What Causes "Flat" Light

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 Sunday, March 30th

Flat light is produced when most of the light from the sun isn't direct but is instead diffuse.  When skies are covered by thick clouds, the sunlight is not direct because it has been scattered by the liquid water and ice particles in the clouds.  As a result, the Earth's surface receives sunlight from all parts of the sky, resulting in a lack of shadows.  This is particularly problematic for seeing contrasts in a white surface like snow.  

High clouds sometimes vary in depth, so on a day like today, there can be variations in the flatness of the light.  Below is a photo I took looking up upper Sleepy Hollow off the Supreme Chair at 11:07 AM.  At that time, the sun was only partially obscured and there was enough direct radiation for the trees to cast shadows and for one to see some texture in the snow, as evident in the foreground of the picture.  

On our next run, at 11:33 AM, the sun was nearly obscured and there was considerably less direct radiation.  Shadows were less apparent and the texture and variability of the snow were harder to see.  

Such conditions though are not anywhere near as bad as they can get.  Add fog and one can have a hard time telling up from down.  Balance becomes difficult and one can even suffer from vertigo.  

Goggles can help some.  Color tints with higher light transmission that enhance contrast are usually best on flat light days.  Skiing closer to trees and other darker objects can also help.  

One of Bruce Tremper's ten commandments of avalanche safety is "thou shalt never go first."  This commandment is especially important on powder days with flat light, as a set of tracks helps to provide some contrast in an otherwise featureless landscape.  Be generous to your friends on these days and let them have first tracks.

Dusty Spring Morning

With yesterday's strong south winds, dust moved into Salt Lake Valley and environs yesterday and lingers this morning with visibility somewhat reduced as one looks to the central Wasatch from the University of Utah.

Source: https://home.chpc.utah.edu/~u0790486/wxinfo/cgi-bin/uunet_camera_explorer.cgi

Observations from the University of Utah show strong south winds yesterday afternoon and evening with PM2.5 levels increasing after about 1600 MDT from 3 to 8 ug/m3.  After settling some through 0200 MDT, they then climbed again to 13 before settling some this morning.  

These numbers are not high and are at good to low-moderate air quality levels, but nevertheless, there's plenty of dust in the air.  

Give the overall flow yesterday, the dust in the Salt Lake Valley was likely from sources to the south and the southwest given the flow direction and not from the exposed Great Salt Lake bed.  The flow direction was such that if there was dust emitted from the Great Salt Lake bed, especially Farmington Bay, it would have been transported to the northern Wasatch Front.  I'm not sure if there could be a some dust from the lake bed mixed in now given the shift to northwest flow last night but I suspect most of this is still from origins to the south and southwest. 

Dry Spring -> More Typical Spring

The work week has proven spectacular with mostly sunny skies and mild temperatures.  "Anything season" is here, meaning that you can do pretty much any type of recreation you desire, sometimes on the same day.  The lower-elevation trails have dried out now in many areas and are quite passable for hikers and bikers. Meanwhile, we still have a deep snowpack at upper elevations.  

Today's sunrise from the Avenues foothills.

We will, however, be transitioning from the dry spring pattern we've experienced the last few days to a pattern that will bring the occasional mild spring storm system to northern Utah.  This transition won't necessarily be abrupt as a couple of weak systems move through Friday and Saturday.  After that, there's a series of troughs that move through.  It's not easy for me to summarize my expectations for the timing and strength of these storms as there's a good deal of variability in the Utah snow ensemble.  Just look at the spread at the end of the 10-day period.  

The best way to summarize this is perhaps simply to say changeable, with the occasional mountain snowstorm interspersed with breaks.  It's a time for adaptation to what Mother Nature brings rather than having strong expectations for what's to come.  Fortunately, spring in northern Utah means a plethora of options.  

Downtime for weather.utah.edu

Due to utility work at the University of Utah data center, weather.utah.edu is expected to be down for approximately two days.  Don't shoot the messenger. 

In the meantime, enjoy the sunny, warm March weather and stop worrying about the next storm.  The forecasts looked like spaghetti anyway, so who the hell knows what's going to happen.  

In the meantime, for your entertainment purposes, there were some beautiful lenticular clouds over the central Wasatch yesterday.  

