Transcript Lect4_surface hoarx

GEOL 410
New material
 Surface hoar
Photo: B. Pritchett
Mountain Snowpack
Surface Hoar
V Surface hoar
Another addition to the
snowpack that is technically
not a new snow crystal but
which can form a significant
layer is called surface
hoar.
 Radiation balance
 Relative humidity and
saturation
Mountain Snowpack
Relative Humidity
Definition:
The actual amount of water
vapor that at airmass at a given
temperature does hold to the
amount it could hold if it were
saturated at that temperature.
The amount of vapor in the
mix varies from time to time
and from place to place.
When the atmosphere
contains little water vapor, it
has low humidity.
When there is a lot of water
vapor present, the air has a
high humidity.
Surface hoar and energy balance
Mountain Snowpack
Relative humidity
When there is so much water vapor
in the air that condensation occurs
and clouds, mist, or fog form, the
airmass is saturated.
How much water it takes for
saturation to occur depends on the
temperature and humidity of the air.
Warm air can hold more water vapor
than cold air.
It takes more vapor to saturate a
warm airmass and less vapor to
saturate a cold airmass.
Mountain Snowpack
Relative Humidity
Temperature and Relative Humidity
When an airmass is saturated, it has reached 100% relative humidity (RH).
This tells us that the air at this place and time is holding all the vapor it
possibly can.
Mountain Snowpack
Dew Point
 The temperature a given airmass
must be cooled to attain saturation
(100% RH).
 If the current temperature of an
airmass is –10ºC, and if cooling it
to -14ºC would bring it to 100% RH.
Then the dewpoint of that airmass
is -14 ºC.
 At a temperature of -14 ºC the
airmass would become fully
saturated with water vapor.
If we cool an airmass the
concentration of water vapor
will rise.
If we cool it enough, it will
eventually become saturated
(even though no water vapor
has been added).
Mountain Snowpack
Formation of Dewpoint
Sometimes, only a very small
portion of the airmass gets
cooled to its dewpoint.
In summer, this occurs where the
air is in contact with a cool
surface (e.g., front lawn or car).
When this happens, we may not
see fog or mist but the thin layer
of air in contact with the lawn or
car will drop moisture onto the
cool surface just like the fogbank
makes your skin feel damp.
When an airmass is fully saturated,
it contains so much water vapor that
anything that it touches will become
damp or wet.
If we cool an airmass just a bit
beyond its dewpoint, condensation
occurs and clouds form.
If this occurs near or at the ground
we would call the clouds mist or fog.
Further cooling (and the presence
of a proper nucleus) will lead to
precipitation (rain if above freezing
and snow if below freezing).
Mountain Snowpack
Formation of Surface Hoar
Put a glass into a freezer, and you
let the glass get very cold, ice will
form on the glass instead of water
when you bring it into the warm
room.
In this case, the water vapor
becomes ice without going through
a liquid phase.
The glass has cooled a very thin
layer of air at the air/glass interface
to the dew point and water vapor in
the air has condensed onto the
cool glass.
Surface hoar is the winter
equivalent of dew.
Mountain Snowpack
Formation of Surface Hoar
Under certain conditions, the
surface of the snow cools a
thin layer of air at the
snow/air interface to the dew
point.
This causes water vapor to
deposit as ice on the
snowpack in the same way
that ice formed on the
freezing-cold glass in the
example above.
Surface hoar is not limited to forming on
snow; it is often seen on trees, bushes,
rocks, etc. and is sometimes referred to
as “hoar frost” in non-technical circles.
The surface hoar you see on
the snowpack in winter
comes from the air that was
in contact with the snowpack.
Mountain Snowpack
Formation of Surface Hoar
V Surface hoar
Surface hoar crystals have a
characteristic “icy” look and
often glitter as they refract
sunlight.
In its classic form, surface
hoar has a feathery V shape
but it can also form as
needle, plate, and hollow six
sided varieties.
Generally, striations are
visible on the crystals; these
are caused by successive
drops of moisture from the
air onto the surface.
Mountain Snowpack
Type of Surface
hoar
Condition of
formation
Form
Forecasting
conditions
Needles
Very cold, T<-21°C
Tiny Needles
Less persistent,
doesn't form thick
layers
Feathers
Normal
Temperatures
Feathers
Persistent, but is laid
down more easily
than wedges
Wedges
Normal
Temperatures
Wedges
Very persistent and
tends to remains
upright
Surface hoar makes perhaps the
perfect avalanche weak-layer.
It's thin, it's very weak, it's
notoriously persistent and it
commonly forms on hard bed
surfaces, which are also slippery.
Photo: T. Murphy
Finally, thin weak-layers tend to fail
more easily because any shear
deformation within the snowpack is
concentrated into a small area.
Surface hoar can also fail in
shear when the first snowfall
lays the surface hoar crystals
over on their side; they remain
as a paper-thin discontinuity in
the snowpack with very poor
bonding across that layer.
These laid-over crystals,
however, tend to bond up more
quickly than the ones that
remain standing on end.
Mountain Snowpack
Conditions that promote surface hoar growth
Cold Clear Nights
Snow surface re-radiates energy (LWR) to atmosphere cooling snow
surface, warm air cools an night and becomes saturated, water molecules
condense to ice from moist air on colder snow surface.
