Getting up high can bring spectacular views of the landscape below and around us, but sometimes Mother Nature can add an extra something that really takes your breath away. Whether it's a stunning display of light, a deceiving shadowplay or a mind-bending optical illusion, hills and mountains often combine higher altitudes with shifting atmospheric conditions to create striking visual effects.

One of the most famous reports of an optical illusion in the mountains came during the Matterhorn tragedy in 1865.

After four climbers died during the descent following their first successful summit of the Matterhorn, Edward Whymper recalled having seen:

What the surviving climbers witnessed would appear to be a combination of a fog bow and a Brocken Spectre, although the apparition of two crosses - which did not seem to be the men's moving shadows - is more difficult to rationalise. While Whymper accepted a more scientific hypothesis, the two guides believed it to be a sign from the heavens.

'Perhaps a particular geometry of ice-crystal can be involved to explain it: yet again, perhaps it was a supernatural event, and we should not try to discover a cause!' wrote John Harries in a 1972 Alpine Journal article. 

Here are some examples of a range of phenomena that can seen and enjoyed during a climb, walk or high camp. If you've seen anything else unusual, let us know in the forums and share your photos!

Light - reflection, refraction, diffraction and scattering

Brocken Spectre & Glory

Arguably the most sought-after mountain spectacle, named after the German peak Brocken in the Harz mountains, where fogs and easy accessibility combined to create frequent sightings and a local legend. 

The Brocken Spectre appears when the sun - or any strong light source - is behind the observer, causing an apparently magnified shadow of their body to be cast onto any cloud of moisture (low-lying cloud, fog or mist) opposite, forming a ghostly figure. Rainbow-like coloured rings called a glory often encircle the shadow due to backscattered water droplets.

In the mountains, they typically occur when a walker or climber is on a summit or ridge above a cloud inversion or fog/mist. These spooky apparitions are thought to have inspired many a mountain myth and legend, including that of the Big Grey Man of Ben Macdui. 

Halo

A halo is similar to a glory, but forms through ice crystals rather than water droplets. They usually occur high up, near the sun or moon, and appear as white rings or arcs, sometimes leading to phenomena such as sun dogs (parhelia, false suns), 22° halos and light pillars.

Ice crystals in plate-like form can fall horizontally and sunlight diffracts at an angle such that an observer sees two 'false suns' either side of the real sun, often connected by a halo of light.

In a mountain environment, the presence of ice crystals can create light shows ranging from the glitter-like 'diamond dust' (tiny sparkly ice crystals that fall through the air), to pillars of light, to these spectacular sun illusions high above summits.

Rain/Fog/Moon Bows

There's no need to explain these beloved sky spectacles in great detail, but the meteorological conditions in mountains can make them both more frequent and photogenic. Being on a cliff-side just makes it a bit more awkward to go looking for that pot of gold...

Rainbows occur when sunlight is reflected, refracted and dispersed in water droplets, separating white light into different colours to create a multicoloured arc. Bonus points for spotting double rainbows - when a second rainbow is visible, colours reversed - or supernumaries, when an extra coloured fringe on the violet end of the rainbow forms due to small raindrops.

Fog bows, often called white rainbows, lack the colour spectrum of rainbows due to diffraction by smaller water droplets, although there are usually faint red and blue bands on the outside and inside of the arc respectively.

A moon bow forms in the same way as a rainbow, but its light source is moonlight rather than direct sunlight.

Fata Morgana

A rare sight, this unusual mirage phenomenon occurs just above the horizon when light is bent as it passes through a thermal inversion with air layers of different temperatures in an atmospheric duct. Ships, icebergs, mountains and other objects can appear to be floating on the horizon, often upside down or just downright distorted, while the image shifts constantly.

The term means Morgan the Fairy in Italian, named after Morgan le Fay in Arthurian legend, as frequent mirages observed in the Strait of Messina were believed to have been conjured by the fairy.

Fata morgana mirages have led to the reporting of non-existent land masses. Notably, in 1818, Sir John Ross abandoned his Northwest Passage expedition in the Lancaster sound when he sighted a distant mountain range directly ahead of his ship, naming them the Croker Mountains. The mysterious range was proved one year later to have been a mirage, which ruined Ross' explorer reputation.

A few weeks ago, a walker on Beinn Dearg (Blair Atholl) saw what she believes to be a fata morgana mirage of a distant mountain range on the horizon. She shared the images on Twitter.

Rays and Shadows

Alpenglow

The mountain photographer's dream. During 'golden hour'-  just before sunrise and just after sunset - on a clear day, a pink or reddish hue graces the highest peaks. The rosy tinge is caused by the longest electromagnetic waves of red light reaching and reflecting off the mountains, while the shorter blue light waves disperse while travelling through the atmosphere (a process called Rayleigh scattering).

Scientists and mountain photographers are divided as to the definition of alpenglow: some define it as occuring through direct sunlight, others indirect - and some apply the term to a glow stemming from both direct and indirect sunlight.

Some mountain regions have their own words for alpenglow. In German, it's called Alpenglühen, while in Italian, the glow is known as enrosadira.

