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There seems to be an assumption in some of the earliest postings that carb icing is in some way related to altitude. Whilst this is not entirely false, it is an odd way to think of the problem.
The real determinant of Carb Icing risk is the ambient temperature of the air, and its water content.
Carb Ice risk is at its highest when air starts of fairly cool, and with high moisture content (note that the warmer the air, the higher its moisture retaining capability)
As the air passes through the throttle butterfly, it expands (and therefore cools). If the cooling effect is sufficient, the ambient temperature is not too high, and there is enough moisture in the airflow, then ice will form around the butterfly and adjacent structure.
So, the worst possible day for carb icing will be one in which the weather is moderate to warm (increasing the water-bearing capacity of the air), and humid.
The worst time in flight is when the throttle is partially closed, because this maximises the expansion of the air passing through the butterfly, increasing ice formation.
This is why we worry about a long low-power descent from altitude on a warm summers day.
As we have heard, cars with carburetors either tend to pre-heat the incoming air by passing it close to the radiator, or have specific heating systems to avoid the problem. We tend not to do the same thing on an aircraft because warm air carries less oxygen and reduces the power of our engines. Small piston engines in aircraft typically need all the power they can get. Chipmunk owners will be familiar with the historic custom of wirelocking the carb-heat on though.
Motorbikes tend to be less sophisticated than cars, and lack the same carb heating options. Carb icing on bikes is therefore far more common.
Worth noting that Icing is not purely restricted to Carbs though. Injection systems are also known to ice, but under rather more extreme circumstances not normally applicable to cars
