels colder than wood despite the fact the thermometer reads the same.