Corner snow accumulation

Could we call this a natural experiment? Nature sets up the conditions and in 4 cases the outcome is almost identical. Excellent repeatability, so good science.

Of course Marcel Minnaert has noticed this many years ago. And has also found a plausible explanation.

Autumn is the time par excellence for studying air currents and the dry leaves that - in their millions whirl all along the roads, are very sensitive wind-indicators. In every ditch, every slight depression of the terrain - eve if it's only an undulation of a few centimeters deep! - the withered leaves accumulate. At such points, the wind has to be significantly weaker.
Other accumulations occur where the air rises temporarily, so that the horizontal component of the airflow is small. Thus one finds sometimes a big pile of leaves in the recessed corner of a building, caused by the whirling ascent of air there.
And look at ribbons of autumnleaves that you can find along any sidewalk and along each wall: as the wind comes closer to the wall the horizontal component of the airspeed decreases and the wind will first drop the sand and only ater that the withered leaves.

Ice fractals

Marcel Minnaert does not write about fractal shapes. In his three-volume encyclopedia of observations in nature he never mentions anything that comes even near.
He wrote his books in the 1940's. Could it be that the concept of "fractal" was still so remote that even this master observer didn't notice it?

My personal theory of this phenomenon:
- swamp gas bubbles rise to the surface, keeping the water in motion and preventing freezing
- the water current created by the bubbles tries to raiate outward from the hole
- it searches for the easiest path, and thus also creating and stabilizing the path.

The same process is seen in lightning and the creation af a river delta (I think).

Looking this up in The Self-Made Tapestry: Pattern Formation in Nature by Philip Ball I'm led to "dendritic patterns" and "lichtenberg figures" - a force forms a path through a medium that resists the force.

From a review of this book (Reuben Rudman):

In the chapter on Branches we find detailed descriptions of dendritic growth of crystals, formation of snowflakes, tree-branch growth, and bacterial colony formation presented in terms of diffusion-limited aggregation (DLA) and fractal geometry. DLA describes systems in which the “rate of growth is governed by the rate of diffusion of particles. It differs from the way regular, faceted crystals grow, in that there is no opportunity for the impinging particles to rearrange themselves so that they pack together most efficiently. Since this takes place at the surface of the growing crystal, it soon becomes jagged and disorderly.”
The details of the final shape are a function of the kinetics of crystal growth, where the branched clusters are nonequilibrium structures. The fractal dimension is a measure of how densely packed the branches are and is a property that is precise, reproducible, and characteristic of the apparently irregular, branched objects. We also learn that the algorithms used to describe the shape of a tree are much more complex than one would imagine; several examples are described.
In Breakdowns, which in some ways is a continuation of the previous chapter, glass fracture, electrical discharges, earthquake-induced crustal fractures, and landscape evolution are analyzed in terms of fractal geometry. The pattern developed as a river forms from its converging streams is described best by a fractal scaling law that is characteristic of most branched networks including cracks and DLA clusters.

Snow drift study

Last weekend, right after snowfall I went out to study snow patterns. I was a delightful walk, that gave me much to think about. Of course I was not the first one - many observers went before me. And they saw and deduced things that escaped me. But I'm happy to rediscover and present their observations. As far as I know, the observations by Marcel Minnaert have never been translated into English before.


John Ruskin - Frondes Agrestes - I have not had the time yet to dive into the works of John Ruskin, but he is cited many times by Marcel Minnaert, so he must be inspiring.

54. In the range of inorganic nature I doubt if any object can be found more perfectly beautiful, than a fresh, deep snow-drift, seen under warm light. Its curves are of inconceivable perfection and changefulness; its surface and transparency alike exquisite; its light and shade of inexhaustible variety and inimitable finish,--the shadows sharp, pale, and of heavenly colour, the reflected lights intense and multitudinous, and mingled with the sweet occurrences of transmitted light....

Marcel Minnaert - The physics of open space - And of course Marcel Minnaert both observes and analyses.

