Media Backlash to My Hurricane Blog

By: 24hourprof , 3:14 PM GMT on September 23, 2013

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Back in 2005, I wrote an essay in the Bulletin of the American Meteorological Society (BAMS) outlining the pitfalls of deterministic medium-range forecasts (this pdf file contains my essay). After my essay was published, BAMS published a response (pdf file) from several television broadcasters. In my opinion, their response seemed to defend "popular" expressions such as "warm air holds more water vapor" and "clouds act like a blanket at night." Both of these expressions are scientifically inaccurate, in my opinion, and I'll add the latter topic to my list of future blogs (here's Alistair Fraser's debunking of the former expression). To me, the most disconcerting aspect of the response by the broadcasting industry to my essay was that, by publishing the response, the American Meteorological Society seemed (at least to me) to tacitly endorse the validity of these two "popular" but scientifically inaccurate expressions.

So I was not surprised when I received an e-mail last week from a media professional, protesting my assertion that relatively strong vertical wind shear doesn't simply blow, advect, carry, transport (whatever similar verb you like) thunderstorms organized around the core of a hurricane away from the storm's low-level center of circulation (here's the relevant blog). Apparently, my argument about the time scale of core thunderstorms, which as you recall, was on the order of 30 to 60 minutes, didn't register with him. Time scales are important here because, by the time thunderstorms would be transported sufficiently far downshear (in the case of Humberto, roughly to the east) so that we would notice on satellite imagery, core thunderstorms would have long dissipated. No, the story is much more complicated than vertical wind shear simply transporting thunderstorms horizontally away from the core.


The 18Z enhanced infrared satellite image of Tropical Storm Humberto on September 16, 2013. At the time, relatively strong vertical wind shear disrupted the symmetry of Humberto's warm core, with the vortex associated with the low-level circulation decoupling from the upper-level vortex. Note high, cold cloud tops of tall thunderstorms downshear (roughly east of) the storm's low-level circulation. Larger image. Larger unannotated image. Courtesy of Penn State.

In my first Humberto blog, I pointed out that relatively strong vertical wind shear destroys the symmetry of the warm-core structure of a tropical cyclone. My goal today is to explain how relatively strong vertical wind shear weakens tropical cyclones in a slightly different way, drawing on satellite imagery so that readers can observe firsthand the point I'm trying to drive home. By way of review of Humberto, check out the 18Z analysis of deep-layer vertical wind shear on September 16, 2013, (at the time, Humberto was a tropical storm). Now focus your attention (above; larger image) on the corresponding enhanced infrared satellite image (18Z on September 16). Note the faint, grayish "doughnut" that indicates the relatively warm cloud tops in Humberto's low-level circulation. Also notice the high, cold cloud tops of tall thunderstorms that were separated from the storm's low-level circulation at this time. Contrary to television explanations that I heard, these storms were not transported eastward away from Humberto's low-level circulation.

For this case, a series of pictures is worth a thousand words. Check out (below) the animated gif of visible satellite images (courtesy of CIMSS; larger loop) showing the separation of the low-level circulation of Humberto from the deep thunderstorms that we typically observe around the core of a mature or developing tropical cyclone. The overarching message I want you to take away from this loop of visible satellite images is that the low-level circulation and the deep thunderstorms decoupled from each other. This decoupling is consistent with my previous argument that relatively strong vertical wind shear destroys the symmetry of the warm core of a tropical cyclone. Let's investigate.


A sequence of visible satellite images on September 16 showing the vortex housing Tropical Storm Humberto's low-level circulation decoupling from the upper-level vortex and the deep convection (thunderstorms) that formed downshear (roughly east of) Humberto's LLC. Larger loop. Courtesy of the Cooperative Institute for Meteorological Studies (CIMSS).

To explain this decoupling, I point out that relatively strong vertical wind shear over the core of a tropical cyclone means that the high-altitude outflow is constrained to a single channel. For example, westerly shear constrains high-level outflow on the western flank of the tropical cyclone (outflow on the western flank becomes limited or "constrained", the that the effective outflow channel lies on the eastern flank of the storm). At any rate, relatively strong winds at high altitudes advect (transport) cyclonic angular momentum over the core of the tropical cyclone. The arrival of cyclonic angular momentum is bad news for the upper-level high-pressure system (anticyclone) sitting atop the tropical cyclone. Indeed, imported cyclonic angular momentum weakens the upper-level high, which, in turn, disrupts (chokes off) the tropical cyclone's high-level anticyclonic outflow. This disruption of high-level outflow feeds back to the ocean surface, where barometric pressure increases, low-level convergence decreases, and eyewall convection weakens. With weakening convection in the eyewall, the vertical transport of cyclonic angular momentum also decreases (weakening updrafts), and the vortex in the lower troposphere eventually decouples from the upper-level vortex. I believe this decoupling is what you're seeing on the animated gif of visible satellite images above.

With regard to vorticity, vertical wind shear produces differential positive vorticity advection downshear of the tropical cyclone (in the case of Humberto, roughly to the east). By "differential" I mean positive vorticity advection at two different pressure levels (500 mb and 1000 mb, for example). This differential PVA promotes low-level convergence and upward motion downshear of the low-level circulation (in the case of Humberto, roughly east of the LLC). It is here that new showers and thunderstorms develop. They are not advected or carried downshear from the core. Rather, new thunderstorms develop in response to the differential positive vorticity advection and the displaced low-level convergence and upward motion.

Yes, I agree, it's very complicated, and I would have never offered such an explanation whenever I was on-air (believe it or not, I was on television for almost 20 years (Weather World at Penn State). And so I have empathy for on-air folks. So a fair question becomes: How would Grenci explain it on television? Given that large-scale convection in the Tropics develops most readily in weakly sheared environments (especially compared to convection in the middle latitudes), I'd be okay with relatively strong vertical wind shear "blowing off the tops of thunderstorms" around the core of a tropical cyclone and new convection developing downshear of the low-level circulation in response to displaced low-level convergence and stronger upward motion.

You might think that I'm nitpicking here, but I firmly believe my shortened explanation is closer to the scientific truth than the one that relies on core thunderstorms simply being transported away from the core by winds associated with relatively strong vertical wind shear.