Snow temperature must be below air temperature
Note:
Large near-surface air temp gradient under clear sky alone is insufficient for
condensation
VERY FEW COLD CLEAR NIGHTS PRODUCE SURFACE HOAR
Mountain Snowpack
Conditions that promote surface hoar growth
Need light breeze 1-3 m/s (NOT 0; NOT 5 m/s)
Best if warm cloudy day followed by cold clear night (high vapor pressure
followed by cold)
High cirrus clouds at night reduce radiation loss and cause decline in
surface hoar formation
Fog reduces radiation loss and inhibits surface hoar formation
Crystal type depends on Temperature
Feathers: -12.5° to -21.0° C
Needles: <-21.0°C
Sector Plates and Needles oriented w/in a few degrees of surface normal
No surface hoar in concavities (reflected radiation in cavity inhibits)
Low strength crystal form
Mountain Snowpack
Conditions that promote surface hoar growth
Elevation
Lower elevations (cold air sinks and is saturated)
May form bathtub ring at boundary between warm and cold air part way
up the mountain above the fog line.
Not where there is fog
Aspect
Formation
North aspect larger temperature gradients and larger crystals
Depends in part on solar exposure
Windward, Lee (different wind speeds)
Presence/absense of trees
Can be destroyed
Wind (windward, lee)
Blow away or sublimate
But what happens if the air in
the valley bottom becomes so
humid it turns into fog?
Study in SW Montana: Compare surface hoar formation in Open, Clearing, Forest
(80% sky cover), Holler (1998).
Open
Forest Clearing
Forest
Tair
warm
Similar to open
Cold (during day; shade?)
Wind Speed
0.8 m/s
Similar to open
0.5 m/s (less wind)
Relative Humidity
Slightly lower
Similar to open
Slightly higher
Radiation Balance
Clear Night
Lower (-60 W/m2)
Lower (-60 W/m2)
higher (- 30 W/m2)
Radiation Input
Longwv in=190W/m2
Longwv in=200W/m2
Longwv in=240W/m2
Vapor Flux
Downward
+0.9 x 10-6 kg m-2 s-1
upward
~ 0 kg m-2 s-1
upward
-3 x 10-6 kg m-2 s-1
Observed
Surface Hoar
No Surface Hoar
No Surface Hoar
If RH increased from
65 % observed
Surface Hoar
Possible Surface Hoar
No Surface Hoar
Mountain Snowpack
Conditions that promote surface hoar growth
Clear skies: promote cooling of the spx through radiation loss that produces
a cold surface for surface hoar growth.
Calm winds: too much wind prevents the air to reach the dewpoint. A very
light exchange of air at the surface promotes growing large surface hoar
quickly as the exchange replenishes vapor supply.
Sheltered terrain: reduces wind effects.
Cooling air temperatures: increases relative humidity.
Calm winds: allows humidity to concentrate undisturbed near the surface of
the snow.
High relative humidity: more moisture available for surface hoar growth.
Proximity of water vapor sources: open water, moist ground, and warm
vegetation. help increase the relative humidity of the airmass.
Mountain Snowpack
Physical processes and surface hoar growth
 Vapor exchange processes between the snow surface and the lower
atmosphere lead to mass sublimation or deposition.
 Sublimation usually occurs during day
 Deposition often co-occurs at night with the formation of surface hoar.
 Measurements indicate that turbulent vapour fluxes (3m above the
surface) are entirely responsible for the mass gain in the snow pack and
thus of the formation and growth of surface hoar.
 Meteo-data suggest that local katabatic winds from nearby slopes during
nights of surface hoar development significantly contribute to the turbulent
fluxes measured near the surface and thus to the growing of surface hoar.
Distribution Pattern of Surface Hoar:
Where we are most likely to find surface hoar after a clear, calm night.
THE EFFECTS OF SLOPE ASPECT ON THE FORMATION
OF SURFACE HOAR AND DIURNALLY RECRYSTALIZED
NEAR-SURFACE FACETED CRYSTALS: IMPLICATIONS
FOR AVALANCHE FORECASTING
S. Cooperstein, Karl W. Birkeland, and Kathy J. Hansen
Presented at 2004 ISSW
Evidence that slope aspect plays a significant role in the
mountain range scale spatial variability of surface hoar.
Explain the differences in crystal
size between N and S slopes?
Explain the TG differences
on N and S aspects?
The wind speed at both sites are close
to the range reported by Colbeck (1988)
and Hachikubo and Akitaya (1997a,b)
for optimal surface hoar formation and,
although the average speed at the
south-facing site was about 1m/s higher,
its role was considered equal.
The relative humidity was measured at
only one location. It was considered to
remain relatively constant throughout
the massif (Figure 6).
Near-surface facets
Jan 13th event
A clear difference between the size and characteristics of surface hoar and
near-surface faceted crystals on two different aspects.
Surface hoar grew larger and showed more striations at the north-facing
site than at the south-facing site
Near-surface facets were better developed at the south-facing site than at
the north-facing site.
This difference was due to the relatively larger shortwave solar gains that
occurred at the south-facing site relative to the north-facing site.