On the beach, we can often note the beautiful shapes created by the sand carried by the wind as this sand falls down around obstacles. Such observations
can also be made after a snowfall, but here the drift figures are more variable. This depends on the snow being fine and powdery, or being composed of heavy flakes.

If the obstacle blocks the wind completely, but has relatively small dimensions, then:
- both deposits and erodes sand in front of the obstacle:
- the wind is slowed at some distance in front of the obstacle, this leads to deposition,
- the wind creates an eddy right in front of the obstacle, this leads to erosion,
- the wind removes sand at the sides of the obstacle,
- the wind deposits the sand in the leeward side of the obstacle as a kind of sand-tail.

The wind also sorts the grains, the coarse grains lie on the windward side, the fine grains on the leeward side.

Shadow cones

Today we had a total lunar eclipse. In theory it could have been a wonderful sight. In practice it was unspectacular because:

1. The moon was near the horizon at the start of the eclipse and it was setting. So most of the sight would have been out of view anyway.
2. It was very cloudy.

When I looked up the coordinates in Stellarium and pointed my camera in the approximate direction this is what I saw. I think the moon was somewhere above the middle street light.

A lunar eclipse occurs when the moon enters the shadow cone that is cast by the earth. This cone has two parts: the umbra and the penumbra. The penumbra is the bigger, widening shadow cone emanating from the earth.

I could not see this effect on the astronomical scale, but I was reminded of a recent "discovery" that I made. The discovery of "urban shadow cones".

A few weeks ago I looked at a street light at just the right angle and I noticed a beautiful, big, pyramidal shadow cone. Somehow I had never noticed this phenomenon before. I had to trudge through the snow a few times before I found a good example that produced a nice clear photograph.

It took some careful observing, remembering and web-searching before I found and verified the answer: new streetlights! During 2010 the city of Rotterdam has been replacing the old sodium lamps with new metal-halogen lamps. These behave more like a point-source and produce less diffuse light - and thus produce much sharper and clearer shadows. I had not noticed these before because they did not exist yet.

It is easy to prove with a picture of the old type of street light - they are still there in the smaller side streets. Notice the combination of umbra and penumbra in this example. But here it is produced by two light sources at different distances from the lamppost that casts the shadows. So the geometry is different from the astronomical case. And just for completeness: an omnidirectional light source casts no shadow at all.

The fuckscale

Last week I came across this art video in the Bonnefanten Museum in Maastricht:

Of course I was immediately reminded of this wise remark by Marcel Minnaert:

If one does not know how a phenomenon should be measured, one can always start with a random, qualitative scale measure for it.

For a star that does not sparkle I use the number "0". The strongest sparkle that I ever saw - close to the horizon - I call "10". Then I distinguish the intermediate values by the other digits.

It is remarkable how useful many of these provisional scales turned out to be in the natural sciences. Much quicker than one would expect it, one gets used to the significance of each number of the scale, and there will soon come a moment when someone finds a way to calibrate the qualitative scale and turn it into a well defined quantitative measure.

Sources:

Wolfgang Fütterer, Ring Frei, 2009 - link and link

Marcel Minnaert, 41. Hoe meet ik het fonkelen der sterren? in De natuurkunde van 't vrije veld. Deel I (1937), Licht en kleur in het landschap [Physics of the natural surroundings, Light and colour in the landscape]

More invisible artworks

Fresh input

Karen Russo sent me an e-mail with links to more invisible artworks. Thanks! Then I had to do more research. And more research always makes things more complicated. These are the preliminary results.

Art hidden behind art

Infrared reflectography can reveal drawings hidden under the layers of a painting. Many old paintings contain underdrawings - hidden ideas and sketches that are invisible in the final version. Paint layers that look opaque to human viewers are transparent in the infrared.

Classic invisible art

The most classic invisible art object is probably "The Invisibile Object" by Alberto Giacometti. It is held in the hands of a disquieting statue.

source

Almost imperceptible art

The most massive almost imperceptible artwork must be Walter de Maria's "Vertical Earth Kilometer". A one kilometre long solid brass rod, five centimeters in diameter. It has been sunk into the earth at the Friedrichsplatz in Kassel, Germany.

  source

Empty space art

Zone de Sensibilité Picturale Immatérielle and Le Vide by Yves Klein.

