Wasatch Weather Weenies

The quality of light strongly affects one's confidence as a skier.  On a sunny day, most of the solar energy (about 85% of it when the sun is high in the sky) is direct, meaning that it is traveling along a straight line from the sun to the Earth's surface.  This results in large brightness contrasts between directly illuminated surfaces and those in shadows.  Even subtle textures in the surface of the snow are easily seen.   Descending a groomed run at Ischgl, Austria on a sunny day dominated by "direct" solar energy. All else being equal, it is much easier to ski on such days.  It is easier to see the steepness of the terrain, variations in the snow surface, and subtle changes in the snow conditions.   However, on day with thick overcast, even high overcast, the light can be "flat." Flat light is a colloquialism for situation in which there are no shadows, as was was the case at times at Alta today (Sunday).   Flat light at Alta on Sun...

'Twas a cold but beautiful Christmas Eve in upper Little Cottonwood Canyon.  Alta was solidly above the clouds through our departure at 2 PM.  The photo below taken at about 11:30 AM from Gunsight shows the low clouds lingering over the Salt Lake Valley and in portions of the lower canyon.   By about 2:15 as we were headed down canyon, patch low clouds had reached the Snowbird base area and producing just enough ice crystals to produce a halo.  Very pretty.   Photo: Libby Snethen The clouds were likely forming just beneath an elevated inversion that had its base near 700 mb (10,000 feet), as suggested by the HRRR analysis for 1800 UTC (11 AM MST).   We normally think of inversions as being near the valley floor, but they are typically elevated when we are downstream of an upper-level ridge.  They descend with time as the ridge moves overhead.  That will happen over the next couple of days.  In fact, if you look at the HRRR fore...

It's pretty rare that stratus clouds, which otherwise are a zero on a meteorological scale of 1 to 10, can make for a great weather day, but they did today.   Morning dawned in the Salt Lake Valley with dreary, grey overcast in the form of stratus clouds covering the entire sky (the image below taken looking south from the U at 8:12 AM).  However, a look toward the Cottonwoods showed a brighter spot, suggesting that perhaps there was hope for sun at high elevations.   Source: University of Utah Department of Atmospheric Sciences Sure enough, the Alta webcam showed clear skies above a Sea of Stratus over the Salt Lake Valley.   Source: Alta Ski Area And the morning satellite loop was really quite incredible with snow capped mountains, especially the central Wasatch, Southern Wasatch, and Uinta Mountains, sticking up above the Stratus Sea.   Source: College of DuPage And, as we started out ski tour from Alta at 9:52 AM, skies were clear, althou...

Smoke has returned to northern Utah and with it degraded air quality and reddish sunrises and sunsets.  Below is last night's sunset showing a deep orange sun and the reddish hue of the smoke.   Many people believe that the sun is yellow, but in reality, the sun is white.  It emits all visible wavelengths of light at sufficient intensities that the sun in space appears white.   NASA photo of the sun and International Space Station in May 2011 So, it is a myth that Superman's powers derive from the yellow sun.  It's a good story, but the sun really isn't yellow. On the other hand, to those of us on the Earth's surface, it certainly appears yellow or, in some instances, orange or red.  This is because of the selective scattering of light by the Earth's atmosphere and constituents, especially aerosols.  Molecules in the Earth's atmosphere scatter shorter wavelength light, such as blues and violets, more readily than longer wavelength light like ...

It was a pretty good day for cloud viewing today.  First, there was a brief display of altocumulus undulatus at Alta.  Such cloud billows were likely produced by a phenomenon known as Kelvin-Helmhotz (KH) instability, which occurs when strong vertical wind shear leads to atmospheric overturning.  Source:  https://www.eoas.ubc.ca/courses/atsc113/flying/met_concepts/01-met_concepts/01b-special-clouds/billow.html Then there was a beautiful sunset, with lenticular clouds hanging over the central Wasatch.   These stationary clouds were generated as the atmospheric flow was forced over the Wasatch Range.  Below is a web-cam video looking south from the University of Utah illustrating the stationary nature of the clouds (apologies if Blogger has coarsened the resolution, but my time is tight).  Finally, there's nothing too exciting about the altocumulus clouds below, but it was a pretty sunset over the Oquirrhs with a weak sun pillar,...

A quick shout out to begin today's post to Mrs. Professor Powder, who celebrated her big five-oh in style today.  I must confess that I did pull the government weather-control lever to ensure bluebird skies and high-speed cruising for her birthday.   She has put up with my weather weenieness for just over half of those years and today was no exception.  So much to see!  Yes, the views were great, but did you spot the 22-degre halo? How about wind transport?  The photo below as taken about 1 PM.  At the time, winds on Mt. Baldy were a rather unimpressive 17 mph with gusts to 32.  No mind, there's plenty of low-density blower to transport and that speed will do it with snow like that.  We departed soon after, but I suspect that wind deposition rates increased further with the winds this afternoon.   With so much dry snow around, not just on the snowpack, but also in the trees, one could practically pick out the gusts by e...

I can't decide if cirrostratus or fog is my least favorite cloud, but after three days of high overcast and flat light, I'm leaning toward the former. Here's how things looked on Thanksgiving.  And here's how things looked today. Yup, cirrus monotony.  Come on Mother Nature, do something already!  Give us snow or give us sun! An irony of thick cirrostratus is that it produces flat light and difficult skiing, but it also results in outstanding visibility.  In fact, the best visibility is usually on days with high overcast.  This is because cirrus clouds reflect some of the incident sunlight back to space, so there's less scattering of sunlight by atmospheric gases and aerosols at low levels.  Below is the view from the Avenues foothills yesterday.  One can easily see Mt. Nebo, roughly 100 km distant, on the horizon to the right of the central Wasatch.

