Now I've heard of the mackerel sky and am pretty sure I know how to identify such clouds. But in looking up sleet on a weather glossary site, I came across a cloud formation of which I'd never read a description. If you didn't know this one, you might enjoy adding it to your list:
MAMMATOCUMULUS
An obsolete term for cumulonimbus mammatus, it is a portion of a cumulonimbus cloud that appears as a pouch or udder on the under surface of the cloud.
I'll come back in a sec and post the url, which I forgot to earmark...
I saw Udders in the Sky & thought, '...and the cow jumped over the moon.'
Ron.
and i thought --Ahh! The land of MILK and honey!
That WW, its a nice site...
Jumping cows and milk & honey, oh, my! Very funny.
I like the site, too, of troy, and have added it to my favorites since the weather is so fascinating. Sublimnation was a point of interest about, oh, two years ago--but I read on this weather site this morning that crystallization is the opposite of sublimnation, which came as a surprise. I hadn't realized all these years that in crystallization gases go from gaseous state to solid state, and bypass the liquid state. Live and learn.
*Plural note: gases and gasses are both acceptable in MW. In other-than-American English, is one preferred over the other?
This is actually my own field (not meteorology, but thermodynamics).
On a phase diagram for water, the vapour-solid equilibrium line is called the "frost line" - the transition from solid to vapour phase is called sublimation.
Unfortunately thermodynamics lacks a standardised term for the opposite transition, from vapour to solid. "Crystallisation" describes a more general phenomenon.
Terms in use include "sublimation", "reverse sublimation" and "deposition".
I usually see (and use) "gases", but I don't know whether "gasses" is acceptable in European English, or whether it is a US variant.
Another useful meteorology term (for pilots, at least) is "virga" - rain or snow that does not reach the ground (it meets a layer of dry air on the way down and evaporates/sublimates). "Virga" is a Latin word for "branch".
in crystallization gases go from gaseous state to solid state, and bypass the liquid state
This is not entirely correct: crystallization can also occur from the liquid state. The term covers any transformation from a disordered array of molecules into an ordered one.
Why do the NASA guys say they are searching for evidence of "liquid water" on Mars?
Cause solid water and gaseous water are neither very important in the support of life.
wsieber, I'm definitely out of my depth here, but love reading about changes in states. I do understand, as hibernicus pointed out, that what we have here is a problem in terminology. However, I thought it most interesting to read on the weather definition site posted above that the term crytallization could mean the opposite of sublimnation. You have added even more information of interest. Would it be correct to expect:
1. Crystallization can sometimes occur when gases go from gaseous state to solid state;
2. Crystallization can sometimes occur when liquids pass to solid state.
This may seem oh-so-obvious to those of you in the field, but I'm just trying to make sure I understand correctly.
Faldage, your comment, as usual, made me laugh!
From WW's site:
VIRGA
Streaks or wisps of precipitation, such as water or ice particles, that fall from clouds but evaporate before reaching the ground. From a distance, the event sometimes may be mistaken for a funnel cloud or tornado. Typically, it may fall from altocumulus, altostratus, or high based cumuonimbus.
I was wondering where "branch" came in, but I guess if you can see streaks, they can look like branches. This def. made me look up altocumulus and altostratus, and sure enough, they're both in the "middle". No listing for tenor clouds, though.
tenor cloudsthey're the good looking ones above everything else...
1. Crystallization can sometimes occur when gases go from gaseous state to solid state;
2. Crystallization can sometimes occur when liquids pass to solid state.
Both correct - as is
3. Crystallisation can occur within the solid phase as the molecular structure of the substance changes.
I would not criticise the weather site for use of the word crystallisation, for want of a better term. But it's not really the opposite of "sublimation". If the phase diagram has three single-phase regions, that gives up to three pairs of phase transitions:
-evaporation/condensation
-melting (or fusion)/freezing
-sublimation/ ?
"Crystallisation" is not really restricted enough in meaning to fill that blank.
Very good, hibernicus. That makes sense to me--and I especially appreciate your coupling up of the opposites.
