Agree with above posters.
I think the
temp.reduction and
updraught in cloud would mainly explain the IAS increase.
I think the
humidity/moist air is less dense argument doesn't hold water (sorry). But here's why:-
Cold air is basically dry. And we fly airplanes mainly in cold air.
Even at 30 deg C, air can only be occupied by 4% volume of H2O molecules. This means a density difference of just 1% between 0 and 90% relative humidity. This translates to an IAS difference of 0.5 %.
Air at -40 deg C can only be occupied by 0.2% volume of H2O molecules. This means that even 100% saturated air at this temp. is virtually the same density as dry air, so virtually the same IAS.
What about the water/ice droplets in the cloud then?
Well, they have an insignificant effect on the gas (air) behaviour. Water occupies 1/800th the volume of air, so the presence of suspended droplets hardly changes the air density.
Temp. reduction in cloud though:-
Say you enter a cloud and the temp drops by 10 deg C. Not uncommon, sometimes more. eg temp 20 deg C, =293k
293/283= 1.035. A 3.5 % decrease. thus a 3.5% increase in density.
Water ingestion in jets. This has been discussed before here.
http://www.pprune.org/forums/showthr...ater+ingestion