Will my tea cool more quickly if it is left out at room temperature, or if my hands are cupped around the mug?
Hillary Shaw
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Newport, Shropshire, UK
Logically, your hands will cool the tea faster. The tea will be at about 80°C (176°F), with the air at say 20°C (68°F). Your hands will be just below core body temperature, say at 35°C (95°F), so not that different from the air. However, water (the main component of human body tissue) is a much poorer insulator than air, so your hands will conduct heat from the cup faster than the surrounding air, as the broken cup on the floor testifies, after you involuntarily dropped it because it was scalding your hands.
Bryn Glover
Kirkby Malzeard,
North Yorkshire, UK
Almost anything conducts heat better than air, so wrapping your hands around the cup will immediately extract more heat. However, hands are also good insulators, especially when wrapped in woolly sleeves, so keeping your hands in place may actually slow down the process.
If you wish to cool the mug more rapidly than in air, I would recommend alternately wrapping your hands around the mug, then cooling them by wafting them in the air, repeating this each time your hands reach their maximum tolerable temperature.
Don Sandom
Reading, Berkshire, UK
Yes, the tea will cool quicker if your hands are clasped around the mug which contains it.
This may seem counterintuitive, since the layer of pudgy digits could be seen as adding thermal insulation to the surface of the mug. The applied hands will, however, have a perspiration function and blood circulation, both acting to keep the core temperature of the drinker’s body at around 36°C to 37°C (97-99°F). Since those are temperatures below that of drinkable tea, I would expect these mechanisms in the hands to actively transfer heat energy away from the higher-temperature tea within the mug.
Significantly, the contact between the mug and hands offers better heat transfer than will occur between your cup and free air, so that heat will be lost more rapidly from the mug when the hands are in position around it.
A way for hands to be used to keep the tea warmer would be to cup them over the top of the mug, rather than around the sides. This will reduce the airflow from the exposed top surface of the tea, which is probably the major source of energy loss, by evaporation as well as convection.
Mike Follows
Sutton Coldfield,
West Midlands, UK
There are too many variables to give a definitive answer. My suspicion is that holding an insulated cup might speed up the cooling a tiny bit whereas cupping a ceramic mug might slow down the cooling. To be sure, this would require an experiment, the sort that might get nominated for an Ig Nobel prize.
With the lid removed from my insulated cup, a hot drink can stay warm enough to be pleasant to consume for up to an hour. Most cooling occurs via the latent heat of evaporation – in which some of the thermal energy in the drink is used to turn some of the contents into vapour – supplemented by thermal conduction to the air above the liquid. Doubtless this air could then undergo small-scale convection.
I suspect holding the cup would risk cooling the contents a tiny bit quicker as I might inadvertently agitate the liquid, increasing the surface area from which evaporation could take place. Moving the air above the liquid by talking or breathing over the surface might increase the rate of evaporation, by replacing humid air with drier air.
Tea cools quicker in a ceramic mug because there is significant heat loss through the walls. I imagine that most heat loss is via radiation from the outside surface, so a darker-coloured mug should cool quickest, as should one with a higher thermal conductivity.
There is a rapid initial drop in tea temperature when the tea is poured into the mug as some thermal energy is used to warm the mug, which stops only when the tea and mug reach thermal equilibrium. This drop is bigger for a mug with a greater heat capacity, which is a measure of the energy needed to raise its temperature.
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