#cloudspotter's guide

We think that clouds are Nature’s poetry, and the most egalitarian of her displays, since everyone can have a fantastic view of them.

We say to all who’ll listen: Look up, marvel at the ephemeral beauty, and live life with your head in the clouds.

Cumulus is the Latin word for ‘heap’, which is simply to say that these clouds have a clumpy, stacked shape. The people who concern themselves with such things divide them into humilis, mediocris and congestus formations–these are known as ‘species’ of Cumulus. Humilis, meaning humble in Latin, are the smallest, being wider than they are tall; mediocris are as tall as they are wide, and congestus are taller still.

Attention all cloudspotters: ‘In the morning mountains, in the afternoon fountains.’

Cloudspotters will be pleased to see this most clearly demonstrated when sailing around a small island on a sunny day. The surface of the island is warmed by the sun’s radiation more readily than the sea around it, and a puffy, white Cumulus cloud can often be seen poised above it, fed by the thermal coming off the ground. South Sea Islanders would use Cumulus clouds as beacons, navigating toward an atoll well before the land itself became visible.

In fact, Cumulus clouds can form above fires. Known as pyrocumulus, these appear as Cumulus mounds atop the plumes of smoke from stubble burning or wildfires.

The darkness of a Cumulus cloud depends, firstly, on whether you are looking at the side in shadow and, secondly, on the brightness of the sky or other clouds behind it. But it also depends on the number of water droplets that the cloud contains, for it is these that scatter the sunlight and prevent some of it from passing through. The more laden with droplets a cloud is, the darker it will appear with the sun behind. Cloudspotters will note that as a Cumulus grows in size from its small humilis form, through the mediocris stage into a towering thick congestus, its base will appear darker and darker as the thickening cloud blocks out more and more sunlight.

CUMULONIMBUS The towering thunderclouds that scare us senseless

The classic shape of a mature Cumulonimbus is a huge vertical column, several miles across and extending up as high as 60,000ft (over 11 miles), which spreads out at the top to resemble a blacksmith’s anvil. This upper canopy is called the ‘incus’ (after the Latin for anvil) and consists of ice crystals, rather than the water droplets that make up the rest of the thundercloud. The anvil can stretch out over hundreds of miles, as it is spread by the strong winds high in the atmosphere. From a distance it can have a calm, majestic beauty.

Cloudspotters can distinguish a Cumulonimbus from its younger brother, the Cumulus congestus, by careful observation of its upper reaches. If the top of the cloud still has the sharp cauliflower mounds found on a fair-weather Cumulus, it is officially known as a Cumulus congestus. It only becomes a Cumulonimbus when the upper region becomes ‘glaciated’, which means that its water droplets have begun to freeze into solid ice particles. A Cumulonimbus anvil of ice crystals is brighter and has softer edges than the top of a towering Cumulus.

Whilst the Nimbostratus doesn’t have anything like the height of a Cumulonimbus, and often spreads out horizontally over hundreds of square miles, it can be hard to distinguish the two from underneath. The weather below a Cumulonimbus is what will give it away. If there is hail, thunder, lightning and strong, gusty winds, then cloudspotters can be confident that they are in the company of the King of Clouds.

From inception to dissipation, an individual Cumulonimbus might last up to an hour or so, leading to a relatively short-lived storm. But thunderstorms can often last much longer than this, since these villains of the cloud world do not always work alone. They have a tendency to form into gangs, which is when they are at their most destructive. As one Cumulonimbus is dissipating, another rises ahead of it. Collectively, they resonate in an enormous self-propagating system of extreme weather that lays waste to whatever is in its path.

Published in three languages, the book was called The International Cloud Atlas and contained numerous photographs to illustrate the ten cloud genera agreed by the committee. Number nine in the list was Cumulonimbus, the tallest of all the types. To be on cloud nine is therefore to be on the highest one.

Just as Cumulus clouds can occur in the different species of humilis, mediocris, congestus and fractus, the Cumulonimbus can be one of two possible species. These are called ‘calvus’ and ‘capillatus’ and they are distinguished by the appearance of the upper, ice-particle region. Cumulonimbus calvus is when the cloud’s anvil is smooth with soft edges. Cumulonimbus capillatus is characterised by an upper region that is fibrous and striated. It is named after the Latin for hair, and can look like the disorderly locks of a child who’s just been in a playground scrap.