When you're away from the mountains, look out for the Belt of Venus, a pinkish glow on the horizon which is a result of the same phenomenon.

Mountain Shadow

Even when the mountain you've climbed is not pyramid-esque, like the Matterhorn - "Ask a child to draw a mountain, they naturally draw the Matterhorn" - the shadow formed at sunrise and sunset appears as a massive, pointy triangle, like a segment of Toblerone. This phenomenon was studied in a 1979 research paper (and many since then):

'Regardless of profile, whether flat topped or pointed, to the summit observer all mountain peaks cast triangular shadows when the sun is low. A theory for such anomalous shadows is developed. The shadow apex angle is shown to depend only on the ratio of the breadth of the mountain to its height.'

This illusion is all a matter of perspective. The long shadows cast by the mountain when the sun is low can stretch for hundreds of miles. A climber or walker is essentially looking out along a tunnel of shadow, which appears to converge at a point. Wherever the shadow reaches its end point, a truer - although still distorted - outline of the mountain would be present, but it's so far away from the observer that the shadow converges. Most buildings or other structures on Earth are not tall enough to create such a long shadow.

Crepuscular Rays (AKA God Rays)

When sunbeams shine through a gap in the clouds and the rays are scattered by haze or dust in the atmosphere, giving the impression that a deity might be about to descend from the heavens. This usually occurs when the sun is low on the horizon and the sky is fairly dark. The name derives from the Latin 'crepusculum', meaning 'twilight'.

The opposite phenomenon is called anticrepuscular rays, when the beams appear to fan upwards towards the sky. This occurs in the opposite direction of the setting sun (east rather than west).

Atmospheric

Temperature Inversion

Another phenomenon that compels a climber or walker to reach for the camera. Typically, air temperature drops with altitude, as the surface of the Earth is heated by solar radiation, warming the atmosphere directly above it, while lower air pressure at altitude also results in lower temperatures.

In a temperature inversion, however, warm air settles above cooler air, often leading to a low-lying foggy layer settling in humid areas. If the inversion is at a high enough altitude, such as in the hills or mountains, cumulus clouds can form below the band of warm air in a valley. Climbers and walkers can then find themselves above a heavenly cotton-like blanket of clouds.

Lenticular Clouds

These UFO-like, wave-shaped layered clouds tend to sit like hats on top of prominent mountain summits or ranges. Latin: Altocumulus lenticularis, 'like a lens'. They form mainly in the troposphere when the air is still and wind blows across a high landform at different heights and from the same direction. The wind causes a series of rising air waves which, if there is sufficient moisture present, can condense to form lenticular clouds.

These clouds are often a sign of high winds at altitude and their presence can forecast snow due since mountains accentuate incoming high-level precipitation ahead of a weather front.

Nacreous Clouds

Nacreous Clouds are an irridescent form of lenticular cloud and can be found downwind of mountain ranges. Forming high up in the lower stratosphere, nacreous clouds are a rare phenomenon outside of high latitudes. They mostly occur when the sun is low in polar regions, although can be spotted in the UK when the stratospheric polar vortex is displaced.

The stunning irridescent colours form due to the way that the ice particles - which are far smaller than those that form more typical clouds - diffract light. Sunlight travels from below the horizon and is reflected by these clouds before dawn and after dusk. Nacreous clouds only form in temperatures below -78 °C and are not to be confused with lower-lying, thin irridescent clouds which occur more frequently but are less spectacular.

As mesmerising as they are to look at, nacreous clouds contribute to the breakdown of the Earth's protective ozone layer, as the ice crystals interact with CFC greenhouse gases in the stratosphere. 

Lightning

A forecast of lightning is a sign to avoid getting up anywhere high and exposed, but there's no denying that watching an epic lightning storm from a safe place is a humbling - and err... striking - experience. 

Lightning is a massive electrostatic discharge between two electrically charged regions in the atmosphere - within clouds - or between the atmosphere and the ground. Mountains create distinct microclimates with rapidly changing weather conditions due to orographic uplift, where air - often carrying moisture - is forced upwards over elevated terrain. The air cools as altitude increases, and moisture condeses as clouds, rain and snow. Since lightning seeks the shortest path to the ground, it frequently strikes summits and other elevated points.

Keep an eye out for developing cumulonimbus clouds, a sign that a storm could be developing. Check out how to keep safe from lightning strikes in the mountains here.

Aurora

Clearly, this phenomena occurs far above mountains and you don't have to be in them to see it, but viewing from dark sky areas in a mountain environment can enhance the experience.

Our galleries flooded with aurora borealis - AKA Northern Lights - photos after a rare UK red alert inspired climbers, walkers and photographers to head out at night last month.

Most commonly visible in the high-latitude regions (the Arctic and Antarctic), auroras are the result of magnetosphere disturbance due to solar storms. This disturbance shifts the trajectories of charged particles, which emit light and produce colourful displays ranging from a mild glow to dancing green and red light.

Further reading

• Atmospheric Optics - atoptics.co.uk
• Alpine Journal, 'Optical phenomena for mountaineers' - John Harries, 1972