How to Interpret Avalanche Reports
Avalanche reports are produced daily by SAIS forecasters. SAIS forecasters carry out a hazard evaluation in the field for part of the day, returning to their base in the afternoon, they will then obtain a specific weather forecast from the Met Office and begin the process of constructing the avalanche report for their region. The area forecaster will discuss the situation with other forecasters and the SAIS co-ordinator after which the report is published.
Reading one avalanche report will provide insufficient information to enable someone to determine the complete avalanche hazard situation for an area for that day.
For a complete understanding of the avalanche hazard prior to your day in the mountains or hills, it is important to take into account the snowpack history: reading avalanche reports from the most recent days, and by monitoring snowpack evolution from the start of the winter.
Avalanche hazard is only one of the factors to consider when venturing into the mountains as a climber, walker, skier or snow boarder. In the decision making process it is important to consider together three important factors: the weather and mountain conditions, individual skill and experience levels, and the type of landscape to be travelled.
The ‘Be Avalanche Aware’ process clearly illustrates a simple process mountain users should utilise before and during their winter mountain and hill excursions.
Interpretation of avalanche hazard reports
The hazard compass rose
The hazard rose is a supplement to the text description and should not be used in isolation as it cannot completely portray the situation on the ground.
The distribution of hazard according to aspect and compass direction by forecasters is generally determined by using observations on the ground during field excursions, and weather forecasts.
When using the hazard rose it is important to consider that in any particular mountain area or corrie many aspects may be encountered. eg The North Face of Ben Nevis or the Northern Corries of the Cairngorms contain most aspects in addition to Northerly ones.
The distribution of snow in our winter landscape is mainly determined by the wind. Areas of wind scoured slopes and ridges and the accumulation of snow into wind sheltered slopes and places presents a situation of great variation. As the snowpack evolves during the winter, layers of snow are built upon by subsequent snow accumulations. This often presents us with a situation where localised weakly bonded areas are distributed in a variety of small locations on an otherwise stable snowpack or often bare ground (see photo). In the photo below even a small avalanche would have serious consequences.
Localised hazard definition
Hazard is termed ‘localised’ because of limited snowpack cover and/or weaknesses in the snowpack being confined to small areas which can release as an avalanche with a loading of one person or more. Even small areas, once triggered, can effect the whole slope because of the increased load and produce avalanches of serious consequence and greater size.
Localised hazard is presented on the hazard rose below as illustrated. Above 600 metres altitude and below 900 metre altitude on north-west through north-east to southeasterly aspects the snowpack has localised areas of moderate hazard. Above 900 metres altitude on north-west and southeast aspects the snowpack has localised areas of considerable hazard. On north to east aspects above 900 metres the hazard is considerable. On all south to west aspects above 600 metres the hazard is low.
Important considerations when interpreting avalanche hazard reports
Identifying avalanche hazard in the hills and mountains throughout the winter is a challenging process.
Constantly changing weather factors, from temperature and snowfall to wind speed and direction can affect the strength and stability of the snowpack.
It is important to keep a close watch on conditions during the season and especially throughout any mountain excursions.
It is recommended that as well as avalanche hazard, other factors should be taken account as outlined in the ‘Be Avalanche Aware’ process.
The planning phase and the information gathered before going into the mountains is the most important and will provide you with 70-80% of your hazard evaluation information.
Other useful information for determining avalanche hazard
Avalanche maps provide up to date information on the location of avalanche activity, providing key information on snow stability in respect of altitudes, aspects, and locations.
They are updated daily by forecasters and from avalanche reports that are provided by the public and provide recent avalanche activity and therefore snow stability information which can be incorporated into any planning.
All avalanche reports are checked before being published to the map.
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