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

Landscape (UK) - - Contents - Words and photograph­y: Ian Cur­rie

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.

Cu­mu­lus clouds

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.

Layer clouds

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.

High fliers

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.

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