Tropical Easterly Waves from Africa: The Media's Loose Use of "Dry" and "Moist"

By: 24hourprof , 3:36 PM GMT on September 04, 2013

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With the conversation still focusing on the lack of Atlantic hurricanes this season, I have noted the media's somewhat loose language describing "dry" and "moist" air and their impact on tropical cyclogenesis (tropical development). First, we're not talking about dry or moist air near the earth's surface, which viewers don't realize because the media rarely offers any quantification. Second, I note that relative humidity is the more correct scientific term to use in the context of tropical cyclogenesis and the presence or the absence of the Saharan Air Layer (see 18Z SAL analysis on September 3 below), which I wrote about in my last blog. By the way, tropical cyclogenesis refers to the development of (or the intensification of) a tropical cyclone.


The SAL analysis at 18Z on September 3, 2013. The presence of the SAL near a tropical wave coming off Africa at the time dramatically reduced any possibility of development in the following 48 hours. Courtesy of CIMSS.

For starters, I point out that one of the criteria for the development of a tropical cyclone is high mid-level relative humidity (of course, there are several other ingredients in the recipe for a tropical cyclone).

When thunderstorms entrain air whose relative humidity is low, evaporational cooling is enhanced aloft, which, in turn, promotes stronger downdrafts. These stronger downdrafts then penetrate into and stabilize the boundary layer, discouraging further development (see the complete explanation in my last blog).

On the other hand, high relative humidity in the middle troposphere limits evaporational cooling and thereby avoids the issue of thunderstorms developing stronger downdrafts (and the associated tropical wave eventually fizzles out).


An idealized closed system with a layer of water along the bottom. The gray "spheres" indicate molecules of water vapor. At this time, the system is in an equilibrium state (saturation), with the rate of evaporation equaling the rate of condensation. The relative humidity is 100%. Courtesy of A World of Weather: Fundamentals of Meteorology.

In general, net evaporation rates increase with decreasing relative humidity. That's because more water molecules can evaporate into the air before the evaporation rate equals the condensation rate. To understand my point, check out the image above, which shows an idealized closed system with a layer of water at a fixed temperature along the bottom of the box. At this point, the air inside the closed box is saturated...the evaporation rate equals the condensation rate, and the relative humidity is 100%. Now imagine that I remove half the water vapor molecules (the grayish "spheres" in the air inside the box). Upon removal, the evaporation rate exceeds the condensation rate (in other words, there's net evaporation) and this state will continue until the system gradually reaches equilibrium (saturation).


And so it is when the middle troposphere over the tropical Atlantic has low relative humidity. In these conditions, thunderstorms associated with tropical waves (disturbances) from Africa develop relatively strong downdrafts when they entrain mid-level air with low relative humidity. This explanation is one of the reasons why the Saharan Air Layer has such a negative impact on tropical waves and tropical cyclones (vertical wind shear associated with the SAL is another reason...revisit my last blog).


The 00Z upper-air station models (500 mb, more specifically) on September 4, 2013 (8 P.M. EDT on September 3). The number in the upper left of the each station (in red) is the temperature in degrees Celsius, and the number below it is the dew-point depression, in degrees Celsius. Larger image. Courtesy of NCEP.

The media's recent use of the words "dry" and "moist" as they relate to tropical waves coming off the coast of Africa are so vague that they are, in my opinion, essentially meaningless. Indeed, such vapid usage does not indicate the altitude of the air in question. These rather "loose" references annoy me because, in the middle troposphere, distinguishing between "dry" and "moist" air boils down to an exercise in futility. To see what I mean, check out (above; larger image) the upper-air station models (at 500 mb) over the western Atlantic at 00Z last evening (8 P.M. EDT on September 3). The red number is the 500-mb temperature in degrees Celsius. The green number below it is the dew-point depression (also expressed in degrees Celsius). Over the western Caribbean, where the National Hurricane Center indicated this morning that there was a 30% chance of development into a tropical cyclone within 48 hours (see tropical weather outlook right here), note that the 500-mb dew points were all lower than 0 degrees Celsius (keep in mind that the dew point equals the air temperature minus the dew-point depression). I'm sorry, but dew points lower than 0 degrees Celsius hardly could qualify as "moist" in my book, especially in the context of tropical systems.

