GEOL 410

New material Surface hoar

Photo: B. Pritchett

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

V Surface hoar

Surface Hoar

Mountain Snowpack

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.

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.

Mountain Snowpack

Surface hoar and energy balance

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

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 SnowpackRelative Humidity

Temperature and Relative Humidity

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).

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.

Mountain SnowpackDew Point

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 SnowpackFormation 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.

Surface hoar is the winter equivalent of dew.

Mountain Snowpack

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.

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.

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

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.

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

Formation of Surface Hoar

V Surface hoar

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

Photo: T. Murphy

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.

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.

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 SnowpackConditions 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°CSector 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 SnowpackConditions 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

Mountain SnowpackConditions that promote surface hoar growth

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 BalanceClear 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 from65 % observed

Surface Hoar Possible Surface Hoar No Surface Hoar

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 SnowpackConditions that promote surface hoar growth

Mountain SnowpackPhysical 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 facetsJan 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.