Asperitas Clouds

From below, asperitas clouds appear like upside-down ocean waves, undulating and cresting and best seen at sunset when they're lit from below. 

The term "asperitas" and the classification is a rare new addition for the official cloud atlas. Here's the official definition of Asperitas clouds:

"Well-defined, wave-like structures in the underside of the cloud; more chaotic and with less horizontal organization than the variety undulates. Asperitas is characterized by localized waves in the cloud base, either smooth or dappled with smaller features, sometimes descending into sharp points, as if viewing a roughened sea surface from below. Varying levels of illumination and thickness of the cloud can lead to dramatic visual effects. Occurs mostly with Stratocumulus and Altocumulus."

Asperitas Clouds on Wikipedia.

Try out my custom-trained James Gurney chatbot. 

Brocken Spectre

A brocken spectre is at atmospheric phenomenon where the observer casts a shadow onto a cloud, and the shadow is visible surrounded by a glowing orb of light. 

For the effect to be visible, usually the observer has to be high up on a mountaintop or in an airplane. In the case if an airplane, the shadow. 

Wikipedia on Brocken spectre

Iridescent Pileus Cloud

Photo by Beckie Bone Dunning

An iridescent pileus cloud forms when tiny frozen ice droplets above the top of a cumulonimbus cloud bend the light into colorful bands.

Below is an even more amazing image claiming to be of an iridescent pileus cloud.

This screengrab is from a viral video posted by Twitter user Science Girl, who later took the image down, not sure of its authenticity. A fact-checking Twitter account called Hoax Eye later determined that the image was manipulated. Already, AI-generated images have started to appear of iridescent pileus clouds

If you're wondering whether a given image is a real, photographic capture or a faked image, one of the best ways to check it is with reverse image search. Drag the image into the search bar of Google Images and you can figure out where the image originated and where else it has appeared.

This is a pretty benign example of a questionable image. With so many photo-real fakes appearing right and left, we have no choice but to be skeptical about the authenticity of everything we see online.

Why Are Reflections of Bright Lights So Elongated?

Nina Garcia emailed: "The other day I was on a walk around my neighborhood pond when I noticed some interesting things about the view."



She continues: "I am trying to understand two things:
1. why is the light that is reflected in the pond so elongated compared to the reflection of the trees?
2. why does the reflected light go straight down (rather than away or towards a vanishing point if that makes sense)?"

Answer: In his 1903 book Light and Water: A Study of Reflexion and Color in River, Lake, and Sea, Sir Montagu Pollock uses this photo to illustrate an observation of reflections on gently rippled water: "The gentle movement of water in the distance elongates and exaggerates the upright lines of the buildings. In the foreground, the individual ripples become visible, breaking up the reflection of the mountains horizontally."

He explains the elongation of the reflection of the buildings by setting up a series of mirrors to represent the far wavelets, and then tilting them toward and away from the observer. Since the area they reflect shifts vertically, they have the effect of stretching the reflected image straight downward.

Light Pillars

Light pillars are optical phenomena where artificial lights appear to form vertical shafts above each source of light. 

They're caused by millions of flakes of ice floating between the observer and the source presenting reflective horizontal surfaces.

They're like an upside-down version a light source reflecting vertically on the surface of water. 

Pillars can form over natural light sources, such as the sun, where it's called a sun pillar.

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Wikipedia on Light Pillar 

Previous post on Sun Pillar

Pyrocumulonimbus Clouds

The Bootleg Fire in Oregon has spawned tornadoes and has generated plumes of smoke that rise to the altitude of jet aircraft flightpaths. 

Image from hdnux.com

The rising currents of air create clouds resembling cumulonimbus formations, but because they're generated by fire, they're known as pyrocumulonimbus clouds.

According to the New York Times, the pyrocumulonimbus cloud over the Bootleg Fire is "similar to a thunderhead. 'It likely reached an altitude of about 45,000 feet,' said Neil Lareau, who studies wildfire behavior at the University of Nevada, Reno. Like a thunderhead, the huge cloud spawned lightning strikes, worrying firefighters because of their potential to start new fires."