(…) ‘fork’ or ‘sheet’ lightning. In actual fact, there is no difference between the two–sheet lightning is merely when the body of the cloud hides the fork lightning from view, and one sees a flickering illumination of the cloud as a whole.

STRATUS The low, misty blankets

(…) distractions occasionally become so profound and sustained that the yogis lose track of their spiritual path altogether. They call it a ‘storm of Maya’. It is one in which illusory ways of thinking and feeling block out the Supreme Light altogether. At times like these, he said, the yogis remind themselves that, beyond the clouds, the sun never stops shining.

WITHOUT THE STRATUS, I would never have experienced the peculiar joy of walking through a cloud. Being the lowest of all the types, whose base rarely forms above 1, 600ft, it is the only one that happily comes down to join us at ground level. Tethered to terra firma like this, Stratus is referred to as fog or mist.

WHEN IS EARTH-BOUND Stratus described as fog, and when is it mist? The official distinction relates to how far you can see through it. If you can see less than 1, 000 yards, then meteorologists call it fog. Visibility between 1, 000 and 2, 000 yards, and they call it mist. (If you can see less than a thousand yards and there is no Stratus, then you are just short-sighted.)

Advection and radiation are the most common types of fog, but they are certainly not the only ones. ‘Steam fog’ appears when cold air flows over warm water (the opposite of advection fog) and the vapour evaporating off the water’s surface instantly cools enough to form into droplets. The swirls of rising droplets are the evaporation process made visible, for water is constantly rising from the sea surface as vapour, but normally you can’t see it. This type of fog is at its most dramatic in polar regions, where it is known as ‘Arctic sea smoke’.

Stratus–at its best when viewed from the top of a mountain.

(…) words by the American poet, James Russell Lowell: Who knows whither the clouds have fled? In the unscarred heaven they leave no wake, And the eyes forget the tears they have shed, The heart forgets its sorrow and ache…

As with any of the cloud genera, a Stratocumulus does not have to be one of these recognised species: if it doesn’t fit one of the above descriptions, a patch of low clumpy clouds is just called Stratocumulus.

Of course, clouds pay little attention to the rules of behaviour we presumptuously ascribe to them. Chaotic to their misty core, they do their best to confound our attempts at classification. How can a body so nebulous, ephemeral and mutable ever be pigeon-holed? Cloudspotters will come to love the cloud’s rebelliousness–just when they think they have identified a formation, it will change and mock their attempt to pin it down.

Contemplating the heavens below, a cloudspotter can always escape–even if for just a few minutes–from the trials and pressures of life. Let others dream of escaping to a place in the sun. Cloudspotters know better.

ALTOCUMULUS The layers of bread rolls in the sky

A cloudspotter should hold up three fingers with an extended arm. If the individual elements of the layer are wider than all three fingers, the cloud is probably of the lower Stratocumulus genus. If they are smaller than the width of one finger, then it is more likely to be a high layer of cloudlets, called a Cirrocumulus. It is most likely to be an Altocumulus layer when the size of the cloudlets is somewhere between the two–smaller than three fingers and larger than one. However, the giving-the-cloud-the-fingers rule doesn’t work if cloudspotters are looking at clouds off in the distance. Their outstretched arm needs to be angled above 30° from the horizontal for it to apply.

The second rule of thumb for identifying Altocumulus has to do with the shading of its cloudlets. When the sky above is clear and the sun shines directly on to the cloud, Altocumulus will have noticeable shading on the sides away from the sun, though this will not be particularly heavy. With the lower Stratocumulus, the shaded parts are often quite dark, while the tiny cloudlets of the high Cirrocumulus show no shading at all.

CLOUDSPOTTERS MUST SURRENDER themselves to the gentle shifts of the clouds’ formations. If they cannot identify a particular cloud form, then so be it–they should just relax and watch it develop. Before long, a familiar formation will appear.

This is not how to look at clouds. Cloudspotters won’t find shapes in them by force of will, nor by searching with half a mind on what the person beside them sees. The best way to find shapes is to look up, empty your mind, and let them find you.

ALTOSTRATUS The mid-level layers, known as ‘the boring clouds’

NIMBOSTRATUS The thick, grey blankets that rain and rain and rain

But just as a Christmas tree actually has its branches pointing upwards, raindrops don’t fall in the shape of tears. Tiny cloud droplets may be pretty much perfect little spheres but, once they’ve grown large enough to fall fast, they are greatly distorted by air resistance and are not shaped like spheres–or indeed teardrops–at all. When they are a couple of millimeters or more across, raindrops actually look like the top half of hamburger buns.

