Carb icing on a warm vs. cold day

[gerardflyagain]

This CAA document states:

"Cold, clear winter days
are less of a hazard [for piston engine icing] than humid
summer days because cold air holds
less moisture than warm air.

I understand that warm air can hold more moisture. However, isn't icing more likely, albeit of a smaller amount of ice, in colder temperatures. Take the following example

Warm day:
25c/24c dewpoint

Cold day:
2c/1c dewpoint

Flying on the cold day, the venturi only needs to cool the air by 2 degrees to produce ice. Flying on the warm day, the venturi needs to cool the air by 25 degrees (although ultimately more ice will form when it does).

What am I getting wrong about the CAA's statement?

[oggers]

... 2ºC with 1ºC dewpoint is going to be very far from a clear day!

[gerardflyagain]

So should I infer from clear day that the relative humidity is low?

Even if that was the case, wouldn't there be more likely to be ice in, say a 12c/1c dewpoint condition, because the venturi only needs to cool the temperature by 12c, rather than by 25c, to produce ice?

[Crash one]

I'll probably get this all wrong but if the lapse rate is 2deg per 1000ft then a difference of one degree between temp and dewpoint amounts to a cloudbase of 500ft
If the Venturi flies into that wet cloud the pressure difference at the Venturi will cause the icing.
Whether that is a hot day or cold the result would be about the same.
I think!!

[rusty sparrow]

Having flown some icing prone aircraft (VW and C90) I have he carb icing chart here at the back of my checklist https://www.atsb.gov.au/publications...urettor-icing/

I check that for the probability of icing - and am always aware that it can happen, even at full power.

[gerardflyagain]

But crash, surely it would take longer on a warm day for the air to reach freezing? Therefore the likelihood of freezing is lower on the hot day?

Thanks for the diagram rusty sparrow. I'll use it to illustrate my question. Look up warm day 25/24 (dewpoint depression = 1) and it shows a third highest risk of icing. Look up 12/2 (since this is already marked on there as the example). This shows the second highest risk of icing. Therefore the risk of icing is higher at the lower temperature, surely?

[foxmoth]

as long as the temp drob in the carb is sufficient to be below zero then you have the problem and it will be worse the more moisture you have, also colder air will form ice faster with more air in and is not so solid (think about airframe icing where very cold water drops freeze instantly but water nearer zero runs back before freezing)

[gerardflyagain]

Great, thanks.

[kghjfg]

A wise man once told me to forget what the weather looks like outside,

"It is ALWAYS Winter inside the carburettor"

and it really is.

[rotorfossil]

Temp drop from the intake to the back of the butterfly at low power is about 25deg C. Hence warm humid days are good for icing.

[oggers]

I think all the CAA are trying to point out is that humidity is the key variable for carb icing, not OAT per se. Because as kghjfg put it 'it's always winter inside the carb'. The idea of icing on a warm day is counter-intuitive and the idea that colder may (or may not) equal less ice also. But that's how it is with carb ice.

[gerardflyagain]

Thanks all. The last 3 posts have driven home the point that the temp drop to freezing is almost a given in any OAT from 0 to 25c. And the CAA's point is that once that has happened, a previously warm temperature will have way more water in it to then freeze than a cold temp would have done.