Erased art

Erased de Kooning Drawing by Robert Rauschenberg.

Exhibition of almost imperceptible art

InVisible: Art at the Edge of Perception.

Exhibitions of invisible art


VIDES (VOIDS) a retrospective exhibition at the Centre Pompidou.


A Brief History of Invisible Art at the CCA Wattis Institute for contemporary arts.

Going to extremes

Also on view was a work that Friedman claims took five years to create, 1000 Hours of Staring, 1992-97, a blank sheet thirty-two and a half inches square, consisting of what the catalogue calls "stare on paper."

In his enigmatic work 'Untitled (A Curse)', Tom Friedman presents the viewer with a simple plinth upon which - it appears - nothing rests. Yet, like Yves Klein's 'Void', Friedman's plinth supports both nothing and something at the same time, for the space above it has been cursed!

And then Jason Metcalf decided to erase the curse by pronouncing a blessing upon the artwork.

Scandal

BRITISH taxpayers have paid £1,500 for a foreign painter to exhibit INVISIBLE pictures.

Technology

Invisible Labyrinth.

Mobile snaps reveal invisible art.

A small enigma

Invisible art detector.

Summary and conclusion

As always ... anything that you start looking into reveals an unexpected complexity. Things that first looked simple become complicated and invite further study. An evening of study brings up tons of material. This would have been impossible before the Internet. Again: invisibility + searching and finding.

Dowsing in the snow

EXCAVATIONS IN ROTTERDAM :: A PSYCHOGEOPHYSICS WORKSHOP WITH MARTIN HOWSE :: psychology as plate tectonics of the mind

On Saturday, December 4th 2010 I joined a psychogeophysics workshop by Martin Howse in  Worm (Delfshaven, Rotterdam). Martin had brought an interesting collection of experimental electronics.

Martin was in Rotterdam with Kathrin Günter, a German artist who does original things with film and photography and who had given a workshop the previous day.

 As evening was falling we went outside with the receivers and laptops - and with a collection of  improvised "dowsing" objects. These were built from random urban debris that were found the previous day.






It is difficult to describe the atmosphere of a Martin Howse workshop. Maybe the best approximation is  the following story from the Internet. It is one of my favourites. Martin Howse's style comes close to the "research" that is described in the story, but it does not use pendulums or dowsing rods. But Martin's electronics simulate dowsing ... and inspire further research.

My favourite dowsing story. I give a few excerpts below. You can read the full story here.

Another "Taos Hum" aspect from Stefan Lofstrand in Sweden

In the fall of 1996 I first noticed the sound that is dubbed "The Taos Hum". Since I have one of the major air force bases in line of sight and major war industries less than 12 kilometres away on the other side of the mountain, my first assumption was that the humming sound emanated from a secret tunnelling operation.

Mysterious sounds and hums are quite common and we even have one in Rotterdam.

... Soon the snow covered the Hunneberg. I did not care for stamping my foot prints on the flat mountain roof, so for the winter I traced the progress of the tunnel boring machine with my pendulum on the map. After a couple of months I had logged the speed of the tunneling rig to some 1.2 km/month.

Dowsing and remote viewing are interesting techniques. Invisible objects are revealed. They may not be real, but they are always surprising.

... I came across a complex of buildings that were not marked on the map - with a function and proprietor that I feel legally prohibited to divulge in a public medium - hidden in the woods close by the tunnel. At the opposite side of the tunnel I found a major asphalt production plant run by the Swedish Road Administration, situated in a day break where macadam evidently had been produced for several decades. There was also an entrance to the underground with a conveyor belt for crushed rock, which lay in huge mounds on the premises. Whether these mounds had a connection with the present tunneling operation, or with earlier excavations, I cannot say. Only that asphalt production is a convenient way to dispose of excavated rock.

