The carburettor is a refrigerator.
Two effects cool the carburettor throat far below the outside air. The venturi accelerates the intake flow, and expanding air cools; then the fuel evaporates into that flow, and evaporation drinks heat the way sweat does. Together they drop the throat 10–25 °C below the day — most at low power, when the nearly-closed throttle plate makes the expansion hardest.
So the carburettor lives in a different season from the airfield: a +20 °C afternoon is a below-zero environment at the throttle plate. Any moisture the air carries condenses on those cold surfaces — and if they are below freezing, it stays, as ice, exactly where the engine breathes.
It needs water, not winter.
The raw material is humidity, which is why the dew point deserves a place in every briefing you fly with a carburettor. Warm air holds far more water than cold air: a humid +20 °C day feeds the ice generously, while a crisp −10 °C day is usually too dry to matter. The dangerous corner of the chart is warm and moist — high dew point, small spread — which is precisely the weather nobody associates with ice.
The envelope your carburettor flies in.
Park the marker on a +20° day with a +13° dew point, then pull the throttle slider back to descent power and watch the red region reach your marker — the descent did that, not the weather. Push the throttle up and the envelope shrinks: a working engine keeps its own throat warmer.
Descent power is the trap.
Everything conspires at low power: the nearly-closed throttle gives the biggest temperature drop, the engine makes the least heat to fight it, and the ice narrows a throat that was barely open to begin with. On a fixed-pitch propeller the tell is an RPM sag you can hear; behind a constant-speed propeller the governor quietly hides it, and the only witness is a manifold pressure that slips lower than the descent explains.
Hence the oldest habit in carburetted flying: carb heat on before the descent, not after the roughness. The classic accident is a long, smooth, throttled-back glide toward the circuit — and an engine that clears its throat exactly once when asked for power on final.
Carb heat: full, early, rough.
Carburettor heat ducts exhaust-warmed air to the intake. Use it fully — partial heat can warm a very cold, dry throat up into the wet freezing band instead of out of it — and use it early, because it must melt ice with whatever engine heat remains. Expect the cure to sound worse before better: melted ice goes through the engine as water, and the roughness that follows the lever is the ice leaving, not arriving. The instinct to push the lever back in at the first cough is exactly wrong.
In your cockpit.
An engine monitor makes the sag visible before the ear catches it: RPM and manifold pressure as trends, not needles, drifting down over two minutes of descent while nothing else changed. Pair that with the OAT and dew point from the morning briefing and the chart above, and carb heat stops being a ritual and becomes what it always was — a response to a number you can see coming.
The same warm, humid air that feeds the ice is also quietly taxing your runway: the takeoff performance article.