Earthquake
An earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves.
Tsunami
A tsunami also called a tsunami wave train, and at one time incorrectly referred to as a tidal wave, is a series of water waves caused by the displacement of a large volume of a body of water, usually an ocean, though it can occur in large lakes.
Tornado
A tornado (often referred to as a twister or, erroneously, a cyclone) is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud.
Floods
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water
Volcanic Eruptions
Volcanoes can cause widespread destruction and consequent disaster through several ways. The effects include the volcanic eruption itself that may cause harm following the explosion of the volcano or the fall of rock.
Snow Structure and Characteristics
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The snow pack is composed of deposition layers of snow that are accumulated over time. The deposition layers are stratified parallel to the ground surface on which the snow falls. Each deposition layer indicates a distinct meteorological condition during which the snow was accumulated. Once deposited a snow layer will continue to evolve and develop under the influence of the meteorological conditions that prevail after deposition.
For an avalanche to occur, it is necessary that a snow pack have a weak layer (or instability) below a slab of cohesive snow. In practice the mechanical and structural determinants of snow pack stability are not directly observable outside of laboratories, thus the more easily observed properties of the snow layers (e.g. penetration resistance, grain size, grain type, temperature) are used as proxy measurements of the mechanical properties of the snow (e.g. tensile strength, friction coefficients, shear strength, and ductile strength). This results in two principal sources of uncertainty in determining snow pack stability based on snow structure: First, both the factors influencing snow stability and the specific characteristics of the snow pack vary widely within small areas and time scales, resulting in an inability to extrapolate point observations of snow layers. Second, the understanding of the relationship between the readily observable snow pack characteristics and the snow pack's critical mechanical properties has not been completely developed.
While the deterministic relationship between snow pack characteristics and snow pack stability is still a matter of ongoing scientific study, there is a growing empirical understanding of the snow composition and deposition characteristics that influence the likelihood of an avalanche. Observation and experience has shown that newly fallen snow requires time to bond with the snow layers beneath it, especially if the new snow falls during very cold and dry conditions. Shallower snow, that can lie above or around boulders, plants, and other discontinuities in the slope, will weaken from the presence of a stronger temperature gradient. Larger and more angular snow crystals are an indicator of weaker bonds within the snow pack, because the sintering process that forms bonds within the snow pack will also cause the snow crystals to become smaller and rounder. Consolidated snow is less likely to slough than either loose powdery layers or wet isothermal snow; however, consolidated snow is a necessary condition for the occurrence of slab avalanches, and will mask persistent instabilities within a snow pack. The empirical understanding of the factors influencing snow stability only places broad predictive bounds on the stability of the snow, consequently a conservative use of avalanche terrain, well within the recommended guidelines of the local avalanche forecasts and bulletins, is always recommended.
