GEOL 410

1. GEOL 410 • New material • Surface hoar

2. Photo: B. Pritchett

4. Mountain Snowpack • 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 Surface Hoar VSurface hoar

5. Mountain Snowpack Relative Humidity 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. Definition: The actual amount of water vapor that at airmassat a given temperature does hold to the amount it could hold if it were saturated at that temperature.

6. Surface hoar and energy balance

7. Mountain Snowpack 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 aircan hold more water vapor than cold air. It takes more vapor to saturate a warm airmassand less vapor to saturate a cold airmass. Relative humidity

8. Mountain Snowpack Temperature and Relative Humidity 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.

9. Mountain Snowpack Dew Point 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.

10. Mountain Snowpack 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). 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.

11. Mountain Snowpack Formation of Surface Hoar Surface hoar is the winter equivalent of dew. 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.

12. Mountain Snowpack 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. 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.

13. Mountain Snowpack 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. Formation of Surface Hoar VSurface hoar

14. Mountain Snowpack

15. 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. Photo: T. Murphy

16. 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.

17. 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

18. 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

19. 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

20. But what happens if the air in the valley bottom becomes so humid it turns into fog?

21. Study in SW Montana: Compare surface hoar formation in Open, Clearing, Forest (80% sky cover), Holler (1998).

22. 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.

23. 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.

27. Distribution Pattern of Surface Hoar: Where we are most likely to find surface hoar after a clear, calm night.

28. 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.

29. Explain the differences in crystal size between N and S slopes?

30. Explain the TG differences on N and S aspects?

31. 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).

32. Near-surface facets Jan 13th event

33. 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.