As the sun sets in the sky, its color often transitions from yellow to orange or even red.  Why? Sunlight is comprised of visible light of all colors and wavelengths.  Superman really didn't get his powers from the "yellow sun" because the sun isn't yellow, it's white. Source: http://www.stellarplanet.co.uk/ The white color reflects the fact that sunlight is comprised of light across the entire visible spectrum, which we can break down using the Roy G. Biv acronym where R=Red, O=Orange, Y=Yellow, G=Green, B=Blue, I=Indigo, and V=Violet.  This also orders the color by decreasing wavelength, with red having the longest wavelength and violet the shortest. As sunlight passes through the atmosphere, the shorter waves are scattered more easily than the longer waves.  This has two important effects.  First, it shifts the color of the sun from white toward the longwave or "ROY" part of the spectrum.  Second, it shifts the color of the sky, which deri...

A spectacular rainbow display could be seen looking toward the east bench this evening.  From my vantage point, the pots o' gold produced by the primary and secondary bows were on the University of Utah Campus, including one near Rice-Eccles Stadium.  No word on whether or not President Pershing was scouring the grounds hoping to add to the U's endowment. The primary bow was spectacular and featured at least one and possibly two supernumerary bows , purple arcs on the inside of the bow. I've had to doctor-up the photo below to bring the most obvious supernumerary bow out.

Yesterday's spectacular display is an example of a halo, not a rainbow Yesterday's spectacular display of colors in the noontime sky caught the attention of many people in the Salt Lake Valley and was widely covered by the media.  Upon further reflection, I'm thinking this was a classic circumhorizontal arc rather than an  infralateral arc .  In either case, the phenomenon is an example of a halo and not a rainbow, despite the rainbow-like color spectrum. Rainbows are produced by the interaction of visible light with water droplets, typically rain drops, although it's fairly easy to make a rainbow using the mist a garden hose if the sun is behind you.  As the light moves through these droplets, it is bent and reflected in a way that yields the classic red-to-violet color spectrum. Halos are produced by the interaction of visible light with ice crystals .  In the photo above, the fibrous nature of the cloud that contains the halo is an indication ...

There was beautiful halo with rainbow colors sitting over the University of Utah campus at noon today. I believe this is either a circumhorizon arc or an  infralateral arc , which form only when the sun is high in the sky (higher than 58º).  As a result, these types of arcs form only near noon from about April to early September at our latitude.  In this instance, I suspect it was an infralateral arc because it curved up slightly from the horizon.  More info at Atmospheric Optics .

I'm on a quick trip for a meeting at the University of Notre Dame and observed a gorgeous sundog during yesterday's sunrise at the Salt Lake City airport. Sundogs, also known as parhelia, are typically produced as light from the sun is refracted by ice crystals in cirrus clouds, as was the case above.  There is some decomposition of the light into a color spectrum, but the decomposition is not as dramatic as occurs with rainbows, which are produced by water droplets. The Atmospheric Optics site  run by Les Cowley is a good place to learn more about sundogs, halos, and similar phenomenon. The sites at Notre Dame are less dramatic and dominated by football.  I'm not a Notre Dame football fan, but did my best to grin and bear it. Not shown here is the hex I put on touchdown Jesus.

The Patch Springs Fire in the Skull Valley west of Salt Lake City put in a great optics display last night, creating a red hue in much of the sky and a blood red sun. The reddish hue is the result of a phenomenon known as Raleigh scattering , the scattering of light by particles that are much smaller than the wavelength of the light.  As you might recall from high school physics, Roy G. Biv (Red, Orange, Yellow, Green, Blue, Indigo, and Violet) describes the visible spectrum arranged in the order of decreasing wavelength.  Colors with shorter wavelengths at the end of the Roy G. Biv spectrum are scattered more by smoke than those with longer wavelengths at the beginning of the spectrum.  Thus, more of the longer, redder wavelengths of light survive transit through the smoke, leading to the reddish hue and the blood red sun. As the sun set, we observed the shadow of the smoke move up the Wasatch Mountains, especially near Mount Olympus.  This shadow had a d...

There was a great rainbow display this evening in the Avenues with very distinct primary and secondary bows.  The primary is the brighter, inside bow, but unfortunately, the pot of gold was located just a bit east of campus.  That's a pity as we're always looking for more funding. To compensate for my poor photographic skills and the fact that it was nearly impossible to take a picture given that wind was coming from the rainbow's direction and driving rain into the lens, I've doctored the photo below just a bit to help with contrast. There was some evidence of supernumerary bows on the inside of the primary bow.  I tried to get a good photo, but alas, it was beyond my skills. For you weather and optics geeks, Les Cowley's Atmospheric Optics site is the cat's meow.  He has a great page on rainbows .  I just learned that the dark area between the primary and secondary rainbow is known as Alexander's dark band  and is named for Alexander of Aph...

Today I was like a kid in a candy shop checking out the clouds and optical displays in Little Cottonwood Canyon.  I could write an entire book on what I observed, but will provide the CliffsNotes version for this post. 1. Context A special sounding collected in West Jordan for the Persistent Cold-Air Pool Study (PCAPS) provides much needed context for the photos and analysis that follows.  The sounding, launched at 1715 UTC (1015 MST), shows two distinct inversions.  The lower inversion was between about 800 and 775 mb and caps a surface-based layer that is nearly well mixed.  The upper inversion was located just above 700 mb, or around 10,000 feet above sea level.  It also shows that the winds throughout the troposphere were very light (10 knots or less below 400 mb).  In other words, there was weak large-scale flow and thermally driven circulations associated with daytime heating were likely to dominate circulations in the mountains. Courtesy NCA...