Lee

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43. georgevandenberghe
4:53 PM GMT on October 03, 2013
By the way about "Clouds act like blankets".
The economical restatement is that they act like radiators. Next question!
Member Since: February 1, 2012 Posts: 18 Comments: 1828
42. georgevandenberghe
4:33 PM GMT on September 30, 2013
Quoting 40. 24hourprof:


It's funny that my blog was about Hurricane Humberto, and there were very few comments about that.

Water vapor stole the show!!!!


A minimal hurricane over water that only affected shipping ( and stimulation starved bloggers,) v.s. the correct repudiation of an idea cherished in too many educated people's minds that somehow air "holds" water vapor in solution or suspension.. I'm not surprised at the ratio of comment topics.
Member Since: February 1, 2012 Posts: 18 Comments: 1828
41. WunderAlertBot (Admin)
3:34 PM GMT on September 30, 2013
24hourprof has created a new entry.
40. 24hourprof
2:34 PM GMT on September 30, 2013
Quoting 38. theshepherd:
ROFL

OK. So, "Water vapor is present in the air".

Thanks Lee.

Next topic????
:)))


It's funny that my blog was about Hurricane Humberto, and there were very few comments about that.

Water vapor stole the show!!!!
Member Since: October 24, 2012 Posts: 91 Comments: 803
39. georgevandenberghe
1:18 PM GMT on September 30, 2013
Quoting 37. 24hourprof:


Yes, air temperatures control condensation rates and water temperatures control evaporation rates.

Just keep in mind that the air temperature is a measure of the average kinetic energy of air molecules. So some molecules move faster than others, but, in the mean, it's all the same.

How's that?


Vapor temperatures control condensation rates. Air and vapor temperatures will admittedly be identical to below measurable differences. (I wanted to say "virtually identical" but the grammatical idea
of an adverb modifying an absolute adjective [ e.g approximately
exact ] bothers me )
Member Since: February 1, 2012 Posts: 18 Comments: 1828
38. theshepherd
11:33 AM GMT on September 30, 2013
ROFL

OK. So, "Water vapor is present in the air".

Thanks Lee.

Next topic????
:)))
Member Since: September 11, 2008 Posts: 9 Comments: 10093
37. 24hourprof
11:20 AM GMT on September 30, 2013
Quoting 36. rpointwx:


My question then is it is the air temperature that is controlling the condensation and evaporation rates and not the temperature of the water vapor? (knowing that they are probably around the same temperature).


Yes, air temperatures control condensation rates and water temperatures control evaporation rates.

Just keep in mind that the air temperature is a measure of the average kinetic energy of air molecules. So some molecules move faster than others, but, in the mean, it's all the same.

How's that?
Member Since: October 24, 2012 Posts: 91 Comments: 803
36. rpointwx
11:17 AM GMT on September 30, 2013
Quoting 35. 24hourprof:


The dew point is the temperature to which the air must be cooled, at constant pressure, for the condensation rate to equal the evaporation rate.

So when the air is cooled ever so slightly below the dew point (not measurable), net condensation occurs (water drops form). By net condensation, I mean that the condensation rate exceeds the evaporation rate.

As a footnote, evaporation and condensation occur almost all the time. It's only when one process exceeds the other are we able to observe something happening (a cloud forms, a puddle of water evaporates, etc.).

Hope this helps.

Lee


My question then is it is the air temperature that is controlling the condensation and evaporation rates and not the temperature of the water being evaporated? (knowing that they are probably around the same temperature). Is air temperature a proxy for the water vapor/water temp?
Member Since: December 27, 2009 Posts: 0 Comments: 10
35. 24hourprof
11:07 AM GMT on September 30, 2013
Quoting 34. rpointwx:
So is it fair to say that the dewpoint temperature is not a measure of air temperature but is the temperature of water vapor where condensation rates = evaporation rates and when the water vapor cools further its evaporative potential decreases leading to net condensation as evaporative rates < condensation rates.


The dew point is the temperature to which the air must be cooled, at constant pressure, for the condensation rate to equal the evaporation rate.

So when the air is cooled ever so slightly below the dew point (not measurable), net condensation occurs (water drops form). By net condensation, I mean that the condensation rate exceeds the evaporation rate.

As a footnote, evaporation and condensation occur almost all the time. It's only when one process exceeds the other are we able to observe something happening (a cloud forms, a puddle of water evaporates, etc.).

Hope this helps.

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
34. rpointwx
3:19 AM GMT on September 30, 2013
So is it fair to say that the dewpoint temperature is not a measure of air temperature but is the temperature of water vapor where condensation rates = evaporation rates and when the water vapor cools further its evaporative potential decreases leading to net condensation as evaporative rates < condensation rates.
Member Since: December 27, 2009 Posts: 0 Comments: 10
33. 24hourprof
8:36 PM GMT on September 27, 2013
Quoting 32. georgevandenberghe:
From Post 30

how can the temperature of atmospheric water vapor be expressed aside from using / substituting it's "neighboring" air temperature? Subsequently, air temperature dewpoint temperature for saturated vapor pressure verses actual / partial vapor pressure, evaporation-condensation rate, and derived humidity values.

The temperature of the water vapor is determined by the averaged kinetic energy of the water vapor molecules. Although it will be virtually the same as the temperature of the rest of the atmosphere, it is independent and is still a definable and mesasurable quantity in absence of the rest of the air.

The rest of the quantities are functions of vapor pressure and the temperature (now we know how its defined) of the vapor and liquid with complications induced by CCN (general ) and solutes (common around hygroscopic CCN, not much of an issue with standing surface water)


Thanks for helping out, George. Good answer!
Member Since: October 24, 2012 Posts: 91 Comments: 803
32. georgevandenberghe
6:06 PM GMT on September 27, 2013
From Post 30

how can the temperature of atmospheric water vapor be expressed aside from using / substituting it's "neighboring" air temperature? Subsequently, air temperature dewpoint temperature for saturated vapor pressure verses actual / partial vapor pressure, evaporation-condensation rate, and derived humidity values.

The temperature of the water vapor is determined by the averaged kinetic energy of the water vapor molecules. Although it will be virtually the same as the temperature of the rest of the atmosphere, it is independent and is still a definable and mesasurable quantity in absence of the rest of the air.

