Goals

1. American Institute for Avalanche Research & Education Level II Avalanche Course Goals 1) Snowpack development and metamorphism. 2) Standardized observation guidelines & recording formats for factors that indicate snow stability.

2. American Institute for Avalanche Research & Education Level II Avalanche Course Video

3. American Institute for Avalanche Research & Education Level II Avalanche Course Objectives • Formation of new snow and surface hoar • Metamorphism of snow on the ground due to direct and indirect weather. • Observation guidelines and recording formats for snowpack factors.

4. American Institute for Avalanche Research & Education Level II Avalanche Course Enable judgment-based decision making. A complete understanding of the ‘science and technology” builds the foundation for practical decision making in the field later. This course is designed to bridge the gap between the basics taught in the AIARE Level 1 Avalanche Course and advanced decision making which is the focus of the AIARE Level 3 Avalanche Course. You will understand how the mountain snowpack forms and evolves, how avalanches are triggered, and how to observe and record snow stability data. A stability analysis and forecasting framework will be introduced. 

5. American Institute for Avalanche Research & Education Level II Avalanche Course • Major points: • Avalanche Types and Characteristics • Avalanche Terrain • Creation of the Mountain Snowpack • Metamorphism of the Mountain Snowpack • Avalanche Danger Scale • Avalanche Danger Factors • Travel Techniques • Planning and Preparation • Decision Making

6. Snow stability Snow stability is defined as: The likelihood that an avalanche will start. When we discuss snow stability we are estimating how easy it will be to trigger a slide and, in general terms, where avalanches might occur. Snow stability does not take into account how large an avalanche might be, its destructive potential, or characteristics.

7. Snow stability • Snow stability is rated on a five step scale: • Very poor • Poor • Fair • Good • Very good stability. • Very poor = least stable condition • Very good = most stable. • Even very good snow stability (e.g., the best stability that can be expected) avalanches are still possible

8. Avalanche Hazard Avalanche hazard is defined as: The likely consequences if an avalanche does start. Avalanche hazard takes into account snow stability and adds a variety of factors such as characteristics, terrain, destructive potential, and what kind of effect an avalanche might have

9. Avalanche Hazard To assess avalanche hazard, one has to take into account snow stability analysis and forecast as well as a variety of other factors that influence the destructive potential an avalanche might have should one occur and the likelihood that people or facilities will become involved in a avalanche. Level 2 course curriculum introduces snow stability analysis and forecasting.

10. Discussion Why separate stability and hazard? Reasonable judgment based decisions are based on analysis and forecasting of avalanche hazard. Stability is a critical part of the process that leads to a hazard analysis and forecast. Stability is a discrete factor in the hazard analysis process. It is essential that we deal with stability as a separate and distinct subject before we delve into hazard.

11. Learning Outcomes The basics of the formation of new snow. Riming and graupel formation. Surface hoar formation. Creation of a layered mountain snowpack. Recognize basic new snow crystals. Recognize surface hoar. Mountain Snowpack

12. Snow crystals form in the atmosphere. These crystals are created when water vapor condenses (deposits) as ice on a crystalline nucleus. Depending on the temperature and humidity in the regions where snow is forming, new snow crystals take a variety of shapes and sizes. Individual particles of atmospheric snow are generically called crystals. Mountain Snowpack The process of vapor becoming a solid is called condensation. Often refer to this process as “deposition.”

13. Mountain Snowpack The word snowflake refers to a larger structure which is formed when individual crystals join together into a “raft” as they descend to the ground. In reality atmospheric (new) snow comes in a variety of shapes and sizes. We recognize a number of sub-classes which reflect the main types of new snow. The classic “stellar” shape is what most people visualize when we talk about a new snow crystal.

14. Mountain Snowpack When a new snow crystal gains enough weight to overcome gravity and any updrafts that might exist in the airmass, the crystal falls from the atmosphere and eventually lands on the ground. This creates what we refer to as the snowpack which is really just the total accumulation of new snow that has fallen to the ground to date in a given winter.

