Exploring the Earth and Sky of the West

Posts tagged “clouds

Halos, arcs, and bows…oh my!

Double rainbow near Gunnison, Colorado

The term “landscape photography” is a misnomer in some ways. After all, many of the most interesting and unique landscape shots are those in which something unique or interesting is happening in the sky: a vibrant sunset bathing the land in a golden glow, an ominous storm looming on the horizon, or a terrestrial scene backed by a sky awash in stars. A colorful sky, interesting clouds, or a stray meteor can single-handedly liven up otherwise passé landscapes. As a longtime astronomy educator, I have a habit of looking up…a habit that often pays photographic dividends. I had an astronomy professor in college who would often remark that “most people just don’t look up”, a sad but true (save for perhaps an occasional glace at the clouds on a stormy day) acknowledgement of just how little attention most of us pay to the sky above us. 

One of the things that we miss by not looking up is a myriad of features that result from the interaction of sunlight with water droplets or ice crystals in our atmosphere. These features lack a catchy collective name, but scientifically are often referred to as atmospheric optical phenomena. Perhaps the most frequently observed example is the humble rainbow: 

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Rainbow over the Wenatchee National Forest in central Washington. This photo was taken in the early afternoon, with the Sun still relatively high in the sky, thus the arc of the rainbow appears quite close to the ground and only a small portion is visible. 

The colorful band of light in a rainbow results from a process known as dispersion. When sunlight passes through water droplets in the Earth’s atmosphere, different colors of light are refracted (or “bent”) by different amounts as they pass through the droplet, thus spreading out the colors that make up “white” sunlight into a rainbow. The water droplets don’t have to be rain; sea spray or the mist of a waterfall can do the trick as well. The geometry of dispersion is such that the center of a rainbow’s curvature will always be directly opposite the Sun. Consequently, the largest and grandest rainbows are seen near sunrise or sunset (see photo below) when our star is low on the horizon. In contrast, when the Sun is higher in the sky, only a small portion of the arc will be visible (see photo above). 

Double rainbow near Gunnison, Colorado. This rainbow appeared not long before sunset, when the Sun was low in the sky, thus nearly an entire semi-circle is visible.

The light source for a rainbow need not be the Sun either. Light from the Moon can also be dispersed through water droplets, with a similar result. The caveat? Even a bright full moon is about 400,000 fainter than the Sun, so the resultant moonbow is much fainter than a rainbow and the colors not bright enough to trigger the color-seeing cones in our eyes. Thus, a moonbow appears monochromatic. I’ve only ever witnessed this eerie phenomenon once: on a damp and muggy evening on the north coast of New Zealand’s South Island, and unfortunately did not have the wherewithal to capture a photo at the time. 

Moving beyond water droplets, the possibilities when sunlight interacts with tiny ice crystals in our atmosphere (such as those that comprise high altitude clouds like cirrus and cirrostratus) are myriad. Depending on the shape, size, and orientation of said crystals, the height of the clouds, and the location of the Sun, a wide range of atmospheric phenomena can result when we see the Sun through these high-altitude icy clouds.  

One of the most common is the 22° halo, a ring of light that encircles the Sun (or the Moon) at a radius of (you guessed it) 22 degrees:

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22 degree halo, Lake Crescent, Olympic National Park, Washington. Blocking out the Sun (with a stick in this case) is often necessary to get a decent photo of these halos.

These halos can be a little difficult to spot (and photograph…) due to the Sun’s glare, but in most locations they are, statistically speaking, much more common than rainbows. Like rainbows, these halos are caused by dispersion as sunlight passes through tiny ice crystals in the atmosphere. If you look VERY carefully (such as in the photo above), you can see that the inner edge of the halo is reddish while the outer edge is bluish.