The rest of the quantities are functions of vapor pressure and the temperature (now we know how its defined) of the vapor and liquid with complications induced by CCN (general ) and solutes (common around hygroscopic CCN, not much of an issue with standing surface water)
Member Since: February 1, 2012 Posts: 18 Comments: 1828
31. 24hourprof
5:45 PM GMT on September 27, 2013
Quoting 30. DocNDswamp:
Good day Lee,

re: 18. barbamz 1:08 PM CDT on September 25, 2013
If I might add to your answer to Barb's question, I can see the problem resulting from strict translation of "vertical", implying up-down direction or motion - like recon flights subjected to strong updrafts / downdrafts! - confuses the definition of vertical wind shear. Given the immense height, or vertical structure of a tropical cyclone, vertical wind shear describes horizontal winds speed / direction affecting varied vertical levels of the storm system - the "with altitude" part of your answer.... or affecting vertical structure alignment, to elucidate the "vertical" reference in vertical wind shear.

Hmmmm... Reading the comment exchange here, I must admit it is with great trepidation I dare address the warm-cold air / water vapor matter. Clearly you've accurately demonstrated process entirely dependent on water temperature but equally clear is the conundrum - language describing such in concise manner for meteorology seems elusive, inadequate. The issue I'm having difficulty resolving in practical application is how can the temperature of atmospheric water vapor be expressed aside from using / substituting it's "neighboring" air temperature? Subsequently, air temperature dewpoint temperature for saturated vapor pressure verses actual / partial vapor pressure, evaporation-condensation rate, and derived humidity values.

Regardless, been enlightening refresher on processes involved reading / absorbing presentations and views here, the interest generated also sent me off on many a "Wild Google Chase" past couple days... so overall, it's served to educate. Found this handy reference tool, the Vapor Pressure Calculator.



With regard to your "language remains elusive," here's what I say to my students.

Cooling the air ever so slightly below the dew point causes water vapor molecules to slow down (air temperature is a measure of the average kinetic energy of air molecules) and congregate around condensation nuclei, resulting in net condensation and the formation of a cloud droplets. (instead of the dreaded "cold air doesn't hold much water vapor").

As a side note, you can see that if I removed all the condensation nuclei from a sample of air, there would be a staggering amount of water vapor in the air.
Member Since: October 24, 2012 Posts: 91 Comments: 803
30. DocNDswamp
4:53 PM GMT on September 27, 2013
Good day Lee,

re: 18. barbamz 1:08 PM CDT on September 25, 2013
If I might add to your answer to Barb's question, I can see the problem resulting from strict translation of "vertical", implying up-down direction or motion - like recon flights subjected to strong updrafts / downdrafts! - confuses the definition of vertical wind shear. Given the immense height, or vertical structure of a tropical cyclone, vertical wind shear describes horizontal winds speed / direction affecting varied vertical levels of the storm system - the "with altitude" part of your answer.... or affecting vertical structure alignment, to elucidate the "vertical" reference in vertical wind shear.

Hmmmm... Reading the comment exchange here, I must admit it is with great trepidation I dare address the warm-cold air / water vapor matter. Clearly you've accurately demonstrated process entirely dependent on water temperature but equally clear is the conundrum - language describing such in concise manner for meteorology seems elusive, inadequate. The issue I'm having difficulty resolving in practical application is how can the temperature of atmospheric water vapor be expressed aside from using / substituting it's "neighboring" air temperature? Subsequently, air temperature dewpoint temperature for saturated vapor pressure verses actual / partial vapor pressure, evaporation-condensation rate, and derived humidity values.

Regardless, been enlightening refresher on processes involved reading / absorbing presentations and views here, the interest generated also sent me off on many a "Wild Google Chase" past couple days... so overall, it's served to educate. Found this handy reference tool, the Vapor Pressure Calculator.

Member Since: September 21, 2005 Posts: 94 Comments: 4794
29. 24hourprof
11:46 AM GMT on September 27, 2013
Quoting 28. EstherD:
Back in the 60's when I was in high school, we were taught that air holds water vapor the same way that liquid water holds dissolved solids, like salt -- that the two systems were more-or-less analogous. So the idea that "warm air can hold more moisture than cold air" seemed to follow quite logically. More recently, when my daughter's science teachers (and textbooks) talked about atmospheric water vapor, they used similar language. However, a few years ago, on one of the websites that debunk "bad science" (don't recall now which), I ran across an explanation similar to the one I'm reading here. I've been struggling with it on and off ever since.

Time for a test. Based on my current understanding, I would say that the statement: "the atmosphere holds the water vapor" is incorrect for the much the same reason that it would be incorrect to say: "the atmosphere holds the argon (or any of the other trace gasses)". In other words, the physical behavior of the water vapor in the atmosphere is independent of all the other atmospheric gasses in the same way that the physical behavior of the argon in the atmosphere is independent of all the other atmospheric gasses. Am I on the right track?

P.S. Thanks for the other examples. I knew that dissolving something in the water reduced its vapor pressure, but I didn't know that smaller droplets had a higher vapor pressure than larger ones. Very interesting!


You made my day, Esther!!!! You have it EXACTLY correct!

Air, even at the earth's surface, is empty space to 1 part in 1000 (by volume). So using the word, "hold," is ludicrous, in my opinion.

Many science teachers (and a few WU readers refusing to see the light) who promote "the holding" concept actively disregard scientific subtlety. They are the same folks who explain a "blue sky" by stating that air molecules scatter (reflect) blue light" (in my early days at WU, I dispelled this notion in a blog...check out my archive).

Alas, there are folks who just can't bear to be wrong, and they keep moving to other lines of defenses that have no validity in the debate. Sad but true.

Thank you so much for allowing me to feel like I taught somebody something. It's difficult for me to feel good about what I try to do here when there are readers who don't even acknowledge great scientists like Dalton who proved the point I'm trying to make here 200 years ago.

I wish universities required a course on the history of science.

Thanks again, EstherD.

