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.
Weather and Avalanche
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Avalanches can only occur in a standing snow pack. Typically winter seasons and high altitudes have weather that is sufficiently unsettled and cold enough for precipitated snow to accumulate into a snow pack. The evolution of the snow pack is critically sensitive to small variations within the narrow range of meteorological conditions that allow for the accumulation of snow into a snow pack. Among the critical factors controlling snow pack evolution are: heating by the sun, radiational cooling, vertical temperature gradients in standing snow, snowfall amounts, and snow types. Generally, mild winter weather will promote the settlement and stabilization of the snow pack; and conversely very cold, windy, or hot weather will weaken the snow pack.
At temperatures close to the freezing point of water, or during times of moderate solar radiation, a gentle freeze-thaw cycle will take place. The melting and refreezing of water in the snow strengthens the snow pack during the freezing phase and weakens it during the thawing phase. A rapid rise in temperature, to a point significantly above the freezing point of water, may cause a slope to avalanche, especially in the spring.
Persistent cold temperatures can either prevent the snow from stabilizing or destabilize a snow pack. Cold air temperatures on the snow surface produce a temperature gradient in the snow, because the ground temperature at the base of the snow pack is close to freezing; unless the snow pack is standing on glaciated terrain, in which case the temperature at the base of the snow pack can be significantly below freezing. When a temperature gradient greater than 10oC change per vertical meter of snow is sustained for more than a day depth hoar will form in the snow pack, through the thermal transport of moisture away from the depth hoar along the temperature gradient, from bottom to top. This layer of depth hoar becomes a persistent weakness in the snow pack, characterized by faceted grains forming either above or below crusts and slabs. When a slab lying on top of this persistent weakness is loaded by a force above the tensile and ductile strength of the slab and the shear strength of the persistent weak layer, the persistent weak layer will fail and generate an avalanche.
Any wind stronger than a light breeze can contribute to a rapid accumulation of snow on sheltered slopes downwind. Wind pressure at a favorable angle can stabilize other slopes. A "wind slab" is a particularly fragile and brittle structure which is heavily loaded and poorly bonded to its underlayment. Even on a clear day, wind can quickly shift the snow load on a slope. This can occur in two ways: by top-loading and by cross-loading. Top-loading occurs when wind deposits snow perpendicular to the fall-line on a slope; cross-loading occurs when wind deposits snow parallel to the fall-line. When a wind blows over the top of a mountain, the leeward, or downwind, side of the mountain experiences top-loading, from the top to the bottom of that lee slope. When the wind blows across a ridge that leads up the mountain, the leeward side of the ridge is subject to cross-loading. Cross-loaded wind-slabs are usually difficult to identify visually.
Snowstorms and rainstorms are important contributors to avalanche danger. Heavy snowfall will cause instability in the existing snow pack, both because of the additional weight and because the new snow has insufficient time to bond to underlying snow layers. Rain has a similar effect. In the short-term, rain causes instability because, like a heavy snowfall, it imposes an additional load on the snow pack; and, once rainwater seeps down through the snow, it acts as a lubricant, reducing the natural friction between snow layers that holds the snow pack together. Most avalanches happen during or soon after a storm.
Daytime exposure to sunlight will rapidly destabilize the upper layers of a snow pack. Sunlight reduces the sintering, or necking, between snow grains. During clear nights, the snow pack can strengthen, or tighten, through the process of long-wave radiative cooling. When the night air is significantly cooler than the snow pack, the heat stored in the snow is re-radiated into the atmosphere.