The name “22° halo” suggests that halos can appear at other distances from the Sun as well, which is indeed the case. Another member of the halo family is the circumhorizontal arc, a halo that takes the form of a brightly colored band circling the horizon at a radius of 46° from the Sun. Unlike 22° halos, circumhorizontal arcs are rarely visible in their entirety. Instead, you’ll usually only see one or two small fragments in areas of the sky where the background consists of the correct type and height of cloud. These fragments look like pieces of a rainbow oriented parallel to the horizon: 

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A portion of a circumhorizontal arc over the Angels Window arch on the North Rim of the Grand Canyon, Arizona

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Portion of a circumhorizontal arc over Ouray, Colorado

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Small fragment of a circumhorizontal arc (upper left) visible over Mt. Rainier, Washington

In order to see a circumhorizontal arc, the Sun needs to be at least 58° above the horizon. This means that, at least for us here in the mid-Northern latitudes, they are only visible in summertime. The rest of the year the Sun simply doesn’t get high enough in the sky to allow them to be seen, even when the right types of ice crystals are present. (Further north, close to the poles, they can’t be seen at all!) Indeed, a quick review of my photo archives shows that all the photos I have of this phenomenon were taken in late June or July. 

Circumhorizontal arcs bear some resemblance to another phenomenon known as cloud iridescence. Unlike the features we’ve discussed so far, cloud iridescence is the result of diffraction (as opposed to refraction and dispersion), which is when light waves are bent around objects. Iridescence occurs at the edges of very thin clouds that are made of very similar sized water droplets or ice crystals. If the cloud layer is thin and uniform enough, light waves bent around these particles interfere with each other, producing a spectrum of colors. Without getting into the nitty gritty details, the colors that you see in an iridescent cloud are more akin to the colors that you sometimes see in soap bubbles or a pool of oil. 

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Close up of cloud iridescence over Walla Walla, Washington

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Cloud iridescence, Whitman Mission National Historic Site, Washington

Despite their similar appearance, cloud iridescence and circumhorizontal arcs are relatively easy to distinguish from one another. Fragments of circumhorizontal arcs always appear (you guessed it) horizontal relative to the horizon and will always have a bright red band on top. In contrast, cloud iridescence can appear in all sorts of shapes and patterns, and you’ll often see the spectrum of colors repeating themselves multiple times as well, as seen in the photos above. 

Like rainbows, many other atmospheric phenomena occur only when the Sun is close to the horizon, just before sunset or just after sunrise. Sun pillars, like those in the photos below, are the result of sunlight being reflected off the surface of flat, hexagonal-shaped, ice crystals and redirected back to the observer: 

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Sun pillar, Grand Canyon National Park, Arizona

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Sun pillar, Yakima, Washington

Once in a great while, conditions will be just right and you’ll get a whole truckload of these phenomena all at once, such as on the evening I took the photo below on a recent neighborhood walk as the Sun set over the Cascade range: 

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Atmospheric phenomena galore! Here, the Sun is surrounded by a 22° halo complete with a tangent arc (the triangular wedge of light at the top of the halo). A sun pillar extends vertically above the Sun, while a sundog (the bright patch on the halo to the right of Sun) is visible at right.  

Sundogs and tangent arcs will have to wait until another day. Until then, keep your eyes on the sky. Chances are you’ll spot a nalo, arc, or bow before too long! 


Views of Tahoma

A green forested valley with a conical volcanic peak in the background. Many different types of clouds in the sky.

White icefields and glaciers at the summit of a peak are visible through some clouds

Light from a nearly full moon illuminates the glaciers and icefields on the summit of Mount Rainier. 

At 14,411 feet, Mount Rainier is the highest peak in Washington and in the entire Cascade Range. British naval officer Peter Rainier never even saw the mountain that now bears his name, but he had a friend that did. Clearly, it paid to have connections in the 1700s. Oddly, Rainier did fight against the Americans during the Revolutionary War, making the fact that we continue to utter his name when referring to this grand peak all the more peculiar. Mount Rainier was originally known as Tahoma or Tacoma by the Salish-speaking indigenous tribes of the Pacific Northwest. There are periodic rumblings about renaming the peak, much like the name of Alaska’s Mount McKinley was officially reverted to Denali in 2015. Hopefully that will indeed happen someday…