Best,

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
28. EstherD
8:11 AM GMT on September 27, 2013
Back in the 60's when I was in high school, we were taught that air holds water vapor the same way that liquid water holds dissolved solids, like salt -- that the two systems were more-or-less analogous. So the idea that "warm air can hold more moisture than cold air" seemed to follow quite logically. More recently, when my daughter's science teachers (and textbooks) talked about atmospheric water vapor, they used similar language. However, a few years ago, on one of the websites that debunk "bad science" (don't recall now which), I ran across an explanation similar to the one I'm reading here. I've been struggling with it on and off ever since.

Time for a test. Based on my current understanding, I would say that the statement: "the atmosphere holds the water vapor" is incorrect for the much the same reason that it would be incorrect to say: "the atmosphere holds the argon (or any of the other trace gasses)". In other words, the physical behavior of the water vapor in the atmosphere is independent of all the other atmospheric gasses in the same way that the physical behavior of the argon in the atmosphere is independent of all the other atmospheric gasses. Am I on the right track?

P.S. Thanks for the other examples. I knew that dissolving something in the water reduced its vapor pressure, but I didn't know that smaller droplets had a higher vapor pressure than larger ones. Very interesting!
Member Since: November 10, 2009 Posts: 2 Comments: 221
27. Levi32
6:22 AM GMT on September 27, 2013
Quoting 24. 24hourprof:


You still don't get the most basic concept here. It's the water temperature that determines the rate of evaporation. Yes, the air plays a role in the water's temperature, but so does solar radiation.

There is one unequivocal principle that you refuse to incorporate into your thinking. The evaporation rate of water at a given temperature in a vacuum is essentially the same as the evaporation rate of water in air. The air is essentially irrelevant. As I have noted, Dalton knew this more than 200 years ago. Please read this reference.

Subtleties make life interesting. So I recommend you think more carefully about this topic.


It's humorous to me that you make such an issue over this terminology. Vapor pressure is the driving process here, so yes, "space" in the presence of liquid water would be the way to teach the science of phase equilibrium. But look, air fills the space in which water vapor exists on Earth! Do you wish us meteorologists and those on TV to start referring to the atmosphere as empty space? It simply isn't so. The atmosphere holds the water vapor. The temperature of dry air, a composite gas outnumbering H2O by about 400 to 1 by mass, largely controls the temperature of H2O in the atmosphere, and thus the vapor pressure equilibrium.

I'll leave this topic now. I never thought it would be beaten into the ground in this manner lol. Thanks for the discussion.
Member Since: November 24, 2005 Posts: 635 Comments: 26654
26. georgevandenberghe
12:46 PM GMT on September 26, 2013
Quoting 20. 24hourprof:


George,

I think you're probably better than I am at convincing folks to change their views. I have failed miserably trying to change the views of television folks. These are mostly good people who are well-intentioned, but the shortcuts they take to make meteorology palatable to viewers often are bad science. I just don't know how to get them to simplify without denigrating sound meteorology.

Any ideas?

Lee


The television problem is a special case. Convincing the weathercaster of the truth is often [yeah not always] straighforward. Finding a way to state it in the same 1.7 seconds it takes to simplify and say "warm air holds more water vapor" is an additional hurdle to overcome. I'll try to come up with an economical correct statement today.
Member Since: February 1, 2012 Posts: 18 Comments: 1828
25. georgevandenberghe
12:36 PM GMT on September 26, 2013
Quoting 22. Levi32:


The word "capacity" is still not wrong to use in my opinion. The air's capacity to hold water vapor is a function of all of the stuff we're talking about (temperature, constituents, etc.). The temperature of the air matters because it controls the temperature of water in the air. In my opinion, using the word "capacity" in relation to moisture content on television is not incorrect science.


Replace "air" with "space" or "volume" and consider the terms in the energy budget that determines the temperature of the vapor and liquid and the chemical properties and shape of the liquid surface, and the concept of a volume's capacity to hold water vapor will then work.
Member Since: February 1, 2012 Posts: 18 Comments: 1828
24. 24hourprof
11:55 AM GMT on September 26, 2013
Quoting 22. Levi32:


The word "capacity" is still not wrong to use in my opinion. The air's capacity to hold water vapor is a function of all of the stuff we're talking about (temperature, constituents, etc.). The temperature of the air matters because it controls the temperature of water in the air. In my opinion, using the word "capacity" in relation to moisture content on television is not incorrect science.


You still don't get the most basic concept here. It's the water temperature that determines the rate of evaporation. Yes, the air plays a role in the water's temperature, but so does solar radiation.

There is one unequivocal principle that you refuse to incorporate into your thinking. The evaporation rate of water at a given temperature in a vacuum is essentially the same as the evaporation rate of water in air. The air is essentially irrelevant. As I have noted, Dalton knew this more than 200 years ago. Please read this reference.

Subtleties make life interesting. So I recommend you think more carefully about this topic.
Member Since: October 24, 2012 Posts: 91 Comments: 803
22. Levi32
11:58 PM GMT on September 25, 2013
Quoting 19. georgevandenberghe:


It's the temperature of the water vapor and the liquid droplets of water (and properties of the CCN and solutes in the liquid) that determine the condensation and evaporation rates. The rest of the air (oxygen, nitrogen, argon, and other stuff including CO2) has nothing to do with it. It could be said that "space" can hold more water vapor at higher temperature but the air is not involved.

As a side point, those pie charts we all saw in elementary school with 78% nitrogen, 21% oxygen and 1% argon, are not correct in a warm humid atmosphere which can be as much as 4% water vapor (75% nitrogen, 20% oxygen in that case)


Dr Fraser covers this well in
http://www.ems.psu.edu/~fraser/Bad/BadClouds.html

My only disagreement is with the difficulty of dislodging incorrect information from someone's mindset. I have NOT found this to be difficult let alone impossible and statements of the difficulty are in my
(not easily verified) opinion, overly pessimistic.


