CLOUDS SESSION 6 2.10. 16 If you need to get some info on coming weather you can always ring the Met Office or check the app on your mobile. Another way is to look at the clouds and see what type they are and what they are doing. Clouds have always been a fascination to many people. Percy Bysshe Shelley, a famous English 19th century poet, wrote a fascinating poem entitled The Cloud. It covers all the things you can see when cloud watching. Many people like watching clouds. A man called Gavin Pretor-Pinney is so enthusiastic about Cloud Watching that he started The Cloud Appreciation Society in 2004 and shortly reached an international membership of 2000.There is a website etc so if you want to look at innumerable photos of clouds looking like horses, elephants, gorgeous sunsets and sunrises famous film stars, hills near you, dinosaurs etc., etc. go ahead and look. We have talked a little about clouds in the other sessions, particularly the WATER session. They are very important in the weather situation and a knowledge of the different types of cloud helps in getting some information about what s on its way. Clouds are formed in different ways and bring their own kinds of weather. They can form when air cools or when more vapour is added to the air. They are only formed when the air is a little dirty! Water vapour only condenses when there is something, dust, smoke salt or industrial pollution on which the vapour can condense. It can take up to a million drops to form one rain drop or ice particle and obviously the ambient temperature must be low enough. Clouds are visible accumulations of water droplets or ice crystals and are found mostly in the TROPOSPHERE and are moved by the wind. 1
They form when water vapour which evaporates from the earth s surface for whatever reason condenses onto minute particles of dust, dirt or salt present in the air. They are caused by a warm air mass rising or warm and cold air masses meeting or by a warm air mass moving over a cold surface. Warm air rises and as it cools it loses heat which diminishes its ability to hold moisture as vapour which then condenses into liquid. Clouds are formed when warm air rises and its temperature drops and/or it meets cold damp air. Those of you who have seen the film Ice cold in Alex will no doubt remember the scene in the bar when they are drinking cold beers and vapour has formed (condensed ) on the glasses. Remember the DEW POINT. Up to the 18 th century, little if anything was known about clouds. The first man to produce a serious description and list of clouds was Luke Howard. Born in London in 1772 he was educated in a Quaker school and apprenticed to a Stockport retail chemist. Whilst developing a successful Chemical manufacturing business, he also was interested in Meteorology. He was one of the several men in the 19 th Century who made very significant contributions to the development of Meteorology as a science. Besides his book on the classification of clouds, he wrote several others on various aspects of the science. He was elected a Fellow of the Royal Society and one of the first members of the British Met. Society which incidentally accepted his classification and is still in use world wide today. Originally he introduced 3 basic cloud types using Latin names with subsections which increased the classifications to 10. When I started my interest in meteorology, 10 was still the main group. Today there between 20-30 groups, sub groups and sub sub groups used by Meteorologists Clouds are defined both by how high they are in the atmosphere and the way they look. CIRRO ( wispy) as a prefix refers to high-altitude clouds,20000ft and more. ALTO prefix is used for medium height clouds STRATUS (blanket or covering) for low altitude cloud. Cloud on the ground is FOG NIMBO(rain) as a prefix or NIMBUS as a suffix means that the cloud can produce precipitation (Rain, hail, snow or sleet). CUMULO (heap) refers to lumpy or piled up clouds and STRATO refers to wide flat layered clouds. 2
The three main types he described are:- 1. CIRRUS CLOUD (A wisp of hair). He called this cloud by this name as he saw it as diverging fibres going in all or any direction 2. CUMULUS CLOUD (Heap). This he described as convex or conical heaps. 3. STRATUS CLOUD (Blanket or covering). He saw this as a wide horizontal sheet, increasing from below. He then used these cloud names in combination for 4 more cloud types. 4. CIRRO-CUMULUS. Small well defined round masses,sometimes referred to as summer clouds 5. CIRRO-STRATUS Thin wispy clouds in horizontal or slightly inclined masses. 6. CUMULO-STRATUS. The cirro-stratus intermixed or blended with the cumulus. NIMBUS or (cumulo-cirro-stratus). This he called the rain cloud. Since then the number of cloud descriptions has increased. There are currently around 20-30 descriptions 7. CIRRUS, CUMULUS & STRATUS These are three main cloud types. The other clouds are named basically because of the heights at which they form and to some degree their physical shape. LOW LEVEL CLOUDS are STRATUS or CUMULUS MEDIUM LEVEL draw the prefix ALTO HIGH LEVEL are CIRRUS Clouds with the pre or post fix NIMBUS or NIMBO are those most likely to bring rain or snow. NIMBO- CUMULUS OR CUMULO- NIMBUS can reach the TROPOPAUSE and are usually also thunderheads. If, however, they build high taller and dark clouds which do reach the 3
TROPAUSE, they tend to flatten out at the top into an anvil shape. They are CUMULO NIMBUS and can bring heavy rain, hail, lightning, thunder and high winds e.g. TORNADOES. This is because the air inside them is becomes rapidly rising and descending air currents. They can be anywhere from GROUND LEVEL TO 70000 FT ALTOSTRATUS CLOUDS are dark & boring and can occur between 6000 to 20000 FT Lenticular Altostratus, a type of this cloud, looks like a number of French loaves etc. STRATUS CLOUDS are usually light or dark grey, flat and boring. They are usually found below 6000 ft.thick stratus clouds can turn into NIMBOSTRATUS which can also produce rain, hail or snow or sometimes all three. NIMBOSTRATUS CLOUDS SURFACE - 10000 CIRROSTRATUS CLOUDS ABOVE 18000 FT CUMULUS CLOUDS are low level, globular, white, usually seen in fine weather and frequently change shape. They are found BELOW 6000ft The highest clouds usually form an anvil shape when they hit the TROPOSPHERE ALTOCUMULUS CLOUDS 6000 20000 FT Altocumulus clouds are sometimes called a Mackerel Sky probably because they bear a slight resemblance to fish scales. CIRROCUMULUS CLOUDS ABOVE 18000 FT. These clouds usually reach the TROPAUSE nd spread out to form a characteristic ANVIL shape. CIRRUS CLOUDS are made of ice crystals, are very thin and high, usually at 18000 FT/20000 FT or more. They are sometimes formed from aircraft s CONTRAILS or condensation trails. Usually they are signs of approaching warm fronts. SOLAR or LUNAR HALOES which are circles round the sun or moon usually signify worsening weather. Haloes are caused by light being refracted by ice crystals in cirrus clouds. They can be white or coloured, full or part coronas. Occasionally, when the sun is low in the sky they can produce coloured circles or part circles. All these clouds have sub sections. In fact there are so many that there are code names for the various types e.g. Cl 1,2 etc. for low sections, Cm 1,2 etc. for medium types & Ch 1,2 etc. for the high types. More sub sections are added when discovered or become noticeable. RAINBOWS are a phenomenon caused by reflection or refraction of light in water droplets. The result is a multi-coloured arc. They always appear directly opposite the Sun. 4
A CLOUDLESS SKY usually signifies settled weather, but in early spring, late autumn or winter, this can mean FROST and /or FOG in early mornings and late evenings. Many people cloud watch as a hobby. The famous 19 th century painter, John Constable, painted more than 100 cloud studies, in the open at Hampstead Heath. He annotated many of them using Luke Howard s classification. Someone once wrote that he found nothing more relaxing and pleasant than lying on his back in the grass on a fine, warm summer s day looking at clouds! XXXXXXX REF CLOUDS 5