Icing probability
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Risk depends mostly on humidity, shown here as the spread between temperature and dew point, the smaller the spread, the wetter the air. Glide/descent power is rated separately from cruise power because a throttled-back engine makes far less heat to fight ice with.
Simplified zones inspired by the FAA carburetor icing probability chart (SAIB CE-09-35), for training awareness, not a substitute for the original chart.
Engine
Quick check
Why a warm engine ices up
Fuel vaporising in the venturi and the pressure drop across it can cool the incoming air by up to about 21°C (70°F) in an instant. That means ice can form with an outside air temperature as high as the mid-30s °C, as long as the air is humid enough, this is not a winter-only hazard.
What's actually freezing
Two effects stack up: the venturi speeds up the air, dropping its pressure and temperature (the same effect that makes a can of compressed air feel cold), and the fuel itself absorbs heat as it vaporises. Ice forms on the throttle plate and venturi walls, narrowing the passage exactly where you need airflow.
Reading the symptoms
Fixed-pitch propeller: RPM drops on its own, without you touching the throttle, often with roughness as ice breaks apart unevenly.
Constant-speed propeller: the governor hides the RPM drop by adjusting blade pitch, so watch manifold pressure instead, a slow, unexplained drop is the tell.
Using carb heat correctly
Apply full heat, not partial, on some carburetors a partially heated position can be warm enough to melt ice into water without being hot enough to prevent it refreezing further downstream, which is worse than doing nothing.
Expect a small, immediate RPM drop from the less dense hot air even with no ice present, that's normal. If ice was there, RPM may dip a little further first as it melts, then climb back higher than where it started as the restriction clears.
When to check
Periodically in cruise when conditions favour icing, and always before reducing power for a descent or landing, glide power makes the least heat exactly when you can least afford a rough or dying engine close to the ground. Fuel-injected engines don't get carburetor ice this way, but always follow your type's own procedures.