Mysterious - possibly secret - building complexes are always worth visiting. We have a few in Rotterdam also.

When you are stalking "secrets" you simply feel and follow the pull of your object of gravitation, until you have reached the sign "Secret". If it is a secret that the object is secret, you will soon be discretely stalked upon. Pulling the spiders web of surveillance always attracts the spider. To consummate my field trip I first picked up a tail, then made a sudden U- turn. Since it is also a secret that you are being observed, this inverts an unwritten rule I did not co-author. Let it be no secret: reckless driving is my hobby. I also carry a formidable weapon. A camera.

Building conspiracy theories and creating an artificial state of paranoia is a positive creative process. I'm not very good at this, but I admire it in others, who are more talented. And who knows ... maybe yhe paranoids notice phenomena that escape us. Let's give Stefan Lofstrand the last word:

Perhaps those who can not hear the hum should have their ears examined. But the cure for not being able to hear the hum is often very simple. I believe it is due to "myopic hearing (myotic?) caused by the urbanized life style. City people are not used to listening. They tend to only detect sounds carrying immediate information from sources near by. When I have instructed people that could not hear the hum to stop listening for a plastic duck sitting on the table in front of them, saying "yap yap", but to listen for a hundred horses running across a wooden bridge five kilometers away, everybody could hear the hum.

Regent's Park temperature walk

Map

On August 2nd, 2010 I walked through Regent's Park with my digital thermometer. These are the results plotted on a map. Further below is the analysis. My walks fit part of the scientific theory but there are some deviations.

Fun and discovery

Maybe I have a strange idea of what “fun“ is, but doing the walk, recording the measurements, making the map and seeing what I can learn from it feel like a “voyage of discovery“. I'm always looking for ways of revealing the “invisible city“ and this walk is another way of showing invisible effects, that would otherwise remain hidden. Also, it is wonderful to see that even such a simple concept like “temperature“ behaves in an amazingly complex manner.

Analysis and surprising complexity

According to Marcel Minnaert the temperature difference between the park and the built-up area should not be that big. But it fits with my measurements of a difference of “a few degrees“.

77. The temperature in the woods
The delicious fresness that we feel in the woods on a hot day makes us suspect that a significant temperature difference with the outside air will be found. But behold! The thermometer inside the beech wood and outside of it, in the shade, shows, even on a warm summer afternoon, a temperature difference of only one degree or a few degrees! [...] In a pine forest, the difference is even less clear. [...] The peculiarities of temperature, humidity, wind speed in town or in the forest, clearly distinguished from those in the rest of the area, are best expressed by speaking of "the local climate" that prevails in these different areas. [1]

Some modern measurements - found in a quick search on Google - also seem to fit my own measurements. The differences are indeed small beyond the park boundary. The temperature differences I measured seem to be too big compared with the literature.

Urban open spaces such as parks can act as urban cool islands. For example, in Seoul, Korea, the many Lee Dynasty royal palaces and tombs act as urban cool islands that reduce the temperature of the park compared with the surrounding urban areas. [...] Results of the investigation show that the average air temperature differences due to the park in the nearby subway station was 1.76°C. [...] The air temperature-reducing effect due to the park decreases rapidly from the park boundary, because of the urban morphology and land use. [2]

However some articles show that the temperature differences can be big indeed. But the London parks are - very visibly - not irrigated, the grass was burnt and yellow, almost desert-like.

In Vancouver, parks are typically 1-2 C, but in ideal conditions can be almost 5 C cooler than their surroundings. Larger PCI (park cool islands) are possible in Sacramento where irrigated greenspace can be 5-7 C cooler. Park type, especially the extent of irrigation and the presence of trees, is important in PCI development. During the day trees may play an important role in establishing a cool park effect, perhaps through a combination of shade and evaporative cooling. At night it appears that the surface geometry and moisture status of the park are important controls on surface cooling. Open parks (with higher sky view factors) that have dry soils (and hence lower thermal admittance) cool the most. Nocturnal cooling in open grass parks is often similar to that at rural sites. The influence of parks on air temperatures appears to be restricted to a distance of about one park width. [3]

But the temperature differences behave in a more complex way than one would expect. Now I see that I also should have looked at the wind direction and wind speed.

