Clouds

A brief guide on the primary cloud forms

High-level Clouds

Cirrus (Ci)

Cirrus is a wispy high level cloud, commonly referred to as Mare’s Tail. It is comprised of ice crystals and occurs at altitudes of around 6-10 km over Ireland

Cirrus over Galway City © Sean Tomkins

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Cirrocumulus (Cc)

A layer of small cumuliform clouds in the upper level of the troposhere, white in colour and without any shadows or dark patches. The individual cells are smaller than Altocumulus and can sometimes exhibit the same optical phenomena as Cirrus. They can be a sign of impending bad weather.

Cirrocumulus over Dublin.
©Elizabeth Smyth

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Cirrostratus (Cs)

A thin continuous layer of cirrus through which the sun is visible, causing optical phenomena such as a halo, mock sun, sun pillar, etc. This cloud is the first sign of an approaching warm front, and is followed by altostratus and eventually nimbostratus.

Cirrostratus over Co. Kilkenny
© Patrick Mullins

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Mid-level Clouds

Altocumulus (Ac)

A mid-level (2,000 – 4,000 m) cumulus cloud, white or grey in colour, with some shading visible in parts. It can occur in sheets (Altocumulus stratiformus), or as individual clouds, with each cloud being smaller than low-level cumulus but larger than Cirrocumulus.

Altocumulus over Co. Kildare
© Moria Vorster

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Altostratus (As)

An extensive and essentially uniform mid level layer cloud of varying opacity. It generally forms when a humid but stable air mass is forced to slowly rise ahead of an approaching warm front. While it can produce some precipitation this tends to evaporate before reaching the ground.

Altostratus over Co. Wicklow
© Maggie Boate

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Nimbostratus (Ns)

Densely lay­ered cloud that pro­duces wide­spread pre­cip­i­ta­tion. It nor­mally devel­ops from thick­en­ing Alto­stra­tus on the approach of large-scale weather sys­tems such as a warm or occluded front. Although classed as a mid-level cloud, it will occasionally be observed at lower levels as the frontal zone it is associated with moves over.

Nimbostratus over Mullingar.
© Rodney Cleary

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Low-level Clouds

Cumulus (Cu)

Fluffy, heaped clouds, with flat bases and generally well-defined edges. They form when warm air parcels rise and cool to their dewpoint, allowing saturation to occur. They are generally associated with fair weather, although they can develop vertically into Cumulonimbus (Cb) if the atmosphere is unstable enough.

Cumulus over the Hill of Tara
© Philip Wall

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Stratocumulus (Sc)

Gen­er­ally a low, flat based cloud of light ver­ti­cal extent which defines it from the more fea­ture­less stra­tus forms. They commonly form from either the break up of stra­tus or the spread­ing out of cumu­lus and is a good indi­ca­tor that the atmos­pheric con­di­tions are rel­a­tively stable. Like stra­tus, stra­tocu­mu­lus is one of the more com­mon cloud forms observed over Ireland

Stratocumulus over Co. Cork
© Madeleine McKeown

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Stratus (St)

Stratus comes from the Latin for layer, and it is just that – any type of flat, layered cloud. Unlike cumuluform clouds, it forms through the very slow rising of an air mass, or by radiative cooling of a warm humid air mass over cooler terrain. It is a pain in the backside of the Irish as it is our most common type of cloud and the reason why more than half of the year is overcast.

Stratus over Co. Kerry.
© Laura Jan Neakon

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Descriptions by Fergal Tierney, Ian Carruthers, Patrick Gordon & Peter O’Donnell
Photos by IWO Contributors