Irrespective of name, Tahoma dominates the skyline from Seattle and much of the Puget Sound region. Tacoma and other towns to the south of Puget Sound are literally built on layers of debris deposited by gigantic lahars (volcanic mudflows) that periodically race down its flanks, filling river valleys on their way to the sea. The threat of future lahars and volcanic activity looms over those who live in its shadow. From my vantage point in the Yakima Valley of central Washington, the foothills of the Cascades obscure all but the uppermost few hundred feet of its glacier-clad summit (and which will, thankfully, block any future lahars). Obtaining a better view requires venturing into the mountains. Recently, we spent a weekend camping high on a ridge about a dozen miles to the south of the volcano’s summit. Our campsite in an old clear cut provided stellar, if slightly obscured views of Tahoma’s bulk.

The weather was quite variable throughout the weekend, ranging from mostly clear (but hazy) upon arrival, to partly cloudy, to overcast, to bouts of dense fog. Our view of the mountain was constantly changing. One evening I decided to capture a time-lapse of cloud movement and formation in the two hours leading up to sunset:

Sadly I did not notice the beer can stuck on top of the tree in the foreground until it was too late. Oh well. On another evening, a spectacular stack of lenticular clouds developed over the summit:

A green forested valley with a conical volcanic peak in the background. Many different types of clouds in the sky.

A series of stacked lenticular clouds above Mt. Rainier.

A series of stacked lenticular clouds above a conical volcanic peak

Closer view of the stacked lenticular clouds.

A nearly full moon provided sufficient light for photographing the mountain after dark:

A large volcanic peak illuminated by moonlight with stars in the background sky.

Tahoma bathed in the light of a nearly full moon.

Not to be outdone by Tahoma, the pinnacle of High Rock just to our west also put on quite the show at sunset, with the light of the setting sun casting an amazing shadow of the peak and it’s summit lookout tower on the foreground mists:

The setting sun casts a pink glow on low clouds above a rocky pinnacle with a lookout tower.

Sunset, High Rock, Gifford Pinchot National Forest, Washington

A nearly full moon rises over a forest of trees

A nearly full moon rises over the forests of the Cascade Mountains.

After this trip and our stunning view of Mt. Adams a few weeks ago, our goal for the summer is now to camp in the shadow of all of Washington and northern Oregon’s stratovolcanoes. Next up: Mt. St. Helens!


And the Thunder Rolls: the Southwestern Monsoon

A 1-minute exposure captures several complex lightning bolts during a storm over Western Colorado on July 7th.

Two complex lightning bolts strike a mesa in Western Colorado in this 1-minute exposure.

One of my favorite things about the southwest is the sheer ferocity of the thunderstorms that arrive like clockwork every summer. It has always seemed to me a particularly violent way of delivering water to the desert. Anyone who has visited Arizona, Utah, New Mexico, or Colorado in the summer knows how an apparently benign, cloud-free, blazing summer afternoon can spawn a multitude of life-threatening thunderstorms in a matter of hours. Known as the monsoon season, late summer in the American southwest is a time during which many areas can receive as much as half their annual rainfall in the span of just a few short weeks. Generated by the arrival of tropical moisture from the south, these are thunderstorms that force one to begin any summer hike involving peaks or ridges in the wee hours of the morning to avoid being caught in an unpleasant situation.  These are thunderstorms that claim the lives of dozens of people every summer, sometimes via lightning strikes, but more often via sudden deluges of water known as flash floods that result when rain falls so fast and so hard that it doesn’t have time to soak into the soil, and instead collects in raging torrents of water, mud, trees, and rocks that can travel vast distances, sweeping unsuspecting hikers dozens of miles away from the nearest raindrop off their feet. These are also storms that produce truly unforgettable memories (the fondness of which is directly proportional to how close shelter is at the time…) and great photographs, again assuming adequate shelter is close at hand.

Thunderstorm clouds swirl above western Colorado

Storm clouds begin to swirl during a late evening monsoon thunderstorm.