The word "capacity" is still not wrong to use in my opinion. The air's capacity to hold water vapor is a function of all of the stuff we're talking about (temperature, constituents, etc.). The temperature of the air matters because it controls the temperature of water in the air. In my opinion, using the word "capacity" in relation to moisture content on television is not incorrect science.
Member Since: November 24, 2005 Posts: 635 Comments: 26654
21. 24hourprof
10:34 PM GMT on September 25, 2013
Quoting 18. barbamz:
Hey, Lee, thanks for the new blog which gives me more items to chew on. I was still digesting your last blog. To be true, as I've followed WU for some years in a foreign language (and with no previous knowledge in metereology) I somehow got the impression, that "shear" always means a horizontal thing, some winds which would be connected f.e. with the jet streams (and of course, there would be no other way for this sort of shear to effect a storm like Humberto than "blowing away" its upper thunderstorms). So I was puzzled to hear something about "vertical shear", and I had to look it up in the internet.

Of course, the german term "Scherwind" would have meant sorts of (temporary) updraft and downdraft for me too, but I didn't make a deliberate connection to "shear" until now. You see, weatherweise you really would have to start from the scratch with me :)

One question, please. We often use those shear maps on WU:



Are they showing mostly vertical shear or horizontal shear? Or both - and is there an easy way to differentiate, when using this map? (easy: *cough", *cough*, lol)

And is "subsidence" a sort of shear then, too? Oh, so many questions ... Thanks from Germany in advance!



When the topic is tropical cyclones, most of the time forecasters are assessing vertical wind shear (a change in horizontal wind direction and/or horizontal wind speed with altitude...yes, it can be confusing, so hang in there).

Horizontal wind shear plays a role with the Middle African Easterly Jet and the formation of easterly waves.

Having said this, all maps you see in the context of tropical cyclones almost always display vertical wind shear.

As I just stated, vertical wind shear is a change in horizontal wind direction and/or horizontal wind speed with altitude. So subsidence is not a vertical wind shear in any traditional sense of the term.

For the record, horizontal wind shear is a change in horizontal wind speed and / or horizontal wind direction over a horizontal distance.

Although vertical motion (up and down movements of air) often changes speed with altitude, we don't classify this as any type of wind shear.

Does this help?

Lee



Member Since: October 24, 2012 Posts: 91 Comments: 803
20. 24hourprof
10:01 PM GMT on September 25, 2013
Quoting 19. georgevandenberghe:


It's the temperature of the water vapor and the liquid droplets of water (and properties of the CCN and solutes in the liquid) that determine the condensation and evaporation rates. The rest of the air (oxygen, nitrogen, argon, and other stuff including CO2) has nothing to do with it. It could be said that "space" can hold more water vapor at higher temperature but the air is not involved.

As a side point, those pie charts we all saw in elementary school with 78% nitrogen, 21% oxygen and 1% argon, are not correct in a warm humid atmosphere which can be as much as 4% water vapor (75% nitrogen, 20% oxygen in that case)


Dr Fraser covers this well in
http://www.ems.psu.edu/~fraser/Bad/BadClouds.html

My only disagreement is with the difficulty of dislodging incorrect information from someone's mindset. I have NOT found this to be difficult let alone impossible and statements of the difficulty are in my
(not easily verified) opinion, overly pessimistic.


George,

I think you're probably better than I am at convincing folks to change their views. I have failed miserably trying to change the views of television folks. These are mostly good people who are well-intentioned, but the shortcuts they take to make meteorology palatable to viewers often are bad science. I just don't know how to get them to simplify without denigrating sound meteorology.

Any ideas?

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
19. georgevandenberghe
6:29 PM GMT on September 25, 2013
Quoting 17. Levi32:
If you take a body of very warm water at vapor pressure equilibrium with the air above it, you have X amount of water vapor in the air. If you now cool the air below the dewpoint, but keep the water temperature the same, what happens? Water vapor in the air condensates into liquid. Why? Because the air is now supersaturated with respect to the atmospheric nuclei/water droplets. The water droplets are going to be at about the same temperature as the air around them. The air is not supersaturated with respect to the water surface, but as long as there is a condensation medium in the air itself, water vapor will condensate in the air until equilibrium is again established with what is now a lower saturation vapor pressure.

This means the "capacity" of the air to support water molecules in the vapor phase relative to its surroundings and constituents has been reduced.

You cite the example of removing condensation nuclei (and condensation surfaces), allowing a greater degree of supersaturation. This does not counteract what I said. The "capacity" of the air to hold water in the vapor phase is a function of multiple variables. Nuclei concentration is one of them. Holding that as a non-zero constant (which for all intents and purposes it usually is), you end up with air temperature as the largest independent variable. "Capacity" is simply a term to describe how much water vapor can be suspended in the atmosphere, given the vapor pressure equilibrium which is determined by several parameters.

The bottom line is that there is science behind the word "capacity." It is just a word used to summarize the effective behavior of the system.


It's the temperature of the water vapor and the liquid droplets of water (and properties of the CCN and solutes in the liquid) that determine the condensation and evaporation rates. The rest of the air (oxygen, nitrogen, argon, and other stuff including CO2) has nothing to do with it. It could be said that "space" can hold more water vapor at higher temperature but the air is not involved.

As a side point, those pie charts we all saw in elementary school with 78% nitrogen, 21% oxygen and 1% argon, are not correct in a warm humid atmosphere which can be as much as 4% water vapor (75% nitrogen, 20% oxygen in that case)


Dr Fraser covers this well in
http://www.ems.psu.edu/~fraser/Bad/BadClouds.html

My only disagreement is with the difficulty of dislodging incorrect information from someone's mindset. I have NOT found this to be difficult let alone impossible and statements of the difficulty are in my
(not easily verified) opinion, overly pessimistic.
Member Since: February 1, 2012 Posts: 18 Comments: 1828
18. barbamz
6:08 PM GMT on September 25, 2013
Hey, Lee, thanks for the new blog which gives me more items to chew on. I was still digesting your last blog. To be true, as I've followed WU for some years in a foreign language (and with no previous knowledge in metereology) I somehow got the impression, that "shear" always means a horizontal thing, some winds which would be connected f.e. with the jet streams (and of course, there would be no other way for this sort of shear to effect a storm like Humberto than "blowing away" its upper thunderstorms). So I was puzzled to hear something about "vertical shear", and I had to look it up in the internet.

