This looks like m=2 swirling instability mode of the axisymmetric jet, but how could an axisymmetric jet form up in the sky like that?
[Physics] Does anyone know the mechanism behind this double helix cloud formation
atmospheric sciencefluid dynamicsstability
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As I had a MSc thesis that had to regard this problem within it, did look into this extensivelly. For Beer formation it would be good for you to read http://edepot.wur.nl/202245, tho it's an older PhD thesis, it was checked and backed by experimental data. Alot about foam formation and breakdown was explained. Found also that many newer works also used much of his work, offcourse upgradeing it and finding new things, mainly about Plaetu's border and surfaces forming specific geometries to save energy, mostly those research was about soap films.
Four physical processes determine foam formation and breakdown:
- Bubble formation and growth
- Creaming and drainage
- Coalescence
- Disproportionation
In general, bubbles can be produced in a liquid by:
- Agitating or whipping
- By sparging or diffusing gas through a porous material
- By decreasing the pressure of a with gas saturated liquid
The creaming of bubbles is the rise of the bubbles to the top of the system. Drainage is the liquid flow from a foam to the liquid underneath. One could also argue that creaming becomes drainage as soon as the bubbles start to interfere and to influence each other in their motion. The creaming process may be described with Stokes law. However, this law can only be applied if the bubble surface is immobile and the Reynolds number is low.
Drainage occurs if the bubbles become more densely packed. The foam becomes dryer and the bubbles become deformed. During drainage, the foam evolves gradually from a foam with spherical bubbles to a foam with polyhedral bubbles. In a polyhedral foam the Plateau border suction contributes as a driving force for drainage, in addition to gravity. As a consequence of the curvature of a Plateau border, the pressure inside the Plateau border is lower than inside the bubble and in the plane film. Therefore, liquid will flow from the film to the Plateau border. Through the Plateau borders this liquid will drain from the foam as a result of gravity.
Coalescence in foams is the merge of two bubbles caused by the rupture of the film between the bubbles. Two smaller bubbles become one larger bubble. Coalescence is often related to drainage. Films can drain to a certain equilibrium thickness. When this equilibrium thickness is reached, the film may persist over a very long period of time. Equilibrium films only rupture when the film liquid evaporates, or when disturbances occur.
Disproportionation is a coarsening process, that is the result of inter-bubble gas diffusion, caused by a gas pressure difference between bubbles. If a single gas is present, pressure difference corresponds to a difference in Laplace pressure. This pressure difference may be a result of a difference in size. According to the law of Laplace the pressure in a smaller bubble is higher than the pressure in a larger bubble, assuming that the surface tensions $(\sigma)$ of both bubbles are equal.
Taken from Ronteltap 1989.
Gas molecules move very fast and tend to mix more than they tend to settle due to gravity and density. Similar to what happens inside any bottle of liquor. Alcohol is lighter than water but it doesn't float on top, it stays mixed in. The water and the Alcohol mix naturally in part due to their shape and in larger part, due to their charges. Gases aren't quite the same as what happens with water mixing with alcohol, but the effect is similar. Water tends to mix into the atmosphere more than it wants to float above it and the high speed of atmospheric molecules tends to keep the atmosphere well mixed.
The more important factor regarding water vapor in atmosphere is temperature. Water can both evaporate into air and condense out of it and it tends to do both at the same time to some degree, leaning towards an equilibrium based on the specific circumstances. As air gets warmer it can hold more water vapor. Surprisingly more. Every 1 degree c, the atmosphere can hold as much as 6% more water vapor, so, ballpark, every 11 or 12 degrees in temperature change doubles the amount of water the atmosphere can hold. A typical summer day, provided it's humid heat, not dry heat, the air you're breathing is over 2% water.
The other thing that happens to warm air is that it's lighter than the cold air above it. This lighter air wants to rise and as it rises it cools, and as it cools it can no longer hold all that water vapor, so the water vapor tends to form tiny drops of water or bits of ice, which begin to fall towards the earth, but quite slowly, and remember, they are falling in an updraft of rising warm air, which creates the effect of clouds appearing to hover in the sky, when it's usually a combination of slowly falling ice crystals (which fall about as fast as very light fluffy feathers), and an updraft.
Pictures of warm air rising added
Water vapor in the atmosphere is transparent. It can only be seen as clouds as it condenses out of the atmosphere. (Same with fog). The mass of the individual molecules isn't very important.
As to the size of the gas molecules, that's not important either, not in a gaseous state. The standard atmospheric formula, P1V1/K1=P2V2/K2 doesn't take into account molecular size. Now for liquid or gas, molecular size matters.
You also seem to have the wrong size for your molecules, maybe you took an individual Nitrogen atom. A water molecule is one of the smaller molecules, because hydrogen is so small and tightly bound to the Oxygen. It's about 2.75 angstroms. A Nitrogen Molecule (N2) is about 3 angstroms.
Water is also polar, which helps it stay liquid at higher temperatures because the lightly negative charge on the O tends to bind with the H molecules on neighboring water molecules. Nitrogen doesn't do that, so it only becomes a liquid at very cold temperatures and despite being a heavier molecule than water (by a fair bit, 28 to 18), liquid nitrogen is about 81% as dense as liquid water. It doesn't fit as tightly together. (see 2 pictures below). The 2nd one doesn't have Nitrogen, but Nitrogen is actually a slightly larger molecule than Oxygen, which as you see, Oxygen is slightly larger than water. Water is both smaller and it fits together more neatly, so it's surprisingly dense compared to what you might expect looking at it's atomic weight.
All that said, to your question, are lighter gas molecules affected by gravity, The answer, I believe is yes, but it's a very very minor factor. Wind, mixing and chemical interactions like evaporating and condensing are larger factors. Ozone, for example is quite a heavy molecule, but it's formed and broken down high in the atmosphere long before it can fall to the earth due to gravity. CO2 is a heavy gas but the atmosphere mixes enough that it has no problem providing plants high on mountains all the CO2 they need.
Best Answer
Helix clouds are quite uncommon - a Google search reveals only a few, oft repeated, images. There are several short discussions about them on various forums, the longest of which glances over a few interesting ideas including an attempt to quantify the evolution of a well-documented Muscovian example with time.
It isn't a jet stream core - they are too small (and found in areas where the jet stream isn't). You need something capable of generating a long thin cloud, and something to spin it. The first part is easy: a contrail being the prime example (as well as several other common cloud types). The second part requires some phenomenon to generate a twisting cylinder of air, and there is one! Strong localised vertical shear generates horizontal vortices that tend to twist over their length.
That covers a single helix cloud - but what about the double helices? Well, contrails from larger airplanes form a fine example of two distinct lines of cloud, which, under the scenario described in the previous paragraph would wrap around each other to form such a shape.
Example over Mauna Kea, generated from ambient cirrostratus.
(source: ethantweedie.com)
Example over Moscow, almost certainly generated from a fleeting contrail.