Tangent Arc?

I got to thinking after taking another looking at my arc photos, that this shot may show a weak tangent arc above the also weak 22 degree halo around the sun.

Possible tangent arc with circumzenithal arc above, also showing left sundog and weak 22 degree halo.

Photos were enhanced as described in my circumzenithal arc post; lower photo is two photos stitched together. Better sundogs and halo can be seen in the circumzenithal post.

Circumzenithal Arc

Driving back from work the other day, Sunday October 28 to be precise, I noticed a bright sundog on the right side of the sun as we came around the corner at Button Point. I looked straight overhead, hoping to see a Kern arc, and there was the so-called circumzenithal arc, which does not make a complete circle around the zenith. When I got home, I ran in, grabbed my camera, took one quick shot of a fairly faded arc before the batteries died and before the arc faded almost completely in fickle cirrus clouds. After changing the batteries, and after the arc came back a couple minutes later, I took the above photo, where the arc is still faint.

By the time I took this picture, two sundogs had formed on opposite sides of the sun (the right one is not visible behind a building to the right; the left one is the bright, washed out spot on the left), and a nice halo had started to develop.

Unenhanced circumzenithal arc looking straight up.

I repeatedly looked straight overhead to see if the rare Kern arc, which completely circles the zenith, would develop. One never did.

Circumzenithal arc looking straight up, somewhat enhanced.

A view of just the circumzenithal arc.

A zoomed in view of the same arc, taken a moment later.

The halo around the sun continued to develop.

Halo mimics the shape of the tree.

Sundog to the left of the sun.

The sun was fairly low in the sky between 4:32 and 4:47 pm PST when I took these photos; it was probably between 10 and 15 degrees above the horizon, a little lower than the ideal 22 degrees.

The following day, I ran outside with my Brunton at about 4:50 pm to measure the sun angle, which was at 10 degrees above the horizon. The days are getting shorter by only 1 to 2 minutes each day, with minimal time difference in sunset between the two days in question, so the sun was probably at 10 degrees or slightly higher.

In fact, photos in which the ground isn't cropped out show that the height of the sun above the horizon is less than half of the 22 degree halo around the sun, although some distortion by the camera cannot be ruled out.

All photos except the one described as unenhanced, were enhanced by increasing color saturation and contrast, and by adjusting the brightness up or down as needed. They were then exported to a smaller size (1000px long dimension). After export, they were minimally sharpened to diminish blurriness imparted during export.

The first two photos both consist of two photos that were stitched together and cropped prior to enhancement, export, and sharpening. My semi-wide angle lens won't show a circumzenithal arc and the lower part of the sky in one shot.

The Sidewalk at Sulphur Works

While at Lassen Volcanic National Park on our first recent visit, which was early last month, we noticed that the sidewalk near the boiling mud pot at Sulphur Works had lots of mineral (and extremophile?) growths in many of the cracks.

Gray, white, and yellowish growths, some of which might be Sulfur crystals.

Closeup of some of the growths.

While bending over to examine these flowery structures or growths, MOH noticed that the sidewalk was hotter than it should be, even for a bright, warm, sunny day. We didn't have our laser IR thermometer handy, so we made plans to revisit the area.

To be continued...

Wave Clouds

Some classic wave clouds, or lenticulars, from the front that went through a couple days ago.

The wave clouds in this photo indicate that winds aloft are blowing swiftly to the east (left). Note the iridescence in the brightest and thinnest parts of the clouds in the upper left. Also note the low rotor clouds below the wave, dark and scud-like, especially the one on the right, which may show hints of the rotation typical of these clouds, and which shows a classic ragged form. When conditions are right—when the wave is strong and the rotor is hitting the ground—winds on the ground will move opposite to the winds aloft, typically back to the west in Nevada (right in the photo). These "backwards" ground winds can be strong, erratic, and turbulent, and can cause unexpected damage to street signs and the like. In Nevada, when the rotor is blowing, it's a good idea to keep an eye out for flying sheet metal.

Lenticulars—which are sometimes described as unique, or even rare—are fairly common in Nevada, in the lee of the Sierra Nevada and the many mountain ranges of the Basin and Range.

Understanding lenticular clouds and mountains waves

Lenticular cloud with irisation (cloud iridescence) at Atmospheric Optics

Lenticular clouds at APOD

Photo taken 23Oct2012, Sparks, NV, from the Sheels parking lot.

Dacite of Mount Helen near Lassen Peak

Dacite in a road cut just below the Lassen Peak parking lot.

While at Lassen Volcanic National Park last month, we pulled over on our way back over the summit of Highway 89 to examine a roadcut that had grabbed my attention earlier in the day. A pullout overlooking Lake Helen on the west side of the road, not far south of the Lassen Peak parking lot, is a great place to stop; see this nearly identical Google Street View view of the same roadcut.

Closer view of the dacite.

Turns out that this is the dacite of Mount Helen, containing what are described here as mafic, phenocryst-poor inclusions. The inclusions are locally quite large and form as much as 20 percent of the rock. It is these large inclusions that grabbed my eye on our first pass through the area. The inclusions or xenoliths were probably plucked from the walls of the dacitic magma chamber or from the volcanic throat through which the dacite erupted.

An even closer view.

The dacite of Mount Helen is middle Pleistocene in age (K-Ar date of 249±12 ka, Clynne and Muffler, 2010) and part of what is called the Bumpass sequence of the Lassen volcanic center, the youngest of several volcanic centers of the area. The several volcanic centers are shown in Figure 4 here, and described more extensively here.

Portion of the Geologic map of Lassen Volcanic National Park and vicinity, California, Sheet 1, courtesy USGS.

The dacite of Mount Helen is indicated as "dh" on the geologic map. Photos were taken just downhill and west of the peak of Mount Helen.

Selected Reference:

Clynne, M.A., and Muffler, L.J.P., 2010, Geologic map of Lassen Volcanic National Park and vicinity, California: U.S. Geological Survey Scientific Investigations Map 2899, scale 1:50,000.