This is the same kind of rubbish I frequently hear during winter, when the media states, unequivocally, that the subtropical jet stream carries "lots of tropical moisture" into the Western states. Really? There's a lot of moisture at 200 mb??? Really?? The standard pressure altitude of the subtropical jet stream is 200 mb, which lies at roughly 12000 meters (12 kilometers). Look at the temperature and dew-point depressions on the display of 200-mb station models over the western Atlantic last evening (00Z on September 4). All the dew points were lower than minus 60 degrees Celsius. Hardly any water vapor at all. Yet the media continues to describe the subtropical jet stream as carrying a lot of moisture. Annoying...Lee gives a deep sigh.

Hopefully, you've gained an appreciation for why the media's usage of the words, "dry" and "moist," in the context of the development of tropical waves from Africa, is, at the very least, too vague, and, at its worst, constitutes questionable science.


The tropical outlook issued by the National Hurricane Center around 18Z on September 3, 2013. At the time, the presence of mid-tropospheric air with low relative humidity worked against development in the following 48 hours. Courtesy of NHC.

My ranting aside, NHC on its tropical weather outlook yesterday afternoon (see above; larger 18Z infrared satellite image from Meteosat), noted that the wave over the Cape Verde Islands had little chance of developing in the next 48 hours. I looked at the GFS model analysis of a skew-T near the Cape Verde Islands at 18Z yesterday (see below; review the basics of skew-T diagrams). Note the relatively wide separation of the temperature (red) and dew-point (green) soundings in the middle troposphere (700 mb to 400 mb, with the single exception at 500 mb). This indicates relative humidity sufficiently low that, if entrained into the updrafts of thunderstorms, it would cause enough evaporational cooling to produce stronger downdrafts that would be counterproductive to tropical cyclogenesis (i.e., development). No wonder NHC gave it a 0% chance of development.


The 18Z GFS model sounding in the vicinity of the Cape Verde Islands on September 3, 2013. Courtesy of NOAA.

So I guess I'm admitting that using "dry" and "moist" air in the context of tropical waves is too vague and sidesteps the correct science. And, yes, I slip up sometimes and do the same thing, but I always try to correct my mistakes.

Here endeth the lesson.

Lee

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14. WunderAlertBot (Admin)
1:45 PM GMT on September 06, 2013
24hourprof has created a new entry.
13. 24hourprof
11:33 AM GMT on September 06, 2013
Quoting 9. Astrometeor:
When TWC began to do "shows on weather" and came out with airplane flying, reef wranglers, and other shows (at least the Coast Guard and Hurricane Hunters are actually connected), there were several of us here that were disappointed in TWC. We were hoping more along the lines of exploring weather phenomenon, charts and graphs, and how meteorologists do forecasts. Whole shows on that, not the 2-minute clips they do currently.

I have a question. In several of the discussions for the invest that became Gabrielle, the NHC cited "mid-level dry air". Where did/do they get that from? The morning soundings from San Juan showed relative humidity values in the 60-70% range, not terribly moist, but not dry. And then the Water Vapor images seemed to show plenty of moisture in the Caribbean, at all three (lower, mid, upper) levels.


Okay, I'll take a shot at your question after finding the reference you referred to.

Rather recurrent southerly winds and anomalously dry weather over eastern Brazil have worked in tandem to regularly inject air with low relative humidity over the tropical Atlantic Main Development Region (MDR). That means that thunderstorms entraining this air can develop strong downdrafts and weaken the parent easterly wave (see my blog on the Saharan Air Layer). Speaking of SAL, it also has contributed to maintaining low relative humidity over the MDR, as the current SAL product from CIMSS clearly shows (Caution: This movie has a relatively short shelf life).