Large green areas have a cooling influence on their surrounding built-up area, thus reducing the stress produced by the heat island. Traverses made on clear nights with light wind show that Chapultepec Park (500 ha) in Mexico City is 2–3 °C cooler with respect to its boundaries and its influence reaches a distance about the same as its width (2 km).
[...] For a recent period of four years, mean monthly minimum temperature differences between a climatological station located in the park and the Tacubaya Observatory (- 700 m south of its southern limits) reach 4.0 °C at the end of the dry season in April, whereas during the wet months they are only 1 °C cooler (in July). On sunny mornings the park heats up more slowly than the built-up section at Tacubaya; but two hours after midday there is no significant difference in temperature as shown by mean maximum temperatures. At this time, the canopy layer is well mixed and Tacubaya being downwind (from N or NE) is then under the cooling influence of the park.
But east and north of the park toward the densely built-up area (where the heat island is located), mean maximum temperatures at the park station are 2–3 °C cooler. [4]

But even inside parks there can be significant differences in temperature, changing between different times of day and night. But I didn't notice large differences in the parks.

This study underlines the differences in summer human wellbeing in an urban park between a grass area and a forested one: during the day the forested area is meanly 3 °C cooler than the grass area, but during the night the grassland is cooler than the woodland of almost the same amount. [...] As the cooling effect of green areas is both during the day (in the wooded area) and during the night (in the grassland area), and as the lower temperatures during the night seems to have very important consequences in human health, this study wants to suggest that both grassland and woodland areas of a park can have different positive effects on human wellbeing during all the hours of the day. [5]

Some more research about the complex temperature (and humidity) behaviour inside of urban parks. In a city always look for parks with high trees!

The research was conducted in three different types of urban parks: a park with grass and a few low trees, a park with medium sized trees and a park with high and wide-canopied trees. The results showed that an urban park that contains high trees with a wide canopy has the maximum cooling effect during daytime, reduces temperatures by up to 3.5 °C and lowers heat stress values despite increasing relative humidity values. An urban park that contains dense, medium sized trees can also reduce temperatures during daytime by up to 2.5 °C as well as slightly lower heat stress. However, during nighttime it can create uncomfortable climatic conditions owing to the reduction of wind velocity and increase in relative humidity. An urban park covered with grass can be warmer and sometimes even more humid than the built-up area during the day, which increases heat stress values. [6]

References

[1] Marcel Minnaert, De natuurkunde van 't vrije veld. Deel II. Geluid, warmte, elektriciteit. W.J. Thieme, Zutphen 1939
[2] Effect of an urban park on air temperature differences in a central business district area
Sang-Hwa Lee, Kyoo-Seock Lee, Wen-Cheng Jin and Ho-Kyung Song, Landscape and Ecological Engineering Volume 5, Number 2, 183-191
[3] The thermal regime of urban parks in two cities with different summer climates, R. A. Spronken-Smith; T. R. Oke, International Journal of Remote Sensing, Volume 19, Issue 11 July 1998 , pages 2085 - 2104
[4] Effects of vegetation on urban and buildings climate, Influence of a large urban park on temperature and convective precipitation in a tropical city, E. Jaureguia, Energy and Buildings, Volume 15, Issues 3-4, 1990-1991, Pages 457-463
[5] AIR TEMPERATURE DISTRIBUTION IN AN URBAN PARK: DIFFERENCES BETWEEN OPEN-FIELD AND BELOW A CANOPY, Martina Petralli, Luciano Massetti and Simone Orlandini, The seventh International Conference on Urban Climate, 29 June - 3 July 2009, Yokohama, Japan
[6] Climatic behavior of various urban parks during hot and humid summer in the mediterranean city of Tel Aviv, Israel, Oded Potchter, Pninit Cohen, Arieh Bitan, International Journal of Climatology, Volume 26, Issue 12, pages 1695–1711, October 2006
[7] How Cities Use Parks for ... Climate Change Management 

Tyburn temperature walk

Last August I participated in the London Psychogeophysics Summit. It was a great happening. Here is a short summary and a few links:

The London Psychogeophysics Summit proposes an intense week-long, city-wide series of walks, fieldtrips, river drifts, open workshops and discussions exploring the novel interdisciplinary frame of psychogeophysics, colliding psychogeographics with earth science measurements and study (fictions of forensics and geophysical archaeology).