Sounds as though it would be good to have a definite separate term for sublimnation's opposite!
I, too, had thought of the process of crystallization as observed in minerals, but didn't want to mix things up so much at that point.
What's very interesting to me is this:
If it's thought to be true that something can move from its gaseous state directly to solid, how can we know for sure? Do we have photography that can slow things down enough that we would know definitely that there is no liquid state? And, if not with some technologically advanced form of photography, do we have others kinds of technology that verify this?
when the weather is right, you can sometimes see rain hitting the hood of your car, and 'disappearing' if the engine/hood is hot enough, the hood doesn't even seem to get wet.. it just becomes smokelike wisps...
the air has to be somewhat dry (so its has to be at the beginning of a rain storm, or cool weather, (since cold air can 'hold more moisture' than hot air)..and you need a nice steady breaze.. (but not windy weather)
you can sometimes see the same 'wisps' of water vapor evaporating from a sheet of ice.. (you need really cold, dry, clear and sunny weather, with some breezes..
the sun will warm up the surface of the ice, (it won't melt, (not when its 15º (f) ) but the combination of sun light and a slight breeze will create small wisps of water vapor.
(of course solid CO(2)-carbon dioxide- goes from solid to gaseous at room temperture.. the block 'melts away' infront of you eyes.. the vapors pour off and diapate.
If it's thought to be true that something can move from its gaseous state directly to solid, how can we know for sure? Do we have photography that can slow things down enough that we would know definitely that there is no liquid state?
This is a very incisive question, and it approaches the limits of what is known about phase transitions. What is known is that for most combinations of pressure and temperature, pure water has a certain equilibrium state, whether that is ice (of which various different phases exist), subcooled liquid or dry vapour. In some special regions, the equilibrium condition can be any mixture of two phases, or even three (at a triple point, such as 0.01degC, 1 bar(a)).
The equilibrium condition is the lowest energy state at that point, and is determined by the "free energy" of the substance. However, it is certainly possible to cool very pure water to below "freezing point" and hold it in an unstable liquid state, until an impurity serves as a nucleation point and rapid freezing occurs. Similarly, if you heat pure water in a long-necked vessel (DO NOT DO THIS!) you can raise its temperature to well over 100degC at atmospheric pressure without boiling, before it suddenly and without obvious proximate cause flashes into steam.
It has been postulated by some of the founders of thermodynamics that the distinction between liquid and gas phases is the result of real-world instabilities, and that in a Platonic world of pure substances there would be a smooth transition at all temperatures (just as there is above the critical point in the real world).
Anyway, it is very possible that although liquid water is not stable at the temperatures and vapour pressures where this phenomenon occurs, droplets of water may form and abide a short while before freezing. No photograph would detect them, but their tendency to refract rather than scatter a beam of light might reveal their presence to the determined experimenter.
cold air can 'hold more moisture' than hot air
"Hold more moisture" isn't right, but I will save you a discussion (nay, a personal crusade!) which I suspect would be compellingly interesting only to me (my previous post has probably already breached some lifetime quota!)
What I will say, however, is that it is warm air which could be described as being able to "hold more moisture" than cold.
You're exactly right about carbon dioxide, and about ice - if left outside (or even in the freezer) in sub-freezing temperatures, the ice will eventually all sublimate away and disappear! Another explanation for the wisps of vapour is that the air blown over the sheet of ice is cooled, and some water vapour condenses to form wispy little clouds.
Virga was a term used frequently by my all-time favorite meteorologist, Larry Green of Channel 4 in Denver, and was important to Denverites not so much because of the phenomenon itself but because virga could also cause localized downdrafts which got pretty intense at times. The evaporation of the water from cloud to gas state cooled the air, which dropped precipitously. These columns of cold air would hit the ground and spread out quite rapidly. 40 MPH (60 KPH) was not uncommon.
It also meant that it wasn't going to rain, a topic always much on the minds of Denverites, who live in the rain shadow of the Rockies.
... with diamonds.
Just means that the weather's gone tits up again, thassall.