Hypothesis on solar effects

Here is something I've long been curious about. I think that many of my very best powder days have fallen between the 6th and 12th of March. Yet, I can probably count the number of great powder days I've enjoyed in April, or really past about March 25th, on two hands.

We all know that solar effects (especially through cloud) seem to matter little through the first 20 days or so of March on north slopes. Then, in the period of just a few days - from about March 20th to March 27th or so - it becomes very difficult to maintain powder snow even on north slopes, and especially with thin clouds or higher humidity. Now throughout this period the rate of change of sun angle is nearly constant and at it's maximum rate for the whole year, with the rate of change slowing only slightly beginning March 21st. But although the rate of change of sun angle in this time period of just 2-3 weeks is say approximately linear, the effect on the snowpack (again especially with thin cloud or high humidity) does not appear to be linear, but more like exponential. So, is it possible that there is a critical solar angle for reflectivity such that the effects of the sun on cloud cover and penetration are magnified after the the sun reaches a certain alpha angle?

This annual pattern seems to vary little with temperature (freezing level) so it becomes hard to understand how a few days can make such a radical difference. Obviously, on completely clear days, this effect doesn't seem to be as great; but then, with clear skies there are other variables involved - radiational cooling and evaporation, as well as snow surface reflectivity.

Any scientists out their with an opinion?

Hi Gary

Holy cow there's a lotta stuff to read before posting nowadays!

I'm pretty sure the sun's rate of change is at its maximum ON march 21st and the rate of change is approximately constant for an equal period in time before and after that date. It's equivalent to the sine of zero, where the rate of change looks the same in either direction.

I suspect your question is about powder that lasts past the first day of good weather. I've managed to find a good powder day in late April or May at least once in each of the last three years, always on north-facing steeps. I doubt there is a critical angle; my guess would be that at some point it's just too warm and there's too much sun.

But let's hear some wild theories!

Cheers,
Sky

It could be as simple as exposure to an upslope wind bringing warmer air up from a deep valley.

Well an interesting topic that should be near to everyone's heart, why the end of the "POW"?  I am grad student that does a bit of snow energy balance modeling and a few things come to mind when spring melt is about to set in.  First spring storms typically have warmer temps, the snowfall at the warmer spring temperatures (usually near zero) take little energy to metamorphosis or melt.  These warmer temps would also lead to larger crystals.  This is especially important in the near infrared wavelengths.  This increases the net energy input to the snowpack, promoting surface metamorphosis and melt.  Second, spring storms typically fall on crust layers.  These crust layers would impede percolation of free water in the upper layer of the snowpack, hence making the powder, heavier.  Also the underlying snowpack is near or at isothermal, so the lower snowpack would exchange energy to the new snow, typically warming it. 

In relation to sun angle, the lower the solar zenith angle the lower the albedo, increasing the net energy input.   So our longer days give us solar radiation at lower angles. 

Sensible heat exchange from warmer air temps can also not be neglected, however these exchanges will be small if there is little air movement.  But the larger the temperature gradient the more sensible heat exchange between the air and the snow.

The one EB component that one may glance over is the thermal radiation from the atmosphere.  If atmosperhic temps are high (in spring) than more moisture can be held.  The more moisture and higher temps up there the more thermal radiation down here.  So if we have longer days, warmer temps, we also get more thermal radiation.  Solar angle does not affect the thermal radiation, only how much sky a slope sees.  So find a slope that is north facing and has a limited skyview...that's where the powder is..

that's my 2cents..adam

Here is something I've long been curious about. I think that many of my very best powder days have fallen between the 6th and 12th of March. Yet, I can probably count the number of great powder days I've enjoyed in April, or really past about March 25th, on two hands.

I suspect that the availabililty of good powder days after mid-March doesn't fall off as precipitously as it may seem. Memory can play tricks on you, and probably none of us are old enough or active enough (even Gary!  ) to have accumulated enough data points to know for sure.

Still, there's no question that the "mean time before crud" (MTBC) gets progressively shorter as we move away from the winter solstice. Perhaps the threshold that Gary describes is that in mid-March the MTBC becomes less than 24 hours. If you're not out there right after the snow falls, chances are it will be crud.

A couple years ago, I had a memorable example of this. John Mauro and I traversed Mt Baker in the middle of June, right on the heels of a cold storm. We found powder on the Park Glacier (see photo below). I doubt that the good snow lasted more than a single day.

having done the watson's traverse exactly one week after Lowell did his trip and found that powder, i can assure you that powder didn't last. by the following weekend, we hit the upper park glacier and found calf-deep elephant snot...only time I've ever felt like i was in quicksand while skiing.
(still a very scenic trip though...)