What defines a hard freeze?

[Mike Shepherd]

This may be a bit of a n00b question, but what exactly defines a hard freeze? Looking at the info on a lot of the snotel sites it looks like many of the lows have been flirting with 32 but not going much below it or at all. Just hearing the term I would guess it means mid 20 temperature wise. It seems with the temps this year that's going to be less and less likely. Does that mean that we are on the tail end of couloir season on account of weather?

[realhillboarding]

Unfortunately there's a lot more to it than just temperature because radiant cooling plays a large role. With clear skies, I've seen it freeze hard with overnight lows in the low 40s, and with cloudy skies I've had it not freeze solid with lows in the low 20s. For the same reason, it's often slushy in the trees but frozen solid once you break treeline. The rate of radiant cooling depend on the temperature differential - if the snow surface is 32F and you have clear skies, the other end of the differential is outer space ( i.e. super cold) and you can freeze deep despite an air temperature of 40. If it's cloudy or there are trees, the other side of the differential is much warmer and you don't get as much radiant cooling, so the air temperature needs to be much colder to get the same level of freeze.

New snowfall also affects the depth of freeze because this has more air in the snow and insulates against a deep freeze. It also tends to freeze deeper on steep snow (not sure why, maybe the water drains out?), so there have been many times where the flats are still somewhat soft but as soon as you get to the couloir it's rock solid. Oftentimes with spring climbing you're flirting with the edge of a good freeze and you just have to bite the bullet, make the approach, and be willing to bail if it didn't freeze.

[San Juan Ron]

From an OFS (older, fatter, slower) guy, there is a joke in there somewhere with ED, Viagra, and go see a doctor after 4 hours......SJ Ron

[Tornadoman]

The National Weather Service seems to use a criteria of 28 degrees but only issue it during the growing season. I guess they don't want to issue it for 8 straight months in the mountains/colder climates! Here is a link to a whole ton of weather terminology from Wiki which includes this and a whole lot more.

http://en.wikipedia.org/wiki/Severe_wea ... _States%29" onclick="window.open(this.href);return false;

[bergsteigen]

It's what didn't happen last night. Clouds insulated warm snowpack, and added another couple inches of snow. Able to sneak in a morning ski before mush. Not the day to climb big lines.

[BillMiddlebrook]

It's what didn't happen last night. Not the day to climb big lines.

Yup, same thing yesterday in the Mosquito Range. We got to 12,800', only to find unsafe snow conditions and lack of a good freeze even though a decent freeze was forecasted. Aborted. Went fishing

[Mike Shepherd]

Bill, your trip report was the reason I asked. Just looking at the numbers things seemed reasonably similar to when I went up it a few weeks back.

[bonehead]

A hard freeze is when no one remembers
what Memorial Day is all about.
God bless those who gave all.

[Mike Shepherd]

Just because we're discussing mountain climbing doesn't mean we don't know the meaning of Memorial Day. Since you're going to presume I don't know what it means, I'm going to presume I spent more time in Afghanistan than you did.

[BillMiddlebrook]

Bill, your trip report was the reason I asked. Just looking at the numbers things seemed reasonably similar to when I went up it a few weeks back.

My Horseshoe Mtn report? Based on the conditions leading up to that climb, I knew we needed a fairly "hard" freeze to stabilize the upper layers of the snowpack. When we got there, we first encountered unfrozen water on the tundra at 12,000', at 8am. The temps seemed to be right around freezing and we continued on to see if cold air coming down off the route and the light breeze had kept the temps down on the upper route. We reached 12,800' before 9am and dug a pit to find that only the top 1/2 inch of snow was frozen overnight. The lower snow had fairly high water content and two weak layers under the most recent storm snow. If the most recent snow had frozen down a few more inches overnight, we would have proceeded. Not.

[Mike Shepherd]

Yes the horseshoe mountain report.

[BillMiddlebrook]

Unfortunately, it looks like we'll be having fairly warm overnight temps in the short-term and with all of the most recent snow on the peaks, wet slide danger will be elevated.