Such clouds are produced by mountain waves, up and down motions produced by the interaction of the atmospheric flow with the topography.  The clouds form where the flow is forced up and dissipate where the flow is forced down.  The layering is produced by vertical variations in relative humidity, which causes saturation in areas of ascent to be reached at different levels of vertical displacement.  

Lenticular clouds have been confused with flying saucers.  Let's not let this start a social media conspiracy theory.  

Avenues Avalanche!

Let's have some fun with a post based on what I'll call "sidewalk science."  

The Hall of Fame Baseball catcher Yogi Berra once said "you can observe a lot just by watching."  This applies for sure to both meteorology and snow science.

This afternoon, right near my house, I encountered an avalanche in the front yard of a house on 16th Avenue in the upper Avenues.

Oh, I didn't actually see the avalanche happen, but I did see the aftermath, which I've summarized in the below.  It contains many of the hallmark characteristics of an avalanche.  The first is the starting zone, which in this case I measured to be 35°.  Most avalanches start on slopes between 30 and 45 degrees, so the slope here was right in that range.

There is also the crown or crown line, which is the upper fracture line of the slab that broke away to create the avalanche, as well as the flank or fracture line along the side of the avalanche.  There is one on the other side, although I have not labeled it.   

Most avalanches that kill people are slab avalanches in which a cohesive layer of snow slides down the slope.  Indeed that was the case here with the slab sliding about 20 inches or 50 cm down the slope as a cohesive layer.  In this case, the length and speed of the avalanche was such that the slab remained intact.  However, eventually it encountered the lower angle sidewalk where the slab decelerated, debris built up, and the toe of the avalanche was found.  

The bed surface for this avalanche was the grass rather than a weak layer in the snow.  The trigger was likely solar heating, which led to a lubrication of the grass by liquid water as the snow melted, weakening the bond between the snow and the grass.  Such an avalanche is referred to as a glide avalanche, which is defined as a release of snowcover as a result of gliding over the ground.  

So much snow science in such a small space!  However, there are real-world applications.  Glide avalanches can be deadly and destructive.  In the Wasatch Range, they are probably most common in Broads Fork, a subdrainage of Big Cottonwood Canyon where glide avalanches commonly occur on steeply sloped rock slabs.  Here's a link to a report about one in Broads Fork: https://utahavalanchecenter.org/avalanche/69596. Note the fully exposed rocks in the starting zone of the avalanche.  

Glide avalanches are hard to predict, but can have deadly consequences.  They are also common in the Alps where the mountains are not only steep, but there are a lot of grassy, high altitude meadows that are grazed in the summer.  The resulting steep, grass slopes can make the perfect bed for an avalanche.  I saw the aftermath of many glide avalanches when I lived in Innsbruck, Austria.  Below is a photo looking down at one during a ski tour on the Gammerspitz, a 2500 meter peak in the Alps south of Innsbruck.  It was on a south facing slope and likely was triggered by solar heating like the avalanche I saw today in the Avenues.  We avoided slopes like this on our tour! 

I found the number of glide avalanches and the expansive coverage of glide cracks in the Alps to be very disconcerting and often made conservative terrain choices because of them.  Fortunately it was easy to avoid avalanche terrain today in the Avenues.

It Shall Get Warm

It's sort of weird after a winter with little valley snow to look out the window and see what might be the deepest snowpack of the season in my yard.  Admittedly, "deepest" in the valley this year is a low bar, but it will be good for the gardens eventually.  It was a wonderful storm for skiing too.

I can summarize the forecast for the next several days in two words: warming trend. Oh, today will be a cloudy "meh" day and Friday night and Saturday we'll have a trough passage that could bring a bit of snow, but after that a big ridge builds into the west.  Below is the GFS forecast valid 0000 UTC 27 March (6 PM Wednesday) showing the high-amplitude ridge parked just upstream of Utah.  

The NWS National Blend of Models maximum temperature forecast for Salt Lake City shows the warming trend nicely after Saturday, with temps climbing through Thursday.  

The median maximum temperature forecast for Thursday afternoon is 81°F, which would be the earliest 81 on record.  I thought this NBM forecast seemed a little jacked, but mid 70s seems doable.  

Bottom line: It shall get warm.