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Mammatus Clouds

Mammatus clouds are striking cloud formations mostly composed of ice with unusual shape due to protrusions hanging from the bottom of the clouds, formed by sinking cold air.
They can extend for hundreds of kilometers and stay visible in the sky for up to a few hours, usually before or after a storm, bringing often rain, hail, or snow, depending on the temperature of the atmosphere.

They are usually associated with cumulonimbus, which is indicative of heavy storms, but they may also be formed from cirrus and altostratus clouds, or even volcanic ash clouds.

http://www.bbc.com/earth/story/20150716-nine-rare-and-beautiful-clouds

Light Pillars

Light pillars are beams of light formed by an optical phenomenon created by the reflection of light from flat hexagonal ice crystals horizontally suspended in the atmosphere.

Different shape and colour depend on the source of light, which can be natural (sun, moon) or artificial (streetlights), on the position of the crystals: the higher they are, the taller the pillars are, and on the number and dimensions of the crystals: the larger and more numerous they are, the more pronounced is the effect.
Due to the low temperature required, this effect is typical of arctic regions.

Borealis & Australis Auroras

Northern lights Alaska — © National Park Service, Alaska Region

The Polar Aurora, well known as Aurora Borealis/Australis or Northern/Southern Lights depending on which hemisphere it occurs, is an optical phenomenon of the atmosphere, characterized by green, blue and red luminous bands with a different range of shapes.

The phenomenon, visible in two stripes around the magnetic poles and more intense and frequent during high solar activity, is caused by the interaction of charged electrons of the solar wind with the earth’s ionosphere (a process similar to the neon lamp light).
The color depends on the atmosphere’s gases: atomic oxygen is responsible for the green, the molecular oxygen for the red and nitrogen for the blue color.

According to old myths, the Northern Lights appear for the Inuits like souls playing with walrus skulls, for Lappish they are created by the tail of a big fox that hits the snow and it is believed that children conceived beneath them are more intelligent and lucky.

In Central Europe, they were considered a bad omen due to the predominance of red in its colours.

Best places to see Northern Lights / Aurora Borealis (the best period is from September to April):

Iceland: Kirkjufell, Reykjavik, Jökulsárlón Lake, Þingvellir National Park
Norway: Svalbard Islands, North Cape, Tromsø, Alta, Karasjok
Finland: Kakslauttanen Arctic Resort, Inari Lake, Sodanklya, Luosto
Sweden: Abisko National Park, Kiruna region
Alaska, USA: Fairbanks, Denali National Park, Juneau, Barrow, Coldfoot
Canada: Mucho Lake Provincial Park, Yukon, British Columbia, Yellowknife
Greenland: Kulusuk, Ammassalik, Kangerlussuaq
Fær Øer Islands
Russia: Kola Peninsula
Scotland: Caithness coast
Ireland: Donegal region, Malin Head, Antrim region

Check Northern/Southern Lights forecasts

Best places to see Southern Lights / Aurora Australis (the best period is from March to September):

Antarctica
New Zealand: Stewart Island, Lake Tekapo, Aoraki Mt Cook National Park, The Catlins
Australia: Mount Wellington (Tasmania), Victoria
Argentina: Ushuaia
South Georgia Island
Falkland Islands

22° Halo (Lunar Halo)

A 22° halo (known also as moon or solar halo) is an optical phenomenon, forming a circle with a radius of approximately 22° around the moon or sun, produced by reflected/refracted light interacting with millions hexagonal ice crystals suspended in the cirrus or cirrostratus clouds (troposphere, 6-13 km high).

No light is reflected towards the inside of the ring, giving it the impression of a big hole in the sky. As the sailors say, when the 22° halo appears in the sky, a storm is approaching.

In clear sky conditions the view of this phenomenon is spectacular: the one in the photo (taken in Thaba-Tseka, Lesotho) is a lunar halo, much rarer than the solar halo.