Fascinating shapes shifting across the skies
Scattered across the sky in a myriad of shapes, sizes and patterns, clouds are a fascinating phenomenon of nature
high in the sky on a July day, wisps of white cloud float across an azure background. In the distance, more clouds, resembling fluffy balls of cotton wool, sit like flocks of sheep in a blue field. Across empty areas of sky, condensation trails indicate aircraft flying to distant destinations. Positioned at a crossroads in the world’s weather systems, the British Isles bear witness to a plethora of activity high above. On one side is a vast capricious ocean, on the other, the edge of a huge land mass. Warm air heading north meets cold polar air flowing south. The result is a never-ending vista high above, which is filled with a panoply of cloud types, textures and colours. Whether thin wisps or towering black masses, all clouds have one thing in common: they start from tiny microscopic nuclei. The atmosphere contains billions and billions of these. The majority are salt particles born from the oceans, but they can also be grains of dust, aerosols from volcanic eruptions or pollutants from human activities. All can be a catalyst in cloud formation. If air cools sufficiently, the vapour it holds will collect on these tiny specks as a film of ice if it is high in the atmosphere or, if it is much lower, as a coating of water. Millions of nuclei are required to make an ice crystal or droplet of water, and many millions of these to create a cloud. Today, clouds are classified into many different types, all based on a system first put forward in 1802 by Luke Howard, a keen amateur meteorologist. He proposed three main categories: cumulus, stratus and cirrus. These romantic sounding names are in fact Latin for heaps, layers, and wisps or curls. From this modest start, the classification of clouds has grown. As well as height levels of high, medium and low, there are also 10 characteristic types, or genera. Still using the original Latin, these include cirrocumulus, altocumulus, stratocumulus and cumulonimbus. Within these types there are 15 further varieties, or species, such as fibratus, meaning fibrous, or uncinus, hook shaped. This is an ongoing process, with the most recent, asperitus, which means roughness or wavy, only added this year.
Formation of clouds
Once it starts to form, a cloud’s shape, texture and even colour are determined by the physical processes that the atmosphere can be subjected to. Most clouds form in the troposphere, the area of the atmosphere that extends upwards for up to eight miles. Strong winds in the upper part of the troposphere will blow cirroform clouds, made up of ice crystals, into long strands, wisps and curls. Clouds at this level are cirrus, cirrostratus and cirrocumulus. They are often located at the boundary between air masses of differing temperature and humidity. This conflict between humidity levels and hot and cold air causes areas of low pressure to form, so these clouds can be a sign of approaching bad weather. Along this frontal margin, air is generally lifted, leading to cooling and condensation and the formation of cloud. Air masses can also produce changes in wind directions and sharp zones of contrasting wind velocities with height. The result appears as delicate billows and dapples in the upper clouds, like fish scales, known as a mackerel sky and consisting of cirrocumulus clouds. Other formations are swirls of suspended hooks and honeycomb like formations, called lacunosis. As the unsettled weather approaches the observer, further cloud thickens above as warmer, less dense air rises over the cooler air mass. The zone of warmer air increases, as does the humidity, and the cloud base lowers. Clouds at an altitude of approximately 3 to 4 miles can now be part ice and water. Altocumulus and altostratus are clouds of this type. Sometimes the sun just manages to shine through the sheet of altocumulus, known as translucidus. At night, the moon may have a watery look. As the cloud-making process continues, enough droplets and crystals coalesce, to fall earthwards as precipitation. At the Earth’s surface, the wind usually blows from west to south. This backing or anticlockwise movement indicates that the area of low pressure and its associated frontal system is heading towards the viewer with increasing amounts of warm, moist air that lowers the cloud base. At the same time, the surface air pressure decreases, as measured by a barometer. Now the cloud is known as nimbostratus, from the Latin for rain and stratus, layer. Anyone below it will experience several hours of rain or snow.
There are days when the atmosphere is relatively dry and is close to an area of high pressure. Sunshine heats up the ground, and bubbles of warm, light air rise. They may ascend high enough to reach the dew point, the level where they cool sufficiently to condensate, forming cloud droplets. However, high pressure often has a warmer zone at high altitude, called an inversion. This inhibits cloud from developing any great height. The result is cumulus, the small, benign, cotton wool-like clouds.
“I wield the flail of the lashing hail, And whiten the green plains under, And then again I dissolve it in rain, And laugh as I pass in thunder” Percy Bysshe Shelley, ‘The Cloud’
The most gentle of these are quite flattened and known as cumulus humilus, the Latin for low. When there is rather more of a breeze, they can be blown into lines of cumuli, which are called streets. In contrast, cumulus cloud can have far less gentle cousins. When the higher atmosphere is cooler than normal, yet surface temperatures are warm and moist, the atmosphere is said to be unstable. Now, thermals, or warmed air from the surface, are able to rise to great heights. Once the dew point is reached, the condensation process provides latent heat energy, allowing further growth. This heat keeps the thermal much warmer than its surroundings, and the warm air, being less dense, continues to rise. This leads to the formation of very large cumulus clouds, cumulus congestus, which usually end in showers or thunderstorms. In turn, cumulus congestus can develop into the superpowers of the cloud world, cumulonimbus. These huge malevolent clouds can unleash sudden deluges of rain and shards of lightning, giving cracks of thunder, and sometimes producing hailstones or even tornadoes. Cumulonimbus can rise to 9 or 10 miles in height. The temperature in the upper regions of these colossal clouds can be as low as -50°C or -60°C. Here, the cloud is composed entirely of ice crystals.
