Avalanche

An avalanche (also called a snowslide or snowslip) is a rapid flow of snow down a sloping surface. Avalanches are typically triggered in a starting zone from a mechanical failure in the snowpack (slab avalanche) when the forces on the snow exceed its strength but sometimes only with gradually widening (loose snow avalanche). After initiation, avalanches usually accelerate rapidly and grow in mass and volume as they entrain more snow. If the avalanche moves fast enough some of the snow may mix with the air forming a powder snow avalanche, which is a type of gravity current.

Slides of rocks or debris, behaving in a similar way to snow, are also referred to as avalanches. The remainder of this article refers to snow avalanches.

The load on the snowpack may be only due to gravity, in which case failure may result either from weakening in the snowpack or increased load due to precipitation. Avalanches that occur in this way are known as spontaneous avalanches. Avalanches can also be triggered by other loads such as skiers, snowmobilers, animals or explosives. Seismic activity may also trigger the failure in the snowpack and avalanches. A popular myth is that avalanches can be triggered by loud noise or shouting, but the pressure from sound is orders of magnitude too small to trigger an avalanche.

Although primarily composed of flowing snow and air, large avalanches have the capability to entrain ice, rocks, trees, and other material on the slope, and are distinct from mudslides, rock slides, and serac collapses on an icefall. Avalanches are not rare or random events and are endemic to any mountain range that accumulates a standing snowpack. Avalanches are most common during winter or spring but glacier movements may cause ice and snow avalanches at any time of year. In mountainous terrain, avalanches are among the most serious objective natural hazards to life and property, with their destructive capability resulting from their potential to carry enormous masses of snow at high speeds.

There is no universally accepted classification of avalanches—different classifications are useful for different purposes. Avalanches can be described by their size, their destructive potential, their initiation mechanism, their composition and their dynamics.

Explanation from: https://en.wikipedia.org/wiki/Avalanche

Hummingbird hawk-moth

The hummingbird hawk-moth (Macroglossum stellatarum) is a species of Sphingidae. Its long proboscis and its hovering behaviour, accompanied by an audible humming noise, make it look remarkably like a hummingbird while feeding on flowers. It should not be confused with the moths called hummingbird moths in North America, genus Hemaris, members of the same family and with similar appearance and behavior. The resemblance to hummingbirds is an example of convergent evolution. It flies during the day, especially in bright sunshine, but also at dusk, dawn, and even in the rain, which is unusual for even diurnal hawkmoths. Its visual abilities have been much studied, and it has been shown to have a relatively good ability to learn colours.

The hummingbird hawk-moth is distributed throughout the northern Old World from Portugal to Japan, but is resident only in warmer climates (southern Europe, North Africa, and points east). It is a strong flier, dispersing widely and can be found virtually anywhere in the hemisphere in the summer. However it rarely survives the winter in northern latitudes (e.g. north of the Alps in Europe, north of the Caucasus in Russia). They have been spotted in Newfoundland, Canada and Malaysia as well.

Moths in the Hemaris genus of the family Sphingidae are known as "hummingbird moths" in the US, and "bee moths" in Europe, which sometimes causes confusion between this species and the North American genus.

Image Credit & Copyright: Malcolm Randle
Explanation from: https://en.wikipedia.org/wiki/Hummingbird_hawk-moth

Interacting Galaxies Arp 87

Arp 87 is a stunning pair of interacting galaxies. Stars, gas, and dust flow from the large spiral galaxy, NGC 3808, forming an enveloping arm around its companion. The shapes of both galaxies have been distorted by their gravitational interaction. Arp 87 is located in the constellation of Leo, the Lion, approximately 300 million light-years away from Earth. Arp 87 appears in Arp's Atlas of Peculiar Galaxies. As also seen in similar interacting galaxies, the corkscrew shape of the tidal material suggests that some stars and gas drawn from the larger galaxy have been caught in the gravitational pull of the smaller one. This image was taken in February 2007 with Hubble's Wide Field Planetary Camera 2 detector.

Image Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
Explanation from: https://www.spacetelescope.org/images/heic0717a/