Hey! What do you know? Another question about hot beverages! (I sense a theme……)
Daroyln asks why cold tea and coffee seem colder than room temperature. Let’s start off with the obvious point: cold tea is NOT colder than its surroundings. Our old friend the Second Law of Thermodynamics sees to that. Also, the data collected for a previous post demonstrates specifically that hot beverages tend towards equilibrium with their environment over time. If things could become hotter or colder without needing outside energy, our world would be very different indeed.
So clearly, the operative word in the question is seems. The main answer is thermal conductivity, that property which makes stone seem cool and plastic seem warm even at the same temperature. Measured in Joules per second-meter-Kelvin, it describes how fast a material allows heat energy to transfer at a given thickness and temperature difference. Think of it as the opposite of insulation (it is, actually. Its mathematic inverse is called “thermal resistance”).
Some relevant values of thermal conductivity (J/s·m·K):
- Air: 0.024
- Porcelain: 1.5
- Water (or Tea/Coffee): 0.58
Getting back to the 2nd Law, your body is not at thermal equilibrium with room temperature (unless you are cold blooded, in which case welcome reptilian readers!) so heat energy will flow from you into whatever you touch at a rate defined by the material’s thermal conductivity. Air at room temperature seems warmer than tea at room temperature because the tea and mug are removing your body heat much faster than air. As a further note, despite having a higher conductivity value, the tea will seem cooler than the mug holding it due to greater contact surface area.
I hope this satisfies your curiosity. Thanks for the question!