Thunder and lightning
One of the most dramatic occurrences in these towering cumulonimbus is lightning. Up currents can reach 80-100mph, and water droplets from lower levels are lifted to sub-freezing levels. Here, they collide with ice crystals. Some grow to form hail, others smash together and shatter. All this can create a positive charge at the top of what is now as an anvil-shaped cloud, or incus. Lower down, there is a negatively charged zone where water droplets predominate. This, in turn, creates an opposite, positive charge on the Earth’s surface. From the cloud, an initial stream of electrons flow to the surface, almost immediately connecting with positive streamers. A blinding flash of fork lightning is formed. This may happen several times along the same channel. These forks are approximately 1in (2.5cm) wide, but appear much wider because the electricity heats air along its path up to 30,000°C, five times the sun’s surface temperature, creating intense light. Three-quarters of all lightning flashes actually occur within differing parts of the cloud or even into the free atmosphere; a true bolt from the blue. When the spark is hidden within the cloud, it is called sheet lightning. Positive-charged lightning occasionally hits the Earth and is five to 10 times more powerful than normal strokes. The sound of thunder is caused by air along the lightning path being rapidly heated, causing a shock wave or sonic boom. The crack-like pitch indicates nearness to the stroke. A low rumbling sound occurs when the flash is further away. In general, a count of five from seeing the flash indicates a mile from the stroke; 10 equals two miles, and so on. It is rare to hear thunder from a stroke more than 10 miles distant. There are 1.4 thousand million lightning strikes a year from approximately 2,000 thunderstorms occurring at any one time on the Earth’s surface. Across England and Wales, thunder can be heard on 10 to 15 days a year, but only on five days in the west of Scotland, where it is cooler and the cumulonimbus clouds are less potent. Places such as Stonyhurst in Lancashire and Huddersfield, West
Yorkshire, have recorded 38 thunder days. Here, they can be at the boundary of cooler Atlantic air and the warmer conditions across the south and east, triggering plenty of large shower clouds. Across much of the UK, storms tend to be more frequent during the afternoon of the summer months, when temperatures reach their peak. Often a sign of impending thunderstorms is indicated by the cloud type altocumulus castellanus. The name indicates turret-shaped or crenulated, and on some occasions they can look like castle battlements. Indicating increasing humidity and instability, it is usually not long before cumulonimbus clouds start to form.
Towering storm clouds, though the most spectacular, are not the most frequent clouds seen in the UK. Instead, this honour goes to stratocumulus, a cloud made of water droplets that may, at worst, give a little drizzle or a few snow grains. Quite often, the atmosphere may be fairly moist close to the ground, but drier above, leading to the formation of this shallow layer of cloud. It often has small gaps within it. In the winter, it may cover the sky, giving rise to gloomy conditions. Stratus, or layer cloud, forms when air is particularly moist near to the surface. When this happens, a low cloud sheet hugs even modest hills, resulting in a murky day, sometimes producing some drizzle. This is the most nebulous of clouds, with no real definition. One of the most eye-catching clouds is a type of altocumulus called lenticularis. This is seen most frequently over and to the lee of hills that run at right angles to the wind direction. A common location is in the lee of the Welsh Hills, the Pennines and the Highlands of Scotland. Lenticularis form when a shallow layer of moist air is forced to rise, due to the general air pressure gradient that creates the wind flow over the region, over the intervening hill range. If a stationary wave pattern in the airflow develops above or to
the lee of the hill or mountain, the cloud remains ‘in situ’. Sometimes, several of them may lie above each other, when it is aptly known as a pile of plates cloud.
All the clouds mentioned thus far occur in the troposphere, where most weather takes place. Above the troposphere lies the stratosphere. Here, it is very dry, but in winter, very small ice crystals can form at a height of 12 miles and temperature of approximately -80°C. The crystals can diffract sunlight into beautiful pastel colours, creating stunning, rarely seen clouds called nacreous, or mother of pearl clouds. Another unusual cloud, this time forming at a very high altitude in part of the atmosphere called the mesosphere, is known as noctilucent cloud. Shining at night, it is bluish-white to yellow, set across the northern sky. Still sunlit when the ground below is in darkness, it lies at a great height of approximately 50 miles. Noctilucent clouds resemble the familiar cirrus that are often seen much closer to the Earth’s surface. They even have billows that look like ripple marks on a sandy beach. They appear to travel more than 200mph from the north-east and can only be seen a month or so either side of the summer solstice. This is because they occur at such a lofty altitude. It is only at this time that, observed from Earth, the sun is high enough to illuminate the cloud. At other times, noctilucent clouds will be too feeble to show up. The south of Britain is approximately the southernmost limit of 50 degrees north to view such clouds. It is not known for certain what causes this cloud, but it may be ice-covered meteoric or volcanic dust, or even frozen methane. Clouds are part of the landscape. There are few people who will not marvel at the sheer multitude of shapes, textures and colours, from a beautiful altocumulus stratiformis at sunset to a towering storm cloud. It is a rare day that there is no cloud in the sky, when this never-ending backdrop fails to entertain, delight and inspire awe in an Earth-bound observer.
The flying saucer shape of lenticular clouds occasionally leads to them being mistaken for alien spacecraft. When formed in layers, they resemble stacked plates.
Small woolly cumulus towers rising in the dry air on a bright afternoon.
Cumulus clouds with a stratocumulus layer above. This latter type results in a typical cloudy day with sunny intervals in summer.
The ice particle canopy that spreads sideways at the top of a cumulonimbus cloud is known as an incus, the Latin for anvil.
A parhelion is caused by the sunlight reflecting though ice crystals in the atmosphere. It is a phenomenon more commonly known as ‘mock sun’ or ‘sun dog’.
Invisible by day, milky-blue noctilucent cloud magically reveals itself after sunset.
Most weather takes place within the troposphere, although rarer clouds may form at higher levels in the atmosphere.