Of course, the german term "Scherwind" would have meant sorts of (temporary) updraft and downdraft for me too, but I didn't make a deliberate connection to "shear" until now. You see, weatherweise you really would have to start from the scratch with me :)

One question, please. We often use those shear maps on WU:



Are they showing mostly vertical shear or horizontal shear? Or both - and is there an easy way to differentiate, when using this map? (easy: *cough", *cough*, lol)

And is "subsidence" a sort of shear then, too? Oh, so many questions ... Thanks from Germany in advance!

Member Since: October 25, 2008 Posts: 55 Comments: 6030
17. Levi32
6:01 PM GMT on September 25, 2013
If you take a body of very warm water at vapor pressure equilibrium with the air above it, you have X amount of water vapor in the air. If you now cool the air below the dewpoint, but keep the water temperature the same, what happens? Water vapor in the air condensates into liquid. Why? Because the air is now supersaturated with respect to the atmospheric nuclei/water droplets. The water droplets are going to be at about the same temperature as the air around them. The air is not supersaturated with respect to the water surface, but as long as there is a condensation medium in the air itself, water vapor will condensate in the air until equilibrium is again established with what is now a lower saturation vapor pressure.

This means the "capacity" of the air to support water molecules in the vapor phase relative to its surroundings and constituents has been reduced.

You cite the example of removing condensation nuclei (and condensation surfaces), allowing a greater degree of supersaturation. This does not counteract what I said. The "capacity" of the air to hold water in the vapor phase is a function of multiple variables. Nuclei concentration is one of them. Holding that as a non-zero constant (which for all intents and purposes it usually is), you end up with air temperature as the largest independent variable. "Capacity" is simply a term to describe how much water vapor can be suspended in the atmosphere, given the vapor pressure equilibrium which is determined by several parameters.

The bottom line is that there is science behind the word "capacity." It is just a word used to summarize the effective behavior of the system.
Member Since: November 24, 2005 Posts: 635 Comments: 26654
16. 24hourprof
4:58 PM GMT on September 25, 2013
Quoting 14. calkevin77:
Hi Mr. Grenci,
Great read. I'm curious but hypothetically speaking, had Humberto been a well established cyclone encountering the vertical shear in the timeframes you stated, would we possibly have seen an eye wall replacement cycle rather than a decoupling from the center of circulation? I'm just wondering this thinking about hot towers around a nicely established inner core and their ability to be retained and recycled back into a system overtaking the shear.


Many thanks for your kind words and your question. It's a good one.

My gut reaction is to say "no" because the inner band of deep convection that was spiraling cyclonically around the core in the ERC on the evening of the 12th and the first half of the 13th (MIMIC loop) would have also weakened, assuming relatively strong vertical wind shear, of course.

Best,

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
15. 24hourprof
4:53 PM GMT on September 25, 2013
Quoting 9. beell:
Good morning, Lee.

"...The bottom line is that the air has nothing to do with the amount of water vapor in the air. It is determined by the temperature of water..."

A dumb question;

What governs the temperature of the water?



P.S. There are no "dumb questions" when it comes to discussing science. To the contrary, yours was a good question.
Member Since: October 24, 2012 Posts: 91 Comments: 803
14. calkevin77
4:42 PM GMT on September 25, 2013
Hi Mr. Grenci,
Great read. I'm curious but hypothetically speaking, had Humberto been a well established cyclone encountering the vertical shear in the timeframes you stated, would we possibly have seen an eye wall replacement cycle rather than a decoupling from the center of circulation? I'm just wondering this thinking about hot towers around a nicely established inner core and their ability to be retained and recycled back into a system overtaking the shear.
Member Since: June 9, 2006 Posts: 0 Comments: 864
13. georgevandenberghe
4:29 PM GMT on September 25, 2013
Quoting 12. 24hourprof:


George,

It was slightly before my time, but there were stories around the department of Fraser and Bohren running up and down the hallways of Walker doing Doppler experiments.


It sounds in character for them but I do not remember it in the period from November 1978 to May 1981 when I was there.
Member Since: February 1, 2012 Posts: 18 Comments: 1828
12. 24hourprof
4:24 PM GMT on September 25, 2013
Quoting 11. georgevandenberghe:


The actual physical quantity determined by the temperature is the kinetic energy of the water molecules which is determined by the quoted quantities.

I also studied under Alstair Fraser decades ago and knew Craig Bohren but never had a class with him. Alstair Fraser really drove these points home to his students and colleagues.


George,

It was slightly before my time, but there were stories around the department of Fraser and Bohren running up and down the hallways of Walker doing Doppler experiments. Is it true? Did you ever see them?

I, too, had this material drilled into me by Alistair and Craig.

But let's face it, "warm air holding more water vapor" is a weed planted in the garden of meteorology that we'll never exterminate.

Like many aspects of our society, there are just too many people who acquiesce and say "it's good enough."

At any rate, thank you for your scientific insights and support. I know I have at least one ally at Weather Underground.

Best,

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
11. georgevandenberghe
12:13 PM GMT on September 25, 2013
Quoting 10. 24hourprof:


Energy budget....incoming radiation versus outgoing radiation...conduction and convection also play a role.

The wind and mixing in larger bodies of water also plays a role.

Evaporation, etc. too.


The actual physical quantity determined by the temperature is the kinetic energy of the water molecules which is determined by the quoted quantities.

I also studied under Alstair Fraser decades ago and knew Craig Bohren but never had a class with him. Alstair Fraser really drove these points home to his students and colleagues.
Member Since: February 1, 2012 Posts: 18 Comments: 1828
10. 24hourprof
11:31 AM GMT on September 25, 2013
Quoting 9. beell:
Good morning, Lee.

"...The bottom line is that the air has nothing to do with the amount of water vapor in the air. It is determined by the temperature of water..."

A dumb question;

What governs the temperature of the water?



Energy budget....incoming radiation versus outgoing radiation...conduction and convection also play a role.

The wind and mixing in larger bodies of water also plays a role.

Evaporation, etc. too.
Member Since: October 24, 2012 Posts: 91 Comments: 803
9. beell
11:26 AM GMT on September 25, 2013
Good morning, Lee.

"...The bottom line is that the air has nothing to do with the amount of water vapor in the air. It is determined by the temperature of water..."

A dumb question;

What governs the temperature of the water?