Perhaps most mercifully though, these are the storms that ultimately temper the stifling heat that dominates the southwest early in the summer. Ironically, this oppressive heat actually brings about its own demise; the intense heating of the land surface in the early summer (May and June) is responsible for causing the monsoon rains that eventually bring temperatures back down to somewhat more humane values by late July and August.

Cloud-to-cloud and cloud-to-ground lightning bolts

A pair of lightning bolts; one cloud-to-ground and one cloud-to-cloud.

Two words are all one really needs to fully describe conditions in the American southwest in the early summer: “hot” and “dry”. Temperatures soar well into the 100s in many locations and relative humidity values in the low single digits are commonplace. As anyone who has ever lived on the 2nd floor of a poorly ventilated apartment building in the southwest in the summer knows: hot air rises. This basic thermodynamic fact can be used to explain just about every aspect of what is formally known as the North American Monsoon, or any monsoon, or just about any form of weather for that matter. As the Sun heats the land surface, warm dry air begins to rise high into the atmosphere due to its lower density. This rising air column leaves a void, a sort of a partial vacuum if you will, behind it, creating an area of low atmospheric pressure over the sizzling southwestern states. This partial vacuum creates a welcoming pathway for warm, moist air from the tropics to slowly begin seeping its way north from Mexico and the Gulf of California.  As the month of June comes to a close, this tropical moisture has begun to saturate the air around the Four Corners region. Gone are the days of single digit humidity values, and by early July, the Sun, instead of heating bone-dry air, is heating air that is rich with moisture.

Cumulus clouds over Mt. Elbert signal a developing thunderstorm.

Large cumulus clouds, the infant stages of a monsoon thunderstorm, hover over Colorado’s highest point, Mt. Elbert. Clouds like these are a sign to hikers to get off the mountain and start heading for shelter; a scene like this can develop into a full-blown severe thunderstorm in as little as an hour.

Now when moist air rises, the water within it condenses into water droplets, first creating puffy cumulus clouds, and eventually enormous cumulonimbus thunderheads that can reach heights of more than 50,000 feet above the Earth’s surface. These storms normally develop in the afternoon, after the Sun has had several hours to warm the surface and generate a robust rising column of air. To give you an idea of just how fast these storms can expand, and since WordPress won’t let me post videos, here’s a fun little animated GIF showing about 15 minutes of growth in a late evening thunderstorm over the West Elk Mountains of Colorado. Notice the stars in the background:

15 minutes of thunderstorm growth in an Animated GIF

While the annual arrival of the monsoon may be predictable, the individual storms that it produces are not in the slightest. The isolated nature of the storms can be incredibly surreal; I’ve been in locations where 2″ of rain and nearly a foot of hail fell in a matter of 30 minutes, while the ground half a mile away remained completely dry. And while the North American Monsoon may not pack quite the same punch as its southeastern Asia cousin, it nevertheless is a significant event, in both negative and positive ways, for all who live in the area. Good in that it provides the southwest with badly needed moisture in the late summer, and bad in that its unpredictable nature never fails to catch those unfamiliar with the weather pattern off guard. In addition, storms during the first few weeks of the monsoon will often generate copious amounts of lightning, but very little rain, sparking numerous wildfires in tinder-box dry forests that haven’t seen rain in months, fires that are nearly impossible to extinguish until heavier rains arrive to douse the flames.

Photographing these storms is most easily accomplished at night, when you can simply point the camera at the storm (provided you and your metal tripod are somewhere reasonably safe), leave the shutter open for a few minutes at a time, cross your fingers, and hope for a few well placed strikes. While the best bolts inevitably occur in the two seconds your camera is processing your most recent exposure, or are located juuuuuuuuuust outside the camera’s field of view, rest assured that the light show that nature puts on will still be exponentially better than anything you could possible see on your computer monitor or TV screen.

The crest of a large thunderhead stops just shy of the Big Dipper

The crest of a large thunderhead stops just shy of obscuring the Big Dipper.