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New York Times: "How Bad Is the Bootleg Fire? It’s Generating Its Own Weather."

Civil Twilight

The term civil twilight refers to the period between the time when the sun goes down and when the natural illumination is so dim that artificial light is needed to distinguish objects on the ground. 

(Photo of Civil twilight in the Mojave Desert from Wikipedia)  

Officially it begins at sunset and it ends when the sun's geometric center reaches 6° below the horizon. 

The designation has some legal ramifications for laws that define when headlights are required or that designate a crime as having occurred in the daytime or the nighttime.

Artists are conscious of the big changes that happen during this period of time when the sun no longer shines on objects on the ground, but the light still touches the higher clouds. Maxfield Parrish made a career of painting during this fleeting time period.

Civil twilight is followed by nautical twilight (above), where the sun moves between 6° and 12° below the horizon. When you're at sea during that period of time you can still distinguish the horizon, but the sky is dark enough to discern many stars for navigation.

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Wikipedia on civil twilight

Polar Stratospheric Clouds

Polar stratospheric clouds appear high in the sky in the polar regions in winter.   

Photos in Iceland. Photos by @ h0rdur c/o Fabulous Weird

They are composed of ice crystals and supercooled water droplets, sometimes mixed with nitric acid.

Wikipedia says: "Particles within the optically thin clouds cause colored interference fringes by diffraction." 

The visibility of the colors may be enhanced with a polarising filter."

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Thanks, Atlas Obscura and Fabulous Weird

Sunsets on Mars

On Mars, the colors of the sunset are reversed from what we're used to seeing on Earth. 

Sunset at Gusev Crater: The Sun sinks below the horizon, 

as captured by NASA's Spirit Mars rover in 2005.

On Earth, we're accustomed to blue skies and warm colors around the sunset. But on Mars, the sky is warm colored, and the thin atmosphere is tinged blue around the setting sun. 

According to NASA, 

"Just as colors are made more dramatic in sunsets on Earth, Martian sunsets would appear bluish to human observers watching from the red planet. Fine dust makes the blue near the Sun's part of the sky much more prominent, while normal daylight makes the Red Planet's familiar rusty dust color more prominent.

"The colors come from the fact that the very fine dust is the right size so that blue light penetrates the atmosphere slightly more efficiently," said Mark Lemmon of Texas A&M University, College Station, a science team member of the Curiosity rover mission. "When the blue light scatters off the dust, it stays closer to the direction of the Sun than light of other colors does. The rest of the sky is yellow to orange, as yellow and red light scatter all over the sky instead of being absorbed or staying close to the Sun."

Heiligenschein

Heiligenschein, is an optical effect where a bright spot appears to surround the cast shadow of the head of the observer. The glowing spot is caused by rays of sunlight reflecting back from individual dewdrops, and the effect is best seen on a cool, clear morning.

The word translates from the German as "saintly illumination" or "holy light." The effect was described in the memoirs of Benvenuto Cellini (1500-1571), so it's sometimes called "Cellini's halo."
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Photo courtesy New York Times, which published the article "How to become an angel in the morning dew"
Heiligenschein on Wikipedia

Are Sunrise and Sunset Colors Different?

A listener to the BBC podcast The Naked Scientists asks: ""Can you tell from a painting, or a photo, whether it's sunrise or sunset?"

My answer: No, there's nothing fundamentally different about the light effects at sunset or sunrise, and there's no way to tell which you're looking at from the light and color effects alone.

The cause of those light effects are the same. Sunlight travels through more atmosphere as the rays approach the horizontal. Passing through more air scatters out more blue wavelengths from the light rays, making the light that remains appear increasingly orange or red. As the sun passes below the horizon, beneath the curvature of the earth, it may briefly illuminate the bottoms of the clouds. Of course this effect happens both at sunrise and at sunset when colors are at their richest.

A single photo or a painting can tell you something about the position of the sun and about the height and distribution of cloud layers. And some art historians have argued that paintings of sunsets after the eruption of Krakatoa in 1883 reveal colors that were more pronounced worldwide. But it can't tell you whether it's morning or evening.