How's that?

Member Since: October 24, 2012 Posts: 91 Comments: 803
12. 24hourprof
10:20 AM GMT on September 06, 2013
Quoting 9. Astrometeor:
When TWC began to do "shows on weather" and came out with airplane flying, reef wranglers, and other shows (at least the Coast Guard and Hurricane Hunters are actually connected), there were several of us here that were disappointed in TWC. We were hoping more along the lines of exploring weather phenomenon, charts and graphs, and how meteorologists do forecasts. Whole shows on that, not the 2-minute clips they do currently.

I have a question. In several of the discussions for the invest that became Gabrielle, the NHC cited "mid-level dry air". Where did/do they get that from? The morning soundings from San Juan showed relative humidity values in the 60-70% range, not terribly moist, but not dry. And then the Water Vapor images seemed to show plenty of moisture in the Caribbean, at all three (lower, mid, upper) levels.


Of course, I believe that NHC should have said mid-level air with low relative humidity, but that's just my opinion. "Dry" and "moist" at 500 mb (mid-level) are not only vague, but, in my view, meaningless at this level. Indeed, dew points are almost always lower than 0 degrees Celsius at 500 mb. What's more important is 500-mb relative humidity. Oh I know, I'm sure that's what they (NHC) meant, but it's just another example of loose language, in my view (again, I slip up sometimes myself, so I'm no saint).

What date, time, and which NHC discussion were you talking about? Remember, you need to separate the environment around the disturbance because high clouds contaminate water vapor imagery. You need to look at the air around the storm. Does it have relatively low or high relative humidity?

Also, remember, water vapor imagery does not sense water vapor in the lower troposphere, especially over the tropics.

Best,

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
11. 24hourprof
10:13 AM GMT on September 06, 2013
Quoting 10. Astrometeor:
Here ya go Lee, I wrote a blog post all the way back in March on TWC's choice of programming:

TWC and shows


Thanks. I enjoyed it.

I have been very frustrated during severe weather here in central Pa. when I want to see Dr. Forbes and there's a rerun of people mining for jewels on mountains.

All I'm saying is that these shows sometimes air when there's important weather happening, and so it's a bit frustrating.

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
10. Astrometeor
4:26 AM GMT on September 06, 2013
Here ya go Lee, I wrote a blog post all the way back in March on TWC's choice of programming:

TWC and shows
Member Since: July 2, 2012 Posts: 101 Comments: 10331
9. Astrometeor
4:13 AM GMT on September 06, 2013
When TWC began to do "shows on weather" and came out with airplane flying, reef wranglers, and other shows (at least the Coast Guard and Hurricane Hunters are actually connected), there were several of us here that were disappointed in TWC. We were hoping more along the lines of exploring weather phenomenon, charts and graphs, and how meteorologists do forecasts. Whole shows on that, not the 2-minute clips they do currently.

I have a question. In several of the discussions for the invest that became Gabrielle, the NHC cited "mid-level dry air". Where did/do they get that from? The morning soundings from San Juan showed relative humidity values in the 60-70% range, not terribly moist, but not dry. And then the Water Vapor images seemed to show plenty of moisture in the Caribbean, at all three (lower, mid, upper) levels.
Member Since: July 2, 2012 Posts: 101 Comments: 10331
8. 24hourprof
7:15 PM GMT on September 05, 2013
Quoting 6. georgevandenberghe:
I think you're looking a little higher than I do looking at moisture. 500MB is too high and cold, always, to have much absolute humidity. It can be close to saturated or bone dry. I'm not sure whether entertainment at this level is more significant than entertainment at 700mb suppressing organized convective clusters. Note this does not mean it isn't well known, it means I don't know it. By the way, that sounding jumps out as dry way below 500mb, actually to below 850mb. Since I don't forecast for that region though I may well be off base or wrong; feel free to correct.

