Hillgoers guide to cloud spotting
Weather is one of the most important factors to take consideration of when heading to the hills. Good or bad the weather ensures no two days on the hill are ever the same and you can see some spectacular and often rare cloud types & optical phenomena as well as the big skies that you don’t get in town.
You don’t need to be a professional meteorologist to marvel at the dynamics of the atmosphere reflected in the cloud formations. We’re certainly not! If you want to learn more about the cloud types in this blog (and many others) we recommend the Cloud Appreciation Society and related CloudSpotter App (for which Sarah is a part-time, amateur moderator).
Cloud names
Luke Howard, a pharmacist with a keen interest in meteorology, provided the basis for the current cloud classification system still in use today, in 1803. Howard claimed that clouds, whilst having many individual shapes have very few basic forms, belonging to one of three families which he termed cirrus (Latin for ‘fibre’ or ‘hair’), cumulus (Latin for ‘heap’ or ‘pile’) and stratus (Latin for ‘layer’ or ‘sheet’). This classification system has been refined and expanded over time. There are now 10 main genera of clouds which are divided into three levels, cloud low (CL), cloud medium (CM) and cloud high (CH) based on the region of the atmosphere in which they are normally found.
These 10 genera are subdivided into species, reflecting the variations in shapes of clouds and the difference in their internal structures. There are also many other cloud varieties, supplementary features, accessory clouds and optical phenomena. With so many to spot you’ll be in the hills for weeks!
The picture below illustrates the 10 main cloud types (orange text) using our own photographs (virga is a supplementary cloud feature and the sun halo is an example of an optical effect, yellow text).
Even now, additions continue to be made to the World Meteorological Organisation's International Cloud Atlas. In 2017 the newest cloud type asperitas, from the Latin for ‘aspero’ to make ‘rough’ or ‘uneven’ was officially recognised.
Cloud spotting in the hills
Hill walking is a fantastic way of observing the clouds from a new perspective and even climbing through and above them, although it doesn’t always feel fantastic if you’re caught in a blanket of mist or hill fog!
Rainbows are always a beautiful sight to see in the hills.
Cloudbows & fogbows are similar to rainbows but very pale, almost white in colour.
Cloud & fog bows appear when sunlight is refracted (bent as it passes through) and reflected (bounced back) by the tiny water droplets that make up low and mid height clouds or fog. These droplets are much smaller than the raindrops that produce rainbows. To see a cloudbow you need to looking towards the cloud droplets, with the sun shining on them from directly behind your line of vision. Such a spectacle is only possible from above the clouds so a mountain is a great place to see them. Fogbows can be seen at lower levels in the fog and mist.
Another optical phenomena that is best seen from a mountain top is a glory, a series of coloured, concentric rings around a shadow of the observer’s head, created by the refracting, diffracting (bent around at an angle) and reflecting of sunlight onto cloud droplets. You need to be above the clouds with the sun shining from behind you to see a glory. Glories are often seen in association with a Brocken Spectre, or Mountain Spectre, the apparently enormously magnified shadow of the observer. The name derives from the Brocken, the tallest peak of the Harz mountain range in Germany. Glories can also be seen from aircraft (where the shadow cast is of the aeroplane) and from tall buildings.
Other optical phenomena you can look out for include the corona, which is a series of coloured rings around the sun (or moon). The corona is created by when sunlight is diffracted through a thin layer of low or medium level cloud. The smaller the cloud droplets the larger the corona.
The 22-degree halo is a large ring or partial ring, 22 degrees in radius, surrounding the sun (or moon). It appears when light is reflected and refracted through ice crystals present in the high-level cirrostratus or cirrus clouds, splitting it into faint rainbow colours.
Corona around the Sun
22 degree halo
The halo is often also associated with sun-dogs which appear as bright spots on one or both sides of the sun, again caused by the refraction of the sun through ice crystals in the high cirrus clouds.
Iridescence, which is caused by uniform drops of water diffracting sunlight to produce luminous, mother-of-pearl colours, can be seen in clouds of all heights.
Sun-dog
Iridescence
A common cloud type is the beautiful lens shaped lenticularis (from the Latin for lentil). These clouds occur at all levels and are formed as moist, stable air flows over the mountains, which is why we see them so often in Scotland. Keep your eyes peeled for the mid-level altocumulus lenticularis which often look like UFO’s!
Most hill walkers will delight in finding out that they will be accompanied by fair weather on their walk and their cloud buddy on such days may be the white, fluffy cumulus cloud which looks like a bit like a cauliflower floret. Despite being a common sight, cumulus clouds are stunning set against an azure blue sky, as far as the eye can see.
At the other end of the weather spectrum are the types of clouds you are less likely to want to be caught out hill walking with, storm clouds! A rarely seen phenomena in Scotland and accompanying the storm cloud is the tuba, this one was photographed by Garry in 2019 in the Cairngorms where the lightening bounced off the mountain tops in spectacular fashion. These spiralling vortexes can be formed by air being sucked into the cloud or sinking from its base due to heavy showers.
As the sun rises or sets on a glorious day you may be lucky enough to spot anti-crepuscular rays streaking out across the sky. They appear when tall clouds in front of sun, cast long shadows across the sky which although parallel, give the appearance of converging to a point on the horizon, as seen with the sun behind you. Anti-crepuscular clouds are less commonly seen than crepuscular rays that shine out as beams of sunlight, scattered and made visible by dust, gases and water droplets in the atmosphere from gaps in or behind low clouds.
Anti-crepuscular rays
Crepuscular Rays
One of the best things about camping out in the hills is getting away from light pollution and having time to gaze at the wonders of the Universe. If you time it right and with a sprinkling of luck you may see two other rare spectacles, the well-known and much sought after Aurora Borealis and the lesser-known Noctilucent clouds, from the Latin for ‘night-shining’.
Aurora Borealis
Noctilucent clouds
The stunning noctilucent clouds, the highest in our atmosphere, form some 30-50 miles above the Earth’s surface and at the limit of our atmosphere. As the sun drops below the horizon on a clear, summers night (at high latitudes, such as in the north of Scotland) these noctilucent clouds catch the sun when the rest of the sky is dark and appear to shine with a bluish-white light.
Whatever the weather and whether or not you know the names of the different clouds and optical phenomena that you see, the sky provides a fabulous and ever changing backdrop to the hills, enjoy!