In the words of St Basil the Great, from the fourth century: Many a man curses the rain that falls upon his head, and knows not that it brings abundance to drive away the hunger.

Or as John Updike, the American novelist, put it: ‘Rain is grace; rain is the sky condescending to the earth; without rain, there would be no life.’

CIRRUS The delicate streaks of falling ice crystals

Nevertheless, to look up at Cirrus clouds is to see snow–well, ice crystals, to be precise–falling too high to reach the ground. Cloudspotters may live in regions too warm for snow but, in the Cirrus clouds, they can still see how it looks from a few miles off.

That Cirrus can be harbingers of a ‘deterioration’ in the weather only adds to their fragile beauty–are not the most delicious things the ones we know can’t last?

THE ATMOSPHERE ITSELF is an ocean–one of air, rather than water. The relationship between the atmospheric ocean and the actual one is close, and of great relevance to the formation of clouds in general.

CIRROSTRATUS The high milky veils that most people barely notice

That first time I saw a cloud smile was on a London street, and no one else seemed to be paying the slightest bit of notice to the sky. I was transfixed, of course, but the passers-by all had other things on their minds. I felt as if I was the only one watching this particular smile. In fact, looking back on it, I can say that I most definitely was the only one. Even if others had been staring up, they would not have seen the same CZA as me. As sunlight passed through the countless crystals up in the cloud, it was being scattered in all directions. But it was only those crystals that sparkled light directly to my eyes that created the light effect for me. Some of them flashed a little red-looking light towards me, others a little blue. Say some of the bustling Londoners had turned out to be cloudspotters in disguise. Had they dropped their shopping and stood beside me to look up at the coloured arc too, the array of crystals sparkling directly into their eyes would have been different ones from mine. They would have seen a different circumzenithal arc. We would each have seen our own smiles.

Despite the stunning range of crystal forms, there is one theme that keeps appearing season after season –the number six. The arms of the stellar dendrites and the sectored plates, the edges of the hexagonal plates, the sides of the columns…when it comes to ice crystals, six, rather than three, is the magic number. This is due to the shape of water molecules, which determines that as they join to form crystals, they do so in a lattice formation of hexagons –a molecular honeycomb.

Even when you know the explanation for crepuscular rays, it is hard not to think of them as somehow divine in nature. In Hellenistic and Roman art, emperors were often depicted with a crown of rays, known as a ‘radiate’. This signified their association with the sun gods, Helios and Sol, and was also used as a sign of posthumous divinity. With the rise of Christianity, the symbol was dropped in place of a circular halo, called a ‘nimbus’. The rays of the radiate were felt to be too pagan in association.

Rainbows are most commonly seen in conjunction with the convection clouds like Cumulus congestus or Cumulonimbus. This is because those clouds are individual precipitating clouds, rather than expansive layers. With gaps in between, there is a fair chance of direct sunlight shining on to rainfall.

The rainbow that a cloudspotter sees standing in one position is never the same as that observed from another one. The droplets that are over in the direction of the arc–perhaps a half to one and a half miles away–each sparkle a bit of sunlight into his eyes. From the drops that fall through the sky off in some directions, it is the yellow-looking part of the spectrum that twinkles at the cloudspotter. From those in other directions, it is the violet, etc. This means that, should the observer change position, different raindrops will be the ones sparkling at him. Hopefully, this will help cloudspotters accept that it is a futile and, frankly, humiliating aspiration to seek the end of a rainbow. It is like driving a speedboat this way and that in an attempt to stop the line of glitters that the sun casts on the sea’s surface from pointing toward you.

Your rainbow is not my rainbow.

Rainbows may be the most familiar of the sky’s optical delights, but how many of us notice the finer points of their appearance? How many realise that the sky within the bow is brighter than that outside it? How many have spotted, on occasion, a secondary bow, outside the primary one, dimmer than it, and with the reverse order of colours? How many have seen ‘Alexander’s Dark Band’? This is not a goth group from Middlesbrough, but the name for the dark region between the primary and secondary rainbows.

THE OTHER CLOUDS The accessory clouds, supplementary features, and stratospheric and mesospheric clouds

CONTRAILS The lines of condensation that form behind high-altitude aircraft

MORNING GLORY The cloud that glider pilots surf