Icing In Cumuliform Clouds

[engineoff]

Have just started reading a book on aircraft icing and was intrigued to see a statement and accompanying chart saying that the icing envelope for cumuliforms was between 4000 - 24000 feet. Below 4000ft even in sub zero temperatures, it was stated that icing would not occur....
Am yet to get into the physics of this however it's certainly something I have never heard of before. Appreciated that the base of CU with <0 temp is not often sub 4000ft but interesting all the same. Any comments?

[BeerFly]

Well absolute nonsense !

All clouds with temp. less than 5 degrees plus can cause icing.
Especially Cu clouds and mainly between temp 0 and -13'.

Who is the author ?

[Sheep Guts]

Ive only had icing around FL200 and up. But Ive done all my flying in the tropics. To give a blanket comment that it definitely wont happen below 4000ft to me is somewhat foolish. Whose the author by the way?And whats the Title?

On a practical note:
If you are in a CU below 4000FT ,I would hope you would be transiting it anyway, or trying to get out of it A.S.A.P. Keep those Passengers smileing. But if the temps are conducive ie. +5 deg or below , and theres is visible moisture ( look at the Windscreen). Theres no reason why not. In my books I bet there plenty of North American and European would back me up on that.

Most of myn icing Ive encountered between FL200 andFL250 ,in CU NS and other forms. I used to fly over water and land , but noticed I allways got more icing over land. I am assuming that different lapse rates were responsible for this. Anyway still unsure why.


Regards
Sheep

p.s. There are alot of books on weather. But there are no hard and fast rules to mother nature and its forces.:)

[purr]

engineoff
It is all pretty straight forward you get icing when the TAT is 10 centigrade or less plus the viz has to be one mile or less.Now if you are say at 280 kts the ram rise would be approx +25 so you would get icing at an oat of say -15 what your height is not really important, also,near the equator the oat at sea level could be +45 or more in europe you would know better.CBs are pretty dicy I once had the oat rise by 20 degrees while passing one at FL 370 it was possibily the same one that injured an cabin crew on an gulfair flight sometime back so read on there is always something new to learn.:cool:

[bluskis]

I've seen icing at ground level, anyone else seen this phenomena?

[Sheep Guts]

Purr you wrote:

It is all pretty straight forward you get icing when the TAT is 10 centigrade or less plus the viz has to be one mile or less.

Are you sure the viz has to be less than 1 mile. Is this in your SOPs? As I have yet to see it written in a Text Book.

So if I have 1.2 Miles Viz, I am ok am I? :p


Regards
Sheep

P.S. As I said previous, there are no hard and fast rules with weather. You cant afford to be too definative as people, will take you literally!;)

[Final 3 Greens]

For what it's worth, I've had airframe icing in SC at 2500'.

Don't know if this counts.

[Tinstaafl]

Icing in Cumuloform at 1000'.

That author's obviously never been north of 60 deg N.

[purr]

sheep guts
its in the 737 AFM

[engineoff]

The book I was referring to is called 'Aircraft Icing' by Terry T. Lankford.

To quote the relevant part...

'The icing envelope for cumuliform clouds is somewhat more narrow than for stratiform clouds. The minimum altitude is 4000ft, and maximum altitude is 24,000 ft. Icing is most frequently encountered (about 40 percent of the time) at ambient temperatures of 0 to -10 degrees C. At lower temperatures, the frequency of icing in clouds is much lower, about 6 percent at -30 degrees C.'

Associated with this comment is a graph showing the icing envelope which also precludes icing below 4000 ft at any temperature....

Aside from this bizarre statement, seems not a bad book.

[purr]

Could be because of supercooled water droplets.I am not too sure .Why dont you walk into your local met office I am sure they would be quiet delighted to get into the nitty gritty.:)

[erikv]

Could this have to do with the classification of clouds and the definition of a cumulus cloud, rather than the ingredients for ice?

Erik.

[dirkdj]

Could it be that the author in that book meant an altitude BAND of 4000ft ? This would make sense. So from the freezing level upwards (wherever that falls) 4000 ft up (at least).

[Dick Whittingham]

Classic airframe icing depends on the presence of supercooled water drops (SCWD) These only exist in OATs below zero, so there's one limit. By the time the OAT is down to about -20º to -25º most SCWD will have frozen, and only small SCWD giving light rime ice will exist, so there's another limit. Finally, by about -45º all water will have frozen, so no icing at all at these low OATs.

The formation of Cb type cloud requires a steep environmental lapse rate, and to get zero surface temperatures and a rapid drop as you climb you have to be in pretty extreme conditions, not in stable cold highs over land but in Arctic and Antarctic conditions.

In temperate climates you would probably have a surface temp in the low pluses, and that would mean a zero degree height in the cloud in the low thousands, and the end of serious icing at about 15,000ft in the winter and say 8000 to 20,000 in the summer. In the tropics, where the zero degree height is about 14-16,000ft the icing would be from 16 to about 30,000ft. Roughly! But in Cb the OAT is all mixed up as the updrafts and downdrafts occur.

Stratocu does not depend on instability for its formation, it often occurs in a stable flow with loe surface temperatures, so the icing can occur lower down. I, too, have experience of severe icing in Sc at 2000ft in the UK in winter.

So the answer is that icing depends on SCWD and the OAT, and given the requirements for the formation of Cb the figures first given are practical values for temperate climates. Icing in Sc can occur at very low height, and, of course, freezing rain can occur right down to deck level.

Dick W