Except over elevated plateaus and mountains, 500mb temperatures are almost always below 0C.

I've also looked at massive cirrus feeds from the subtropical jet and
taken pause at people who call this massive moisture transport esp.
when there is a high mountain range in the path such as the Rockies.



Otherwise whether one is evaluating the "moist" status of 0C air
depends on why. It's important at the surface when determining radiative flux, evaporative cooling, or fog, frost or dew probabilities
(and in horticulture, dessication risk when plants can't transport water from frozen soil.. a splinter point agreed)

Calling it moist for "moisture transport" purposes is.. yeah misleading.
or not relevant.


Absolutely agree, George. Thanks.
Member Since: October 24, 2012 Posts: 91 Comments: 803
7. 24hourprof
7:13 PM GMT on September 05, 2013
Quoting 5. georgevandenberghe:



Alistair Fraser also stated that once an incorrect idea or concept was implanted in someone's mind it was virtually impossible to dislodge it. However I have just not found this to be true and my experience has been that adults can be educated to lose the misconceptions they learned in their youth. The point remains though, when teaching it's far better to get it right and complete the first time. Since I'm not a teacher perhaps I should not pontificate too much though.

I suspect that if a broadcaster is simplifying his explanation for his audience (reasonable) and he does it repetitively, too many times for a few years, he forgets the more complex real situation and only remembers the simplified one and that's just human nature.


That's a reasonable position, George. And I agree that adults can be educated. That begs the question...if you're going to try to do some science on TV, why not do it right? Why resort to a simplification that cuts corners and doesn't ring exactly true?
Member Since: October 24, 2012 Posts: 91 Comments: 803
6. georgevandenberghe
6:56 PM GMT on September 05, 2013
I think you're looking a little higher than I do looking at moisture. 500MB is too high and cold, always, to have much absolute humidity. It can be close to saturated or bone dry. I'm not sure whether entertainment at this level is more significant than entertainment at 700mb suppressing organized convective clusters. Note this does not mean it isn't well known, it means I don't know it. By the way, that sounding jumps out as dry way below 500mb, actually to below 850mb. Since I don't forecast for that region though I may well be off base or wrong; feel free to correct.

Except over elevated plateaus and mountains, 500mb temperatures are almost always below 0C.

I've also looked at massive cirrus feeds from the subtropical jet and
taken pause at people who call this massive moisture transport esp.
when there is a high mountain range in the path such as the Rockies.



Otherwise whether one is evaluating the "moist" status of 0C air
depends on why. It's important at the surface when determining radiative flux, evaporative cooling, or fog, frost or dew probabilities
(and in horticulture, dessication risk when plants can't transport water from frozen soil.. a splinter point agreed)

Calling it moist for "moisture transport" purposes is.. yeah misleading.
or not relevant.
Member Since: February 1, 2012 Posts: 18 Comments: 1826
5. georgevandenberghe
2:43 PM GMT on September 05, 2013
Quoting 4. 24hourprof:


Boy that's a great question. There are some on-air folks who don't have degrees in meteorology. I also believe that "weeds" get planted in the garden of learning by teachers / instructors who have misunderstandings of scientific concepts. Once planted in the minds of students, they stay there forever. That's why I always took teaching so seriously. I was a gardener, and I tried my very best to keep the weeds out of the garden.

People who are self-confident don't mind being wrong. Indeed, they are pleased when their errors are pointed out to them lest they continue to propagate them. Most of us are wrong some of the time. I know I am. Learning science (or anything) is an iterative process. Hardly a day goes by that I don't change my mind about something I thought I understood. But some TV weathercasters lack the self-confidence (not to mention the firm scientific training) to correct their errors, to constantly be strive for clearer, more correct, less misleading explanations.

Instead, I believe some talk down to their audience, coming up with slick analogies or explanations they think are palatable for their audience. In the process, they take a short-cut and plant weeds.

Don't get me wrong. TV folks provide important information to their viewing audience. They can save lives. But their science sometimes takes a backseat to the truth.

