Technical information, news, research, and opinion on avalanches, snow safety, and winter backcountry travel.

Friday, January 8, 2010

Weather Observations: Trend Analysis

( 1 + x ) × ( 1 + y )—Ms. Steffens

NOTE: Backcountry avalanche forecasting is composed of four interconnected elements: goal, people, awareness, and uncertainty. Four types of observations, human, terrain, snowpack, and weather are used in backcountry avalanche forecasting. { Edited on March 7th, 2010. }

Backcountry avalanche forecasting relies on observations of the terrain, snowpack, and weather inside a single valley or backcountry ski run. Observations of terrain, snowpack, and weather are made to increase awareness and manage uncertainty.

Weather Observations: The Direction Of Instability
The primary difficulty with weather observations is the quantity of data, wide area of influence, and the uncertain relationship between observations and instability. Despite these complexities, weather observations focus on discerning the presence of, and effects of, three key factors that create and influence snowpack instability. These factors are listed below in order of importance relative to snowpack instability:
  • Loading
  • Warming
  • Cooling
Warming, cooling, and loading are trends that can increase or decrease snowpack instability depending on the current structure of the snowpack and the magnitude, rate, and duration of the trend across space and time. Individual variables such as cloud cover, slope aspect, local terrain, weather patterns, and radiation balance contribute to these factors, but ultimately warming, cooling, and loading are the practical concerns.

Warming, cooling, and loading create new snowpack instabilities while simultaneously altering the current snowpack in ways that can increase or decrease instability. A primary source of confusion is the circular nature of trends related to these factors, especially the relationship between these trends and snow metamorphism. Temperature changes are a good example. A warming trend can create instability by melting the snow and decreasing its hardness. However, when subject to a gentler warming trend, the direction of instability will eventually reverse itself because warm temperatures promote settlement and strength gains throughout the snowpack in general. Trends for cooling and loading also create circular variations in the direction of instability.

The direction of instability depends on the magnitude, rate, and duration of the trend relative to the current state of the snowpack. In simple terms, it is important to understand:
  • Magnitude. How much cooling, warming, or loading is expected?
  • Rate. How fast will the cooling, warming, or loading occur?
  • Duration. How long will the cooling, warming, or loading last?
You are probably aware that avalanche formation is much more likely at some times than others. Trends and their magnitude, rate, and duration are significant sources of uncertainty with respect to the formation of avalanches across space and time. These trends are responsible for the development of both long-term and short-term instability.

Table 1.1. Partial list of key weather observations mapped relative to instability and stability. Do you notice the potential for each factor to increase and decrease instability depending on the magnitude, rate, and duration? The circular nature of these trends, and their effects on both new snow and the existing snowpack, along with times when more than one trend is active ( cooling + loading ), are strong sources of confusion and uncertainty in backcountry avalanche forecasting. Make a list of your own and experiment.
LoadingMediumFast2 Hours+
LoadingLargeSlow22 Hours-
WarmingLargeFast18 Hours+
WarmingSmallSlow14 Days-
CoolingLargeSlow12 Days+
CoolingMediumFast4 Hours-

Table 1.2. Contributors to warming, cooling, and loading.
PrecipitationAir TemperatureAir Temperature
SkiersCloudsClear Skies

The current state of the snowpack, and magnitude, rate, and duration of current trends, are extremely relevant to backcountry avalanche forecasting. Are you preparing to descend a steep slope covered in fresh snow even though temperatures have been rising quickly for the past two hours?

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