15. Sub-class Shape Growth Environment Columns Short prismatic crystal, solid or hollow High supersaturation at -3o to -8o C, below –22 o C Needles Needle-shaped, approximately cylindrical High supersaturation at -3 to –5o C Plates Plate-like, mostly hexagonal High supersaturation at 0o to -3o C, -8 to-25o C Stellars (dendrites) Sixfold, star-like, planar or spatial High supersaturation at -12o to -16o C Irregular Crystals Clusters of very small crystals Varying environments Graupel Heavily rimed particles Super-cooled water in airmass causes riming Sub-classes, shape, and atmospheric conditions of snow

16. Each of the sub-classes in turn has numerous variations. More than one sub-classes and/or variation can form in a single storm as the temperature and humidity regimes change. It is not uncommon to see several different types of new snow during a single storm, that change back and forth over short periods of time (minutes to hours). Mountain Snowpack Sub-classes Columns Needles Plates Stellars (dendrites) Irregular Crystals Graupel

17. Mountain Snowpack When a new snow crystal gains enough weight to overcome gravity and any updrafts that might exist in the airmass, the crystal falls from the atmosphere and eventually lands on the ground. This creates what we refer to as the snowpack which is really just the total accumulation of new snow that has fallen to the ground to date in a given winter. Sub-classes Columns Needles Plates Stellars (dendrites) Irregular Crystals Graupel

18. New snow, stellar

19. Under some conditions, tiny water droplets form in the atmosphere and remain in a liquid state at temperatures below 0o C. These water droplets are described as super-cooled. When a super-cooled water droplet comes into contact with any surface or object, it immediately adheres to the surface or object and freezes, forming a small spherical piece of ice. Mountain Snowpack Riming This process is called riming. The tiny ice spheres are referred to as rime.

20. New snow, plate (rimed)

21. New snow, needle (rimed)

22. New snow, capped columns

23. Mountain Snowpack The most visible form of rime is when super-cooled water is driven against a surface by wind. Under these conditions, rime accretes on the windward side of the surface and creates a kind of icy stalactite formation which grows larger as additional rime is added. These rime formations are often seen on rocks, trees, communication towers, etc. in wind exposed areas, especially in maritime climates. Riming Rime can also deposit on the surface of the snowpack where it often takes the form of a white, crunchy crust.

24. Mountain Snowpack Under heavy riming, a new snow crystal can accumulate so much rime that its original form becomes completely obscured, eventually forming a roughly spherical (seldom a perfect ball) pellet. Sometimes referred to as “pellet snow” this is what we call graupel. Atmospheric snow simply called new snow and use one symbol for all types of new snow when making field notes. The notable exception is graupel. Graupel We will identify sub-classes when we can clearly identify the grain type and its significance in terms of stability

25. Mountain Snowpack These are the most common sub-classes. Additional specialized symbols for new snow sub-classes are in the American Avalanche Association’s Observation Guidelines (SWAG) book. The SWAG book is on-reserve. Symbol used to identify new snow in field notes +New snow +rRimed new snow Graupel A lower case “r” is added when riming is prevalent but the grain is still recognizable as new snow.

26. 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. To understand surface hoar formation, we need to understand the concepts of relative humidity and saturation, dew point, and the formation of dew. Surface Hoar VSurface hoar

27. Mountain Snowpack One of the components of the atmospheric mix is water vapor. The amount of vapor in the mix varies from time to time and from place to place. When the atmosphere contains little water vapor we say it has low humidity. Conversely, when there is a lot of water vapor present, we say 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.

28. When there is so much water vapor in the air that condensation occurs and clouds, mist, or fog form we say the airmass is saturated. How much water it takes for saturation to occur depends on the temperature of the air. Warm air can hold more water vapor than cold air so it takes more vapor to saturate a warm airmass and less vapor to saturate a cold airmass. Mountain Snowpack Relative humidity

29. Mountain Snowpack When an airmass is saturated, we say 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. When a given airmass is less than fully saturated with water vapor, its RH is less than 100%. For example, if an airmass contains only half the water vapor required to bring it to saturation, we would say it has 50% relative humidity. Relative Humidity Misconception: The air doesn't "hold" water vapor in the sense of having some attractive force or capturing influence. Water molecules are actually lighter and higher speed than the nitrogen and oxygen molecules that make up the bulk of the air, and they certainly don't stick to them and are not in any sense held by them.

30. A warm airmass, say one at 30oC, that is at 50% relative humidity will have more water vapor in it than a cool airmass, say one at -10o C that is at 50%. In both cases, when the air becomes saturated it will be at 100% RH even though the total amount of water vapor will be different. That’s why we call it relative humidity. Mountain Snowpack Relative Humidity Relative humidity is a ratio that describes the actual amount of water vapor that at airmass at a given temperature currently holds compared to the amount it could hold if it were saturated at that temperature. Water is a a certain amount of water vapor can be resident in the air as a constituent of the air. of dry air can be expressed as volume percentage