If there are differences between sunrise and sunset, they're qualitative and subjective. In some environments, humidity and dust may be stirred up at the end of the day because of evaporation and turbulence, and these effects can increase the saturation of the colors. But you wouldn't be able to guess that from a single image. Emotional subjectivity also plays a part in our human perception of sunsets and sunrises as we experience them in time. While a sunset builds gradually to a dramatic crescendo before quickly transitioning to twilight, a sunrise starts off with a blast of color and, as Wordsworth says, the "vision splendid" fades "into the light of common day."
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M. Minnaert, author of the book "The Nature of Light and Color in the Open Air," addressed this topic and here's how he puts it: "Are there any differences between dawn and dusk? If any they are so small that it is not possible to mention any really typical differences. One important thing, however, is that the eye is completely rested in the morning and sees the light-intensity increase continuously, so that it is more sensitive to dawn phenomena than to dusk phenomena. The latter have generally a greater richness of colour on account of the greater humidity of the air, and because the air is a little more turbulent, and contains more particles of dust than in the morning." (page 280, paragraph 193)

Listen to the Q and A on The Naked Scientists Podcast: Can you tell if it's sunrise or sunset?

Glitter Path

A glitter path is a vertical reflection of a very bright light source on water, extending from the horizon straight down to the water near the viewer.

Glitter path, photo by Harald Edens

Typically the source is the sun or moon, so sometimes it's called a "moon-path." The glitter path widens where the water is disturbed, and it narrows in the areas where the water is calmer.

Study by Peder Krøyer

Wavelets present many small reflecting surfaces at a variety of angles. Wherever those surfaces are just the right angle to reflect the sun, a spot or dash of light appears.

The effect fascinated Danish artist Peder Krøyer (1851-1901), who did many studies of it, and included it in some of his most famous paintings.

In Howard Pyle's magical story, The Garden Behind the Moon, the "moon-path" appears one night and a young boy discovers that he can walk out on the water:

"There was the moon-path, and there was the wave, and there was this bar of moonlight right a-top the wave. I stepped out again, and this time I wasn't afraid. This time, would you believe it, I didn't fall into the water at all. All the same I had to jump off that wave on to another, for the moonlight was sliding under my feet. It was as slippery as glass."

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Books: How to Read Water: Clues and Patterns from Puddles to the Sea by Tristan Gooley
The Garden Behind the Moon by Howard Pyle
Online: Glitter Path, explained in Backyard Optics 

What happens to light in clouds?

Let's take a look at how sunlight interacts with clouds.

Photo New Scientist, photographer Mahrt Fabian

Although there is a light side and a shadow side on most clouds, most of the light enters the cloud rather than bouncing off the surface. If the cloud is thin enough or fragmentary enough, a light side and shadow side are not distinguishable.

But in a large, dense cloud like a thunderhead, the light that enters the cloud is scattered and dispersed within the mass of water vapor or ice crystals. After its random journey, the light eventually exits the cloud, lightening the shadow side.

 Note that the white cloud is darker than the white house.
Photo Home Buyer

This subsurface scattering gives clouds a different character than an opaque white surface such as plaster or painted stucco. The cloud's light side is darker than the solid surface (because it's absorbing light), and the shadow side is lighter (because of subsurface scattering).

The value of the shadow side of the cloud is therefore a combination of internal scattering and external sources, such as the blue light of the sky or reflected light from the ground.



Simulating clouds turns out to be a computational challenge for artists using 3D digital tools. A recent research paper by Disney's team of computer engineers discusses some improvements in the rendering of light behavior within clouds.

They used machine learning techniques to speed up the process of volumetric path tracing, following the complex pathways of the light within the cloud. (link to video)

Read more
Disney's research paper (PDF)
My book Color and Light: A Guide for the Realist Painter (link to Amazon) gets deep into this topic. If you live in the USA you can get the book signed on my website, shipped within 24 hours.
Previously: Subsurface Scattering

Ten Tips for Painting Rainbows

Here are some tips for painting rainbows in oil: 

1. Plan the scene so that the lighting is frontal, with the antisolar point at the center of the rainbow's circle. 

2. Lightly pencil the arc using a homemade beam compass (basically a long wooden bar pivoting on a sharp nail).

3. Paint the scene around it the arc. Don't paint the colors of the rainbow yet. Leave the area of the rainbow's arc whitish and lighter than the background, but still a little transparent so you can see forms through it. Sometimes you can use a rag on your beam compass to lift pigment out of the arc. 