I'm gradually presenting my results here and I've even opened this new blog for it. Below I'm replicating and mapping the thermometer experiments of Marcel Minnaert. This is a nice demonstration of temperature differences in the urban microclimate.

It was easy to trace our path through the city streets using the pictures I had taken of the thermometer at each location. But it was surprisingly difficult to trace our path through the park. Using Google Maps and Bing Maps it took me almost an hour to locate the pump in Hyde Park. From the air it is just a tiny vague blob. But mapmaking and staring at satellite pictures is fun - especially when it's a puzzle.

A book of non-existent books

Does a theory of "searching" exist?
Does a theory of "unintentional finding" exist?

They do - but they are not entirely satisfactory for this case. The most classic book about searching is Part 3 of The Art of Computer Programming by Donald Knuth. And "unintentional finding" is part of the theory of synchronicity as introduced by Carl Gustav Jung. More research is necessary because searching and finding invisible or previously unknown phenomena is the main subject of this blog.

Today I was searching for a book in the paranormal section of the Rotterdam library. It describes a non-existent phenomenon that I will write about in the near future. But someone had left a random, non-paranormal book in the stack. I realized it was a book about non-existent books.

Net niet verschenen boeken Samengesteld en ingeleid door Gummbah De Harmonie September 2010 ISBN 978 90 6169 954 5 Not quite published books Compiled and introduced by Gummbah

The book has been compiled by Gummbah (Gertjan van Leeuwen), a Dutch absurdistic cartoonist. It reproduces the covers and excerpts from a collection of totally failed fictional books. The most hopeless poetry, experimental art and horrible modernistic novels. Very funny if you appreciate intentionally bad art.


I liked the following fictional book best, because it fits the scope of the blog. It contains topography, invisibility, experimental travel and high weirdness. I would buy it if it really existed.



From the introduction to this fictional book - Met een rugzak stampvol porno dwars door Aken:

Straight through Aachen with a backpack chock full of porn
Author: Epi Duist - Editor: Van Granen


The city of Aachen has always fascinated me for some reason. Here history and hospitality go hand in hand. It produces it's own "haves" and "have-nots". Because that is also an aspect of Aachen.
Life in Aachen is forever changing yet still stays the same. After a few hours the city starts to weigh heavily on the visitor. The modern inhabitant of Aachen does not have a high profile in arts and sciences, but he is always quick of understanding and as a guest you enter a city that is light and stiff af the same time.
But after leaving this city, all your problems suddenly seem very futile and for a few days you only hear the infinitely gentle crackling of the city echoing in your mind.
Therefore I was pleasantly surprised when Epi Duist presented me his book about Aachen. He has crossed this city in one day, walking at a trot, carrying a heavy 15-kilogram backpack of which the reader knows the content, but about which the people on the street have no idea. Thus creating a strange, but very welcome tension.
The Publisher


Writing a collection of fictional books is not entirely new. One of the best examples is a collection of book reviews by Stanislav Lem called A Perfect Vacuum:


A Perfect Vacuum (Polish: Doskonała próżnia) is a 1971 book by Polish author Stanisław Lem. It is an anthology of imaginary reviews of nonexistent books. A Perfect Vacuum can be seen as a compilation of Lem works: some of the reviews remind the reader of drafts of his science-fiction novels, some read like philosophical pieces across scientific topics, from cosmology to the pervasiveness of computers, finally others satirise and parody everything from the nouveau roman to pornography, Ulysses, authorless writing, and Dostoevsky. Reviewing nonexistent books is not a theme unique to Lem (consider Jorge Luis Borges' Investigations of the Writings of Herbert Quain), but the idea of an entire anthology of such pieces is rather novel.

