Hoy clouds

This cloud spotter’s guide comes from cloud catching photographer Jeff Clark from the Rackwick Weather Station. If you like looking up and watching the sky be sure to check out the Cloud appreciation Society.

Nimbo strata

Strata / Nimbo strata

Stratus most commonly occurs as a single, grey, fairly uniform, featureless layer of low cloud (left). Nimbo strata is a grey cloud layer, often dark. It is thick enough to block out sun. This cloud is associated with precipitation. 

Cumulus

Detached clouds, generally dense and with sharp outlines, developing vertically in the form of rising mounds, domes or towers, of which the bulging upper part often resembles a cauliflower.

The sunlit parts of these clouds are mostly brilliant white, their base is relatively dark and nearly horizontal. Sometimes cumulus is ragged. Cumulus occurs from 1,000 feet up to 6,000 feet. This cloud can develop to produce showers of rain, sleet, hail or snow. 

Cumulonimbus

Heavy and dense cloud, of considerable vertical extent in the form of a mountain or huge towers. At least part of its upper portion is usually smooth, or fibrous or striated, and nearly always flattened; this part often spreads out in the shape of an anvil or vast plume.

These are storm clouds with a base of 1,500 to 3,000 feet and their tops can reach 18,000 feet. This is coded 9, the highest cloud coding used by the Met Office. This cloud can be a danger to aviators and can be so high that it is the origin of ‘being on cloud nine’.

STRATOCUMULUS

Grey or whitish patch, sheet or layer of cloud which almost always has dark parts, composed of rounded masses, rolls etc. This a low cloud with a base between 2,000 and 6,000 feet.

ALTOSTRATUS

Greyish or bluish cloud sheet or layer of striated, fibrous or uniform appearance totally or partly covering the sky. Altostratus prevents objects on the ground from casting a shadow. Altostratus lowers to become nimbostrata with precipitation.

Altocumulus

White or grey, patch sheet or layer of cloud, generally with shading and composed of laminae (fingers), rounded masses or rolls which are sometimes partly fibrous or diffused and which can be merged. Altocumulus can sometimes produce descending trails of fibrous appearance – this is called virga. Coronae and irisation are also often observed in thin parts of the cloud. Heights between 8,000 and 15,000 feet. It is rarely that precipitation falls from this cloud type.

CIRROSTRATUS

Transparent whitish cloud, veil of fibrous or smooth appearance, totally or partly covering the sky, and generally producing halo phenomena. Usually forms a veil of great horizontal extent, without structure and of a diffuse general appearance. It is composed almost entirely of ice crystals and can be so thin that the presence of a halo may be the only indication of its existence. Shadows will normally continue to be cast when a high sun is shining through this cloud. No precipitation is associated with this cloud. This cloud occurs between 20,000 and 30,000 feet.

image shows a halo see other phenomena below

Cirrocumulus

Thin white patch, sheet or layer of cloud without shading, composed of very small elements in the forms of grains or ripples, merged or separate, and more or less regularly arranged. It is rippled and subdivided into very small cloudlets without any shading. It is composed almost exclusively of ice crystals and can include parts which are fibrous or silky in appearance. This high cloud forms at between 20,000 to 30,000 feet. 

This cloud is more commonly known as a mackerel sky and goes with the weather prediction of ‘not long wet, not long dry’. 

Cirrus

Detached clouds in the form of white delicate filaments, or white or mostly white patches or narrow bands. These clouds have a fibrous hair-like appearance, or a silky sheen, or both. Cirrus is whiter than any other cloud in the same part of the sky. Cirrus, like the opther high clouds, is composed of ice crystals. These clouds occur between 20,000 and 30,000 feet. 

cirrus with cirocumulus

Some cloud species

LENTICULAR CLOUDS

These clouds have the shape of a lens or almond, often very elongated and usually with well defined outlines. They occasionally show iridescence and often form above mountains. They are usually an indication of strong high atmospheric winds.

  

CUMULUS CONGESTUS 

Cumulus clouds which are markedly sprouting and are often of great vertical extent. 

Duplicatus or plate clouds

These cloud formations occur when air is passing over high ground such as on hills or mountains, the cloud duplicates layers.


Optical phenomena & other features

Halo

A halo of 22 degrees from the sun is the most frequent halo phenomenon and appears as a luminous ring. This halo is probably due to the refraction of light through hexagonal prisms among ice crystals in the cloud. There is what is known as an arc of contact at the top of the halo.

Irisation

Colours, predominantly green and pink, often with pastel shades, that sometimes appear on cirrocumulus, altocumulus or stratocumulus. The colours may appear as bands nearly parallel to the margins of the clouds, or as a mosaic pattern

Virga

Trails of precipitation (fall streaks), that do not reach the earth’s surface, attached to the under surface of a cloud.

MAMMA

Downdraughts can sometimes cause udder-like protuberances to form on the under surface of many cloud types. They are usually a sign of unstable atmospheric conditions.

RAINBOW

The primary rainbow is a coloured bows which appears on a screen of water droplets when light from the sun (and sometimes the moon) falls upon them.It is rarely that all the colours of the rainbow – red, orange, yellow, green, blue, indigo and violet – are observed. Outside the primary rainbow there may be a secondary bow much less bright.

CREPUSCULAR RAYS

These take the form of pale blue or white rays diverging from the sun when it is behind a cloud. Sunbeams piercing small gaps in cloud layers – sometimes known as ‘Jacob’s Ladder’- and shadows cast by clouds near the horizon at twilight are also called crepuscular rays. These rays can appear to come down or sometimes upwards.


Some special clouds

Nacreous clouds

These clouds resemble pale cirrus or lenticular altocumulus and show very marked irisation, the most brilliant colours ocurring when the sun is just below the horizon. They are sometimes called Mother of Pearl clouds. These are rare clouds, in the 30 years I have been making observations in Rackwick, I have only seen this cloud twice. They have been observed mainly from Norway and Scotland at altitudes of 15 miles above the earth. ‘There is a dark side to these clouds, their chemical composition assists the production of chlorine atoms, which in turn contributes to the depletion of the ozone layer, so sadly, these most beautiful and benign looking clouds turn out to have a powerfully destructive environmental impact (Richard Hamblyn, Cloud Book)’. 

Noctilucent clouds

These clouds resemble thin cirrus, but are usually bluish or silvery, sometimes orange to red, or reddish when on the horizon. They are extremely rare, usually observed on clear midsummer nights between latitudes of 55 and 65 degrees north. They become visible at the same time as the brightest stars and appear more brilliant after midnight. They can occur at nearly 50 miles above the earth. Particles collected by rockets in 1962 provided strong indications that these clouds consist of ice crystals. 

You will find a history of weather forecasting at Hoy Heritage Centre along with Jeff’s weather records, the original Rackwick Weather Station observation chart and some useful weather books.

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