4. Remember that the rainbow is composed of light added to the light of the scene, so it should be lighter than what's beyond it, even after you add the color pigment.

5. Let the background painting completely dry. Then use your brush with the beam compass to glaze colors in individual bands along the arc.

6. Let the edges between the bands blend into a smooth gradation. You can do this by strapping to the beam compass a flat white synthetic brush which is wide enough to span the full width of the rainbow.

7. Colors should start with red on the outside edge of the arc, then orange, yellow, green, blue and violet on the inside.

8. If you want to add the secondary rainbow, remember that is should be weaker than the primary one, with reversed colors.

9. Plan for the region between the rainbows to appear relatively darker (Alexander's Dark Band). In effect, that means that the region inside the primary rainbow should be just a bit lighter than the area just outside the primary rainbow.

10. Objects overlapping the rainbow should partially occlude it, depending on how far they are from the viewer, and how much illuminated atmosphere there is between the viewer and those objects.



Think of the rainbow not as a solid "thing" occupying space but rather as a region of added light—light that is bouncing back to your eye from millions of hovering raindrops.

Previously on GurneyJourney

The Science of Rainbows

What Do Rainbows Mean?
Alexander's Dark Band
Circumzenithal Arc

Books with more info

Color and Light: A Guide for the Realist Painter

 Light and Color in the Outdoors by Minnaert, Marcel (1995) Paperback by M. Minnaert 

Water Reflections vs. Ice Reflections

In water refections, the reflected image mirrors the subject at a slightly darker value, deepening the colors of whatever it's reflecting. 

Even in still water, distortions begin changing the reflected image. Verticals remain legible, but they're typically blurred in the vertical direction. Thin horizontal lines disappear.

Tiny ripples introduce wobble into the image, but the components of the image—in this case branches, tree trunks, and sky—are still  legible as separate elements.

How is this different with reflections on ice?

Here are three different photographs taken of a pond at the same time of morning on different sunny days. 

• In #1, the open water is a little more disturbed than in the previous picture, so the ripple distortions are greater. 

• In #2, a thin layer of smooth ice has formed. The range of values of the reflection is less than with the water surface. Where the ice refroze and formed a thicker edge in the middle, it reflects more deep blue color from the sky. 

• In #3, the ice has aged several days, roughening the surface and making it less reflective. The value range is even narrower.

Below are three photos of ice reflections on overcast days. In all three, the overcast conditions reduce the contrast of warm and cool colors, and they all appear more gray.

• #1 is ice with a thin layer of water on top of it. The ice raises the values of the deepest darks, but the water offers a clear reflection of the trees.

• In #2, ice at the edges vs. open water in the middle of the pond shows the difference between the two.

• #3. Older ice reflects the trees as soft dark verticals against a light sky all the way to the far shore. 

The bottom line: Ice reflections are less definite than water reflections. They are blurrier, and they should be painted with a narrower range of values. If you're not bold enough with water reflections, they tend to look like ice reflections.

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My book Color and Light: A Guide for the Realist Painter discusses reflections, atmospheric effects, and a lot more. You can get it from Amazon or at my website. 

Previous series: 

Water Reflections, Part 1

Water Reflections, Part 2

Water Reflections, Part 3

Circumzenithal Arc

A few days ago I photographed this circumzenithal arc in the sky above the Hudson Valley. These halo phenomena are sometimes called "smiles in the sky" or "upside down rainbows."

Unlike a normal rainbow, which describes a circle centered around the antisolar point (directly opposite the sun), this light effect curves around the zenith. The colors appear on the section of the circle closest to the setting sun.