Another non-existent book is Life, a monumental, many-volume work by Baron Bodissey whose work is frequently quoted in the books of Jack Vance. And of course there is the magnificent Tlön, Uqbar, Orbis Tertius which is highly recommended. But here we stray into another kind of non-existence.

Invisible artworks

If you look closely - you will see that these people are enjoying an art exhibition. But where is the art?

The art is invisible - a description of the artworks is presented by audio. This is not the "ultimate white cube" of idealized gallery space. The space is full of art - but it's in the imagination - of both of the artist and the "viewer".

It was a very popular exhibition. People were queuing up to "see" the art. I was too late to "see" any of the artworks - it was the last day of the exhibition.


Here is what I might have "seen". The titles are interesting. Maybe it's just as well that the artworks stay in my imagination:

1. Introduction 
2. What defines an unfinished work? (Krukowski) 
3. When is an artwork finished? Alberto Giacometti and Paul Cezanne 
4. Exhibiting the great nothing ..... David Hammons 
5. Artworks finished by others, Kafka's Castle 
6. The interpretation of the dead composer's manuscripts 
7. The Croce and Collingwood theory 
8. Creating the impossible work of art, Urs Fisher 
9. The unseen film of the genius, Clouzot's L'enfer 

The idea of an audio tour as a piece of art is not new. Janet Cardiff has made a fascinating series of audio walks that you can listen to or even view. It's a pity that you need to install Quicktime for this - I hate Apple products. But of course there are ways to circumvent this. Here are some direct links to the audio:

http://www.cardiffmiller.com/media/walks/14_Long_Black001.mp3
http://www.cardiffmiller.com/media/walks/15_Long_Black.mp3
http://www.cardiffmiller.com/media/walks/16_Long%20Black.mp3

This will also send you on an invisible journey of the imagination.

Reference:
The artworks you have never seen - an audiotour by Margo Onnes

First attempt at a taxonomy of invisibility

In my personal world model there are many different kinds of invisibility. All of them are interesting. All of them are worthy of research. Each one hides interesting discoveries:

Things that are in plain view but are passively overlooked:	graffiti, paving structures, chimneys, numbers on lampposts.
Things that are in plain view but are actively ignored:	litter, garbage tins, dog shit, beggars and homeless people.
Things that are out of view:	underground cables, pipes, drains, alleys, back-sides of buildings.
Phenomena that are out of view:	ground water levels, geologic strata, the Mohorovicic discontinuity.
Things that have disappeared into history - but are still present in memory - more or less:	ice age soil structures, settlement history, vanished buildings, the snow of yesteryear (Jacques Villon).
Phenomena too faint for the human senses:	gravity variations, magnetic deviations.
Phenomena that are outside the scope of the human senses	infra-red light, ultraviolet, infrasound, supersonics, radio waves.
Things that happen while you are elsewhere:	wild animals that roam the city streets unnoticed while you sleep, the car crash that happened just around the corner.
Things that exist but are too big or abstract to see:	the power- and economic structures that determine how a city grows, the global economy. (Like the names on maps that are too big to notice. (E.A. Poe)).
Things that are kept hidden on purpose:	crime, espionage, corruption, fraud.
Things that exist only in fiction - but can be imagined at a specific spot:	site an itinerary descriptions in literature (Malte Laurids Brigge), mentions of real places in fictional accounts (Rotterdam is mentioned in V by Pynchon and the Demon Princes series by Jack Vance).
Personal memories:	the site of the first kiss, the sites that remember what I played on my mp3-player when I was there once.
Paranormal phenomena:	ghosts, EVP, the "stone tape" hypothesis.

Each of these modes of invisibility can be used to experience the city in a new inspiring way. Each one should be tried - at least once.

I am fully aware that his is not a proper taxonomy. But it is workable starting point. Feel free to add your own items.