[Chris B]

Okay, as queries are going around...

As I review the alpine forecasts around Vancouver, I note that there are going to be some funny inversions during the day - below freezing from sea level to 1500m, above freezing 1500m to 2000m and then below freezing above 2000m. At night, everywhere would be below freezing.

So, my question is, does this have any impact in creating future weak layers in the snow? I would expect there to be greater surface hoar formation on the "inverted" area, but am not sure.

[Rebob]

This condition could create some interesting layers, depending on how warm the inversion is. With only a 500m layer, it probably won't be too warm, but combined with aspect, this could help facilitate suncrust development.

We had the same condition last week, and during that time a suncrust did develop just at treeline on due south exposures. This weak crust has persisted beneath the 15cm of snow we received after the inversion reverted to normal patterns.

As far as surface hoar development is concerned, the fact that temperatures quickly revert to normal gradients after the sun goes down will probably minimize this effect, but this will also be affected by the humidity of the air parcel at the inversion layer.

If the inversion doesn't revert at night, there is some likelihood of surface hoar formation. For example, today, at 1900m in the Fitzsimmons Range (SE aspect), at 1300 hrs, air temperature was -0.5C, surface temp -3.5C, and 10cm down was -8.5C. If the steep gradient is able to persist during the day, there will be a very cold layer next to the snowpack as soon as the sun goes off the snow.

Interestingly enough, one hour after sunset, the temp at this elevation had returned to -11C. A moderate N wind didn't hinder this quick transition.

[Gary Brill]

Good analysis, Rebob.

Relative humidities, cloud cover and wind all affect surface hoar formation. Humidities are routinely measured at weather stations, mountain or otherwise. If you can't find relative humidities, you can find a meteorology chart called a "Sounding" in which atmospheric temperatures and dewpoints are plotted on a graph from sea level to high elevations. Under clear skies, the closer the temperature to the dewpoint, the more likely surface hoar formation is. At the dewpoint or very close to it, it is likely cloudy or foggy. Inversions are readily obvious from these "soundings".

Wind and clouds inhibit surface hoar formation, clouds by limiting radiational cooling; wind by mixing the atmospheric layer adjacent to the snow surface, removing the vapor pressure gradient that drives surface hoar formation.

Of course, this is academic, surface hoar formation is routinely reported on avalanche forecasts.

[Rebob]

Chris, further to your report from last night--the new forecast is in and shows a stronger trend than today's (which didn't materialize, due to winds in the alpine, in Whistler).

ALPINE RECREATION FORECAST FOR THE SOUTH COAST MOUNTAINS ISSUED BY ENVIRONMENT CANADA AT 2 PM PST TUESDAY 4 JANUARY 2005.
THE NEXT MOUNTAIN FORECAST WILL BE ISSUED AT 5 AM WEDNESDAY.

Synopsis..Clear skies and brisk northeast winds are expected through Wednesday as a ridge of high pressure over the BC interior dominates the weather pattern. Temperatures will moderate somewhat with an above freezing layer developing this afternoon at Mount Seymour and on Wednesday at Whistler.

Whistler - Blackcomb.
Tonight..Clear. Alpine temperature minus 14. Freezing level surface. Mountain top winds northeast 50 to 80 km/h.
Wednesday..Sunny. Alpine high 2. Above freezing layer between 1200 and 3000 metres. Mountain top winds northeast 40 to 60 km/h.

Now an above freezing layer of 1800m thickness could create quite interesting conditions, but the forecast high is only 2C, and that is unlikely to really accomplish much, especially since this is a short-lived event and current temps are well below 0C in the alpine and predicted to drop to -18C within 24 hours.

What would concern me much more in this circumstance are the predicted 40-80km/h winds. This will undoubtedly cause more redistribution onto S and SW aspects (there is currently from 20-40cm foot penetration in the alpine) and, with the temp increase, facilitate more windslab creation on an aspect that doesn't normally slab and that already has low density snow on it, two buried surface hoar layers, and a weak suncrust with facetting beneath it and another raincrust (December 19th) 20cm below that with 10cm of facets.

In addition, backcountry skiers will be drawn to the sun and southern aspects as that is where untracked snow will be and some relief from -18C temps and a NE Arctic wind. This will bear watching.

[Valdez Telehead]

FWIW the Avalanche Handbook(USDA) defines temperature inversion as "the result of nocturnal cooling of the surface by raditaion loss"

[Gary Brill]

The inversion doesn't show up in the Cascades, but here is today's sounding from Spokane, showing a slight inversion at 830mb, or around 5500'. http://www.atmos.washington.edu/cgi-bin/latest.cgi?upperair/OTX

Meteorology Canada should have a similar chart for the Vancouver area and probably for Revelstoke.

Note that the temperature (on the right) and the dewpoint (on the left come close together around that same elevation, meaning that with a little lifting there may be a cloud deck with tops at 5500'. Otherwise, the atmosphere is very dry. Low humidities cause evaporation, no surface hoar and cool snow surface temperatures despite above freezing air temperatures in the alpine in your area.

[Valdez Telehead]

Pictures alway help. Here we see T.Pass enshrouded in "fog". This occurred after a heavy warm snow into the evening, followed by a very quick overnight cooling. Throw a little April radiant sunrise and this happens very quick. This inversion layer lingered most of the day. About 2pm, the winds picked up allowing mixing of the airs at ground and up high til the temperature gradient measured at different elavations became more or less equal. Note the precence of high cirrus, and indicator of cold air aloft.

But knowing about inversions allows you to know what lays above the ground, while others stay on the ground.