31. Mountain Snowpack Say we take a cubic metre of air that is -10oC and we know it to be saturated. If we extract all the water vapor from it, weigh the water vapor, and we find that there was 200 grams of water vapor. We now know that it takes 200 grams of vapor to bring air at -10 to 100% RH. Relative Humidity Actual amount of vapor 200 g = Could hold 200 g at saturation 200 g X 100 = 100% 200 g

32. Mountain Snowpack If, at another time or place, we take a different cubic metre of air that is at -10oC, extract all the water vapor from it and find only 100 grams of vapor. We can deduce that the RH in this second example is 50%, because it actually contains only 50% of the vapor it could contain at saturation Relative Humidity Actual amount of vapor 100 g = Could hold 200 g at saturation 100 g X 100 = 50% 200 g

33. In our examples, we assume that only one variable is changed at a time. Thus, the air temperature remains constant or the amount of vapor remains constant when changing temperature. There are other factors that affect RH (e.g., atmospheric pressure) but are significant in in the formation of surface hoar. Mountain Snowpack Relative Humidity • Relative humidity changes when: • Water vapor is removed from an airmass (RH decreases) • Water vapor is added to an airmass (RH increases) • The airmass is warmed (RH decreases) • The airmass is cooled (RH increases)

34. In our examples, we assume that only one variable is changed at a time. Thus, the air temperature remains constant or the amount of vapor remains constant when changing temperature. There are other factors that affect RH (e.g., atmospheric pressure) but are significant in in the formation of surface hoar. Mountain Snowpack Relative Humidity • Relative humidity changes when: • Water vapor is removed from an airmass (RH decreases) • Water vapor is added to an airmass (RH increases) • The airmass is warmed (RH decreases) • The airmass is cooled (RH increases)

35. Mountain Snowpack Dewpoint 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 that 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.

36. 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.

37. Mountain Snowpack The droplets of dew you see on your lawn on a summer morning comes from the air that was in contact with the lawn during the night. How much dew you get (how many droplets of water there are on the grass and how large the droplets are) depends primarily on how much water vapor was in the air and how cool the lawn got. Formation of Dewpoint Place a glass in a refrigerator. When it has cooled, bring the glass into a warm room. Dew will form on the surface of the glass where it is in contact with the air. What’s happened is 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.

38. 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. Surface hoar is the winter equivalent of dew. Formation of Surface Hoar What is surface hoar?

39. 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.

40. Surface hoar layer on surface

41. 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 vee 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

42. 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 vaporsources: open water, moist ground, and warm vegetation. help increase the relative humidity of the airmass.

43. Snow climates Maritime Continental Intermountain There are three main snow climates, each of which has particular weather, snowpack, and avalanche characteristics.

44. Continental Intermountain Maritime Weather High Rate Large Accum. High Riming Low Rate Small Accum. Little riming or surface hoar Mod-High Rate Med–Large Acc. Much surface hoar formation. Precipitation Wind Transport Much pre storm Much in-storm Little post-storm Little pre Some-much in Much post Little-some pre Some-much in Some post Temperatures Cool Warm Cold Snowpack Shallow - mod, var. early winter. Deep - uniform later Shallow, variable Deep, uniform Depth/Distribution Uniform Rounded Strong over weak Faceted Variable, faceted early, more uniform, rounded later. Layering Temperatures Cool Warm Cold

45. Maritime Continental Intermountain Avalanches Avalanche Danger “Direct action” Many in-storm events, associated with significant storms. Some post storm events, usually ending within 24 – 36 hours “Delayed action.” Some in storm events, often associated with minor storm. Many post storm, days or even weeks later, often associated with little or no significant weather. Direct and delayed action Quick to rise Quick to fall Quick to rise Often slow to fall early season; quicker to fall late season. Slow to rise Often very slow to fall

46. Snow climates Since the different sub-classes new snow often fall during a storm and since each of these may have significantly different characteristics, it is not unusual to see different layers form in the snow that falls during a storm. Even if the storm snow is homogeneous, in most cases it differs from the surface of the snowpack it falls onto. This forms the first of what may be many layers in the mountain snowpack, the interface between the storm snow and the old snowpack surface being the boundary. Maritime Continental Intermountain

47. Snow climates Riming may occur and the snow climate has an influence on the type of snow that forms, weather conditions under which it is deposited, and the likelihood that surface hoar will form. Successive storm snow deposits, the weather conditions present during and between storms, riming, surface hoar deposits, and the snow climate combine to create a succession of layers in the snowpack as it develops over the winter. . Maritime Continental Intermountain Since there are layers in the snowpack, and if they are different from one another, the layers may not bond to each other. It is this layering that is the basis for the formation and release of avalanches.

48. What is my name?

49. What is my name?

50. What is my name?