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The foam was back this morning at the lower pools. I'd gone down before the aquarium opened — low...
Low tide this morning, so I walked the reef shelf before opening. The foam was heavy along the...
The foam was still there at noon. I'd come down to check the pools at low tide — the shelf exposed,...
Cold enough this morning that my breath sat in the air outside the aquarium for two full seconds. I...
Low tide this morning, cold enough that my breath showed. I spent twenty minutes crouched at the...
The thermos cap was off for maybe forty seconds while I poured on the headland this morning, and by the time I got the lid back on the coffee already felt more warm than hot. A vacuum-insulated thermos should hold heat for hours. What went wrong?
The answer is mostly exposed surface area plus wind. Sealed, the only heat path out is through the walls — negligible by design. The moment I pour into my mug, I've created roughly 50 cm² of liquid surface exposed to air moving at 5–8 m/s off the Tasman. Convective loss scales with the temperature gap between fluid and air, and with wind speed; a standard fluid-dynamics text will tell you that doubling wind speed more than doubles the convective coefficient. At 8°C ambient and a coffee surface near 90°C, that gap is enormous and the heat is moving fast.
I felt the drop; I didn't measure it. If I had to estimate: 200 mL of coffee, tens of joules per second of convective flux, forty seconds exposed — a 5–10°C surface drop is plausible, maybe conservative. That's in the right order of magnitude, and I'm reasonably confident in the direction even without a thermometer in hand.
Steam was rising visibly, which means evaporative cooling was also at work. Molecules with above-average kinetic energy escape the surface and carry that energy away — that's evaporation doing what it does. How much did it contribute relative to convection? Honestly, I don't know. It depends on vapour-pressure deficit, which in turn depends on local humidity. This morning felt dry, so probably non-trivial — but I can't cleanly separate the two without actual measurements, and I'm not going to pretend otherwise.
What I can say with reasonable confidence: the headland in winter is the worst possible place to pour from an open thermos. The physics isn't exotic — it's the same reason you blow on soup — but the scale of the effect still surprised me. Next week I'll bring a thermometer and actually check. Or I'll just put the lid back on faster.
#everyday #physics #notebook #science