Member Since: September 11, 2007 Posts: 143 Comments: 16729
8. 24hourprof
10:56 AM GMT on September 25, 2013
Quoting 5. Levi32:
The statement "warm air holds more water vapor" is mid-way between "warm air always contains more water vapor" and "warm air has a higher capacity for water vapor."

By itself, the first statement doesn't necessarily imply either of the others. The last statement is the scientifically correct one. I don't think most TV meteorologists actually mean the second statement when they say the first one.


As I noted last night, the "last" statement is not correct.

First, The simplest, scientifically sound statement is that the evaporation rate of water (rate at which water molecules escape their liquid bonds and enter the atmosphere as a gas) increases with increasing temperature. That's what I told my students. The warmth of the air has nothing fundamental to do with the evaporation rate. The evaporation rate is determined by the temperature of the water. Period. Moreover, the presence of the air is almost completely irrelevant to the evaporation rate, a fact which was known to Dalton more than 200 years ago.

Here's a counterexample to what you wrote (I'm expanding on one of George's ideas). Consider the saturation vapor pressure of pure water and the saturation vapor pressure of an aqueous solution, both at the same temperature. The saturation vapor pressure over pure water is greater that the saturation vapor pressure over the aqueous solution. In other words, there's more water vapor over pure water than there is over the aqueous solution. Following your argument, we would have to say that the same air has a "higher capacity for water vapor" when the air just happens to be above pure water instead of the solution. That's just nonsense.

Here's another counterexample. Let's get rid of the impurities in the air by filtering out all particles, including condensation nuclei. Somehow getting rid of nuclei makes the air open up its arms and take in much more water vapor. We can easily prepare in the laboratory clean air that can "hold" a staggering amount of water vapor. And the temperature is irrelevant!

Can you explain to me why you think "warm air holds more water vapor?" Please don't invoke the old and incorrect argument that heated air expands and thereby creates "more room." This is the explanation often given and it is 100% rubbish. Air, even at the earth's surface, is empty space to 1 part in 1000 (by volume).

I hope I've changed your mind, but I'm not "holding" my breath. The television broadcasters who responded to my BAMS essay believe it's sound science and good communication (at least that's what they wrote).

The bottom line is that the air has nothing to do with the amount of water vapor in the air. It is largely determined by the temperature of water.

The defense rests.

Best,

Lee



Member Since: October 24, 2012 Posts: 91 Comments: 803
7. 24hourprof
11:37 PM GMT on September 24, 2013
Quoting 6. georgevandenberghe:


The air has no capacity for water vapor. Water vapor behaves independently of the rest of the air. The saturation vapor pressure is higher at higher temperature. Until the partial pressure of water vapor reaches the saturation vapor pressure at the temperature of a surface of liquid water, more water will evaporate than condenses out into the water. At saturation the rates are equal. At higher than saturation partial presssures, the net evapopration is negative and water vapor condenses out. The temperature of the other parts of the air has nothing to do with it, it is the partial pressure and temperature of the water vapor and the temperature of the water that determine net condensation and evaporation rates. Presence of solutes in the water (e.g salt or ammonia) and the curvature of the water surface also affect these rates and the balance.




George,

Right on! I have some ideas I'll express tomorrow (I gotta run right now).

Until then, all I'll offer are thoughts from Craig Bohren who believes that saving appearances is a common ploy used by people who cannot bear to be wrong. And so they change a word here or there or go through some other form of verbal gymnastics in an attempt to prove that they're right. Unfortunately, "warm air having a high capacity to hold water" is such a ploy.

So Levi32, while I appreciate your interest, you're missing the point completely, and I will try my best to convey the correct science tomorrow (George's comment is impeccable, by the way).

Real scientists, in the words of Craig Bohren, are confident about what they know, humble about what they don't know, and always ready to change their views when they see through explanations that "don't hold water" (excuse my pun). This is Craig's (and my) measure of a true scientist. Levi32, I'm hoping you're a true scientist and are prepared to change your views tomorrow. See you then.

Best,

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
6. georgevandenberghe
9:44 PM GMT on September 24, 2013
Quoting 5. Levi32:
The statement "warm air holds more water vapor" is mid-way between "warm air always contains more water vapor" and "warm air has a higher capacity for water vapor."

By itself, the first statement doesn't necessarily imply either of the others. The last statement is the scientifically correct one. I don't think most TV meteorologists actually mean the second statement when they say the first one.


The air has no capacity for water vapor. Water vapor behaves independently of the rest of the air. The saturation vapor pressure is higher at higher temperature. Until the partial pressure of water vapor reaches the saturation vapor pressure at the temperature of a surface of liquid water, more water will evaporate than condenses out into the water. At saturation the rates are equal. At higher than saturation partial presssures, the net evapopration is negative and water vapor condenses out. The temperature of the other parts of the air has nothing to do with it, it is the partial pressure and temperature of the water vapor and the temperature of the water that determine net condensation and evaporation rates. Presence of solutes in the water (e.g salt or ammonia) and the curvature of the water surface also affect these rates and the balance.


Member Since: February 1, 2012 Posts: 18 Comments: 1828
5. Levi32
7:37 PM GMT on September 24, 2013
The statement "warm air holds more water vapor" is mid-way between "warm air always contains more water vapor" and "warm air has a higher capacity for water vapor."

By itself, the first statement doesn't necessarily imply either of the others. The last statement is the scientifically correct one. I don't think most TV meteorologists actually mean the second statement when they say the first one.
Member Since: November 24, 2005 Posts: 635 Comments: 26654
4. 24hourprof
6:23 PM GMT on September 24, 2013
Quoting 3. georgevandenberghe:

I'd also be interested in your thoughts about "warm air holds more water vapor" or "Cold air holds less water vapor." As you know, Alistair Fraser and Craig Bohren opposed these expressions (which, by the way, the AMS seems to endorse).




"Opposing" implies two sides and that the point opposed has any merit. That isn't
the case here. You, Alistair and Craig are correct, the opposition isn't.

THe "simplification" argument can be made that the wording describes a relationship that is correlated with what happens. But it's bad science.

I'll try to expound later. No time now. What is the AMS defence of endorsing
this inaccurate science?


