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.

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