I'm probably coming off like a know-it-all. I'm not. I make mistakes all the time. But what keeps me going is that never-ending thrill of life-long learning and the striving to get things right.

Did this help? I hope I didn't offend you or any of my readers. It was not my intention. I'm just trying to convey my opinion.

Lee



Alistair Fraser also stated that once an incorrect idea or concept was implanted in someone's mind it was virtually impossible to dislodge it. However I have just not found this to be true and my experience has been that adults can be educated to lose the misconceptions they learned in their youth. The point remains though, when teaching it's far better to get it right and complete the first time. Since I'm not a teacher perhaps I should not pontificate too much though.

I suspect that if a broadcaster is simplifying his explanation for his audience (reasonable) and he does it repetitively, too many times for a few years, he forgets the more complex real situation and only remembers the simplified one and that's just human nature.
Member Since: February 1, 2012 Posts: 18 Comments: 1826
4. 24hourprof
9:17 PM GMT on September 04, 2013
Quoting 1. PuntaGordaPete:
Hi Lee,

I really love your blog. Just the right amount of depth for me. I am a retired physicist, so I understand the physics, but I do not have much depth in the weather context.

My question is about the "talking heads" in the media that get this sort of thing rather muddled up. Many of the TV weather reporters claim to be meteorologists with degrees in meteorology. It seems that the topics and level of discussion in your blogs should have been covered in an undergraduate program.

Am I wrong about what is taught in meteorology school? Did these TV weather forecasters simply not pay attention?

P-G-P


Boy that's a great question. There are some on-air folks who don't have degrees in meteorology. I also believe that "weeds" get planted in the garden of learning by teachers / instructors who have misunderstandings of scientific concepts. Once planted in the minds of students, they stay there forever. That's why I always took teaching so seriously. I was a gardener, and I tried my very best to keep the weeds out of the garden.

People who are self-confident don't mind being wrong. Indeed, they are pleased when their errors are pointed out to them lest they continue to propagate them. Most of us are wrong some of the time. I know I am. Learning science (or anything) is an iterative process. Hardly a day goes by that I don't change my mind about something I thought I understood. But some TV weathercasters lack the self-confidence (not to mention the firm scientific training) to correct their errors, to constantly be strive for clearer, more correct, less misleading explanations.

Instead, I believe some talk down to their audience, coming up with slick analogies or explanations they think are palatable for their audience. In the process, they take a short-cut and plant weeds.

Don't get me wrong. TV folks provide important information to their viewing audience. They can save lives. But their science sometimes takes a backseat to the truth.

I'm probably coming off like a know-it-all. I'm not. I make mistakes all the time. But what keeps me going is that never-ending thrill of life-long learning and the striving to get things right.

Did this help? I hope I didn't offend you or any of my readers. It was not my intention. I'm just trying to convey my opinion.

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
3. 24hourprof
8:46 PM GMT on September 04, 2013
Quoting 2. Some1Has2BtheRookie:
Another excellent blog, Lee. Thanks!


You're quite welcome.

I enjoyed writing it.

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
2. Some1Has2BtheRookie
8:11 PM GMT on September 04, 2013
Another excellent blog, Lee. Thanks!
Member Since: August 24, 2010 Posts: 0 Comments: 4745
1. PuntaGordaPete
7:10 PM GMT on September 04, 2013
Hi Lee,

I really love your blog. Just the right amount of depth for me. I am a retired physicist, so I understand the physics, but I do not have much depth in the weather context.

My question is about the "talking heads" in the media that get this sort of thing rather muddled up. Many of the TV weather reporters claim to be meteorologists with degrees in meteorology. It seems that the topics and level of discussion in your blogs should have been covered in an undergraduate program.

Am I wrong about what is taught in meteorology school? Did these TV weather forecasters simply not pay attention?

P-G-P
Member Since: September 5, 2006 Posts: 0 Comments: 44

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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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