Whereas the regular rainbow is the result of sunlight bouncing back to the eye in suspended raindrops, this effect occurs when sunlight refracts through plate-shaped hexagonal ice prisms floating in a horizontal position in cirrus clouds. Therefore, it often appears interrupted as it intersects the parallel tendrils of the clouds.
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Wikipedia on circumzenithal arc

More at Atmospheric Optics
Classic book on light/atmosphere phenomena: The Nature of Light and Colour in the Open Air

Chromatic Snow Sparkles

Looking at this photo that I just took, you might assume that it's a view of the stars and planets in the night sky, but it's not that at all.

It's a view looking down at the snow outside our breakfast window. The colored lights are sparkles of sunlight reflected and refracted internally inside snow crystals. Like a prism or a rainbow, the hexagonal crystals break the light into component wavelengths, giving the observer a variety of colors sampled from the rainbow: red, magenta, yellow, cyan, blue, and violet.

I took the photo with a digital SLR underexposed enough for the colored sparkles to register.

The conditions today are perfect for chromatic snow sparkles. It snowed three days ago, and has stayed below freezing since then, allowing the crystals to grow larger. This morning the temperature was five degrees Fahrenheit. 

It appeared to me that the colored sparkles are most pronounced about 45 degrees away from the sun. In the photo above, that band passes through the right central part of the picture, with the sun coming from the left. 

If you look at the sparkles on an angle farther away from the sun—or closer to the sun, the chromatic effect is much less noticeable.

Related previous post: Annular Highlights

Related topic: Sun dogs (rainbow effects on ice crystals in the sky near the sun)

Sun Pillar

We saw a good sun pillar last night, though my photo didn't come out as well as this one from the web.

A "sun pillar" or "solar pillar" is a vertical shaft of light extending upward from the sun. It appears when the sun is at or below the horizon.

The light in the pillar reflects off the bottom surfaces of ice crystals that are suspended in the air. Those crystals are like tiny hexagonal dinner plates, floating in a horizontal position. Their flat surfaces act like mirrors to reflect the rays of sunlight down to the viewer. (Left: photo by W. A. Bentley)

When such a pillar appears over an artificial light at night, it's called more generally a "light pillar."

Detailed explanation (with photos and diagrams) from Weather Doctor

Sun pillar (with more photos) from Wikipedia

The best book on this sort of subject is: The Nature of Light and Colour in the Open Air (Dover Books on Earth Sciences)

Slit-Scan Video

(Link to YouTube video) Slit-scan video transforms everyday scenes by elongating, compressing, and twisting elements into trippy dreamscapes. This one was made with an inexpensive Mac app.

Kamil Sladek explains how it works on Gizmodo:

You can make your own slit camera out of any video capable digital camera with a regular sensor and a regular lens. All you need to do is the following:
1. record a video of your action
2. extract each frame as an individual image (the opposite to what you would do for a time lapse)
3. extract a vertical single pixel wide line from each image (for example a line from the center)
4. stack those lines horizontally from left to right to form an actual "slit scan" image
This can be automated by tools like e.g. ImageMagick and the longer your initial video was, the wider your image will be. In fact, the width of your slit scan image will have exactly the same amount of pixels as your initial video's frame number.
Now, to go one step further you can proceed for all the other vertical lines of your images and create one slit scan image for each particular set of vertical lines. This will give you a set of as many slit scan images as your initial video was wide in pixels. Combining that set of slit scan images to a video (this time exactly as in a time lapse) your result can look like this.

Via BoingBoing

Reality vs. Screen Illusion

Rob Legato creates visual effects for big-budget movies.


(Video link) In this TED talk, he shares how he created effects sequences for Apollo 13, Titanic, and Hugo. In the first two cases, actual documentary film footage exists of some of the scenes he was visualizing. The real footage served as a reality check against the cinematic invention.

One of the surprising revelations is that our sense of what looks real is greatly influenced by the emotional processes of our memories, which reorder reality into a composite fiction.

That emotionally tinged version of reality is what moviemakers need to bring to life if they want to create convincing illusions. The same general principle applies to painting. Often it's necessary to go beyond optical or photographic realism in order to achieve psychological realism. It's the difference between mere accuracy and true believability.
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Rob Legato on TED: The Art of Creating Awe