Definition of a super cooled water droplet?
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Definition of a super cooled water droplet?
Anyone have definition of a super cooled water droplet? I understand that it is a droplet in liquid stage existing in temperatures below 0 degrees C. How does it manage this?
Join Date: Jun 2000
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You are quite right a SCWD is a droplet of water that exists at or below 0 degrees c.
It occurs because for freezing to occur and thence a change of state with its subsequent release of latent heat,a hygroscopic nuclei needs to exist.You see , in this state it is "unstable".When it locates this nuclei ie an ice particle ,it latches onto it and becomes stable.
At the top of a CB you will find virtually no SCWD , but plenty if ice particles.Why no SCWD?Because they have latched onto all the ice that exists.
Dug out from a while ago,hope it helps!
Autothrottle
It occurs because for freezing to occur and thence a change of state with its subsequent release of latent heat,a hygroscopic nuclei needs to exist.You see , in this state it is "unstable".When it locates this nuclei ie an ice particle ,it latches onto it and becomes stable.
At the top of a CB you will find virtually no SCWD , but plenty if ice particles.Why no SCWD?Because they have latched onto all the ice that exists.
Dug out from a while ago,hope it helps!
Autothrottle
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To add a few things to auto's answer :
The SWDs cause clear ice, the nr 1 ice-related problem in flight.
They can exist from -40 to 0°C, being largest between -10 & 0°C
They have a size proportional to the vertical development of their cloud (ie small in shallow lyr cloud, large in CB & thick NS).
For physics freaks, it has been demonstrated that about 1/80th of each SWD freezes on impact for each 1°C below 0°C, eg :
* at -4°C : 4/80th of the SWD will freeze on impact & 76/80th will flow back => the clear ice amount is a major factor
* at -34°C : 34/80th will freeze on impact & 46/80th will flow back => less clear ice => safer flying conditions
MF
The SWDs cause clear ice, the nr 1 ice-related problem in flight.
They can exist from -40 to 0°C, being largest between -10 & 0°C
They have a size proportional to the vertical development of their cloud (ie small in shallow lyr cloud, large in CB & thick NS).
For physics freaks, it has been demonstrated that about 1/80th of each SWD freezes on impact for each 1°C below 0°C, eg :
* at -4°C : 4/80th of the SWD will freeze on impact & 76/80th will flow back => the clear ice amount is a major factor
* at -34°C : 34/80th will freeze on impact & 46/80th will flow back => less clear ice => safer flying conditions
MF
Hmm, dredging up old memories ...
The water molecule, H²O, has the shape:
O
/ \
H H
It's bipolar nature means that when it cools to a solid, it forms a crystal lattice, with six surrounding molecules instead of the expected eight - which is why water expands when it freezes into a solid (the only substance to do so).
Because it needs to form this crystal in order to enter the solid state, it is possible for droplets of water to cool below the freezing point, while still remaining as a liquid. If the droplet has a starting point for crystal formation - then it can begin to build upon that.
If it doesn't then it has to wait for a radom chance alignment to begin crystal formation.
[ 16 October 2001: Message edited by: Checkboard ]
The water molecule, H²O, has the shape:
O
/ \
H H
It's bipolar nature means that when it cools to a solid, it forms a crystal lattice, with six surrounding molecules instead of the expected eight - which is why water expands when it freezes into a solid (the only substance to do so).
Because it needs to form this crystal in order to enter the solid state, it is possible for droplets of water to cool below the freezing point, while still remaining as a liquid. If the droplet has a starting point for crystal formation - then it can begin to build upon that.
If it doesn't then it has to wait for a radom chance alignment to begin crystal formation.
[ 16 October 2001: Message edited by: Checkboard ]
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As stated the SCWD needs a nuclei round which to freeze. An a/c performs this job exceptionally well, so getting iced up.
Similar thing ocurs with what could be called 'super cooled water vapour', inasmuch that water vapour needs a nuclei around which it can condense to form a cloud. Usully there are so many particles in the air (smoke, dust etc)that this happens with no problems. However in very clean,cold enviromnents, such as Antartica, there are sometimes not enough particles to allow condensation to occur. When you get out of your hut on a crystal clear morning and start up the generator, the exhaust smoke allows the condensation to occur, and cloud quickly forms.
Similar thing ocurs with what could be called 'super cooled water vapour', inasmuch that water vapour needs a nuclei around which it can condense to form a cloud. Usully there are so many particles in the air (smoke, dust etc)that this happens with no problems. However in very clean,cold enviromnents, such as Antartica, there are sometimes not enough particles to allow condensation to occur. When you get out of your hut on a crystal clear morning and start up the generator, the exhaust smoke allows the condensation to occur, and cloud quickly forms.