George,

I'm not sure what BAMS was thinking when they published the letter from the television folks. BAMS came to me and asked me to write the essay (no honorarium involved...I wrote the essay in an honest attempt to improve weather communication).

I still get a knot in my stomach when I read that letter. In case you didn't notice, they never once addressed the issue of deterministic versus probabilistic medium-range forecasts, which was the main theme of my essay.
Member Since: October 24, 2012 Posts: 91 Comments: 803
3. georgevandenberghe
6:08 PM GMT on September 24, 2013

I'd also be interested in your thoughts about "warm air holds more water vapor" or "Cold air holds less water vapor." As you know, Alistair Fraser and Craig Bohren opposed these expressions (which, by the way, the AMS seems to endorse).




"Opposing" implies two sides and that the point opposed has any merit. That isn't
the case here. You, Alistair and Craig are correct, the opposition isn't.

THe "simplification" argument can be made that the wording describes a relationship that is correlated with what happens. But it's bad science.

I'll try to expound later. No time now. What is the AMS defence of endorsing
this inaccurate science?
Member Since: February 1, 2012 Posts: 18 Comments: 1828
2. 24hourprof
5:52 PM GMT on September 24, 2013
Quoting 1. georgevandenberghe:
Concerning your 2005 essay in BAMS, it would help a great deal, at not much airtime cost, if the tombstones included uncertainty information in the temperature numbers, for example, either the standard deviation for that particular ensemble cycle, or, failing that, just one from climatology. Precipitation, particularly heavy precipitation beyond a few days is hopeless but probability information of headline events is reasonable to present that far out (Is a 2' snowfall or a 5" rainfall within the 10 day ensemble envelope?.. if so make clear it's probably low probability but start
getting ready) [ By the way you covered the within the envelope possibility and advisement in the last paragraph on page 1538 of the BAMS essay.. I missed that in the first read.]

Otherwise I often tell people 10day deterministic forecasts have the
(yes significant) entertainment value of a fortuneteller.

I'll discuss my take on the fortunate 1993 March snowstorm forecast
in another comment.



LOL!!! about the fortuneteller.

I'm sure you heard about the 30-day tombstone forecasts now being offered to the public.

Agree with you completely about the idea of introducing probabilistic forecasts into any presentation.

I'd also be interested in your thoughts about "warm air holds more water vapor" or "Cold air holds less water vapor." As you know, Alistair Fraser and Craig Bohren opposed these expressions (which, by the way, the AMS seems to endorse).

I asked Craig to respond to the remark of a well-known scientist (I'm omitting his name as a courtesy) who says it's okay to say "warm air holds more water vapor."

Here is Craig's response (he said I could pass it on). I happen to agree with him.

"Ask him (the scientist) why warm air holds more water vapor? Is it because the air expands, thereby creating more room? This is the explanation often given and it is 100% rubbish. Air, even at the surface, is empty space to 1 part is 1000 (by volume).

"More water vapor can evaporate into warm air" is better but still misses the essential point. In particular, what is special about "warm air?" The evaporation rate (by which I do not mean the net evaporation rate, the difference between evaporation (outgoing) and condensation (incoming) increases with the temperature OF THE WATER. This is a physically correct statement and has a fairly simple molecular interpretation.

Also ask him (the scientist) the following. Suppose I have two large sealed containers. One container has pure water in it, the other the same amount of a salt solution. The temperature is the same in both. Air is in both. What is amazing that the air above the salt water "holds" less water vapor that the air above the pure water. Now how on earth does the air know the difference? Just to forestall any irrelevant criticism I note that the salt does not evaporate, at least to any easily measurable degree.

Now consider something else. Suppose I have two sealed containers of air, both at the same temperature. In one container I suspend a very small droplet. The vapor density is such that this droplet neither shrinks nor grows. In the other container I suspend a much larger droplet. Again, I adjust the vapor number density so that this droplet neither shrinks nor grows. And again we discover something amazing (or not depending on how your brain works). Air around the small droplet "holds" more water vapor than the air around the large droplet. How is this possible if you think air as a hotel the number of rooms in which increases with temperature?

And now the final argument. Note that the statement "warm air holds more water vapor" is not qualified at all. Hence we have no choice but to believe that if we have two samples of moist air at different temperatures, the air with the higher temperature ALWAYS has a higher water vapor concentration. No exceptions. This is idiotic. All I have to do is search through climate records and find temperature and dew points in some locations and at some times such that the higher temperature air has a lower concentration of water vapor than at some other location where the temperature is lower. With 5 minutes of effort I found an example.

I can write the vapor density as

n = r * es/kT

r is the relative humidity, es is the saturation vapor pressure at absolute temperature T, k is Boltzmann's constant (which we can ignore)

The relative humidity and temperature in Ridgecrest, CA are 33% and 18 C. In State College, the corresponding values are 58% and 12 C. Thus I calculate that the colder air in State College is miraculously "holding" about 20% more water vapor than the air in Ridgecrest. This is impossible according to the scientist.

I don't know how to make the point more forcefully, but I know that no arguments will ever move people like this scientist."

George...I feel the same way about media folks who continue to practice questionable science. In a future blog, I plan to address "clouds act like a blanket at night," which is another media favorite. It's scientifically inaccurate, but it passes for "science" on television.

Best,

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
1. georgevandenberghe
4:47 PM GMT on September 24, 2013
Concerning your 2005 essay in BAMS, it would help a great deal, at not much airtime cost, if the tombstones included uncertainty information in the temperature numbers, for example, either the standard deviation for that particular ensemble cycle, or, failing that, just one from climatology. Precipitation, particularly heavy precipitation beyond a few days is hopeless but probability information of headline events is reasonable to present that far out (Is a 2' snowfall or a 5" rainfall within the 10 day ensemble envelope?.. if so make clear it's probably low probability but start
getting ready) [ By the way you covered the within the envelope possibility and advisement in the last paragraph on page 1538 of the BAMS essay.. I missed that in the first read.]

Otherwise I often tell people 10day deterministic forecasts have the
(yes significant) entertainment value of a fortuneteller.

I'll discuss my take on the fortunate 1993 March snowstorm forecast
in another comment.

Member Since: February 1, 2012 Posts: 18 Comments: 1828

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Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.

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