A cloud is defined as ‘a visible aggregate of minute droplets of water or particles of ice or a mixture of both floating in the free air’. Each droplet has a diameter of about a hundredth of a millimetre and each cubic metre of air will contain 100 million droplets. Because the droplets are so small, they can remain in liquid form in temperatures of -30 °C. If so, they are called supercooled droplets.
Clouds at higher and extremely cold levels in the atmosphere are composed of ice crystals – these can be about a tenth of a millimetre long.
Clouds form when the invisible water vapour in the air condenses into visible water droplets or ice crystals. For this to happen, the parcel of air must be saturated, i.e. unable to hold all the water it contains in vapour form, so it starts to condense into a liquid or solid form. There are two ways by which saturation is reached.
(a) By increasing the water content in the air, e.g. through evaporation, to a point where the air can hold no more.
(b) By cooling the air so that it reaches its dew point – this is the temperature at which condensation occurs, and is unable to ‘hold’ any more water. Figure 1 shows how there is a maximum amount of water vapour the air, at a given temperature, can hold. In general, the warmer the air, the more water vapour it can hold. Therefore, reducing its temperature decreases its ability to hold water vapour so that condensation occurs.
Method (b) is the usual way that clouds are produced, and it is associated with air rising in the lower part of the atmosphere. As the air rises it expands due to lower atmospheric pressure, and the energy used in expansion causes the air to cool. Generally speaking, for each 100 metres which the air rises, it will cool by 1 °C, as shown in Figure 2. The rate of cooling will vary depending on the water content, or humidity, of the air. Moist parcels of air may cool more slowly, at a rate of 0.5 °C per 100 metres.
Therefore, the vertical ascent of air will reduce its ability to hold water vapour, so that condensation occurs. The height at which dew point is reached and clouds form is called the condensation level.
There are five factors which can lead to air rising and cooling.
1. Surface heating. The ground is heated by the sun which heats the air in contact with it causing it to rise. The rising columns are often called thermals.
2. Topography. Air forced to rise over a barrier of mountains or hills. This is known as orographic uplift.
3. Frontal. A mass of warm air rising up over a mass of cold, dense air. The boundary is called a ‘front’.
4. Convergence. Streams of air flowing from different directions are forced to rise where they meet.
5. Turbulence. A sudden change in wind speed with height creating turbulent eddies in the air.
Another important factor to consider is that water vapour needs something to condense onto. Floating in the air are millions of minute salt, dust and smoke particles known as condensation nuclei which enable condensation to take place when the air is just saturated.
Types of clouds
In 1803 a retail chemist and amateur meteorologist called Luke Howard proposed a system which has subsequently become the basis of the present international classification. Howard also become known by some people as ‘the father of British meteorology’, and his pioneering work stemmed from his curiosity into the vivid sunsets in the late 18th century following a series of violent volcanic eruptions. They had ejected dust high up into the atmosphere, thereby increasing the amount of condensation nuclei, and producing spectacular cloud formations and sunsets.
Howard recognised four types of cloud and gave them the following Latin names.
Cumulus heaped or in a pile
Stratus in a sheet or layer
Cirrus thread-like, hairy or curled
Nimbus a rain bearer
If we include another Latin word altum meaning height, the names of the 10 main cloud types are all derived from these five words and based upon their appearance from ground level and visual characteristics.
The cloud types are split into three groups according to the height of their base above mean sea level. Note that ‘medium’ level clouds are prefixed by the word alto and ‘high’ clouds by the word cirro (see Table 1). All heights given are approximate above sea level in mid-latitudes. If observing from a hill top or mountain site, the range of bases will accordingly be lower.
Table 1: The 10 main cloud type
Low clouds Surface – 7,000 ft
Medium clouds 7,000 – 17,000 ft
High clouds 17,000 – 35,000 ft
Cumulus (Cu) Height of base: 1,200-6,000 ft Colour: White on its sunlit parts but with darker undersides. Shape: This cloud appears in the form of detached heaps. Shallow cumulus may appear quite ragged, especially in strong winds, but well formed clouds have flattened bases and sharp outlines. Large cumulus clouds have a distinctive ‘cauliflower’ shape. Other features: Well developed cumulus may produce showers.
Cumulonimbus (Cb) Height of base: 1,000-5,000 ft Colour: White upper parts with dark, threatening undersides. Shape: A cumulus-type cloud of considerable vertical extent. When the top of a cumulus reaches great heights, the water droplets are transformed into ice crystals and it loses its clear, sharp outline. At this stage the cloud has become a cumulonimbus. Often, the fibrous cloud top spreads out into a distinctive wedge or anvil shape. Other features: Accompanied by heavy showers, perhaps with hail and thunder. By convention Cb is usually reported if hail or thunder occur, even if the observer does not immediately recognise the cloud as Cb (it may be embedded within layers of other cloud types).
Stratus (St) Height of base: surface-1,500 ft Colour: Usually grey. Shape: May appear as a layer with a fairly uniform base or in ragged patches, especially during precipitation falling from a cloud layer above. Fog will often lift into a layer of stratus due to an increase in wind or rise in temperature. As the sun heats the ground the base of stratus cloud may rise and break becoming shallow cumulus cloud as its edges take on a more distinctive form. Other features: If thin, the disc of the sun or moon will be visible (providing there are no other cloud layers above). If thick, it may produce drizzle or snow grains.
Stratocumulus (Sc) Height of base: 1,200-7,000 ft Colour: Grey or white, generally with shading. Shape: Either patches or a sheet of rounded elements but may also appear as an undulating layer. When viewed from the ground, the size of individual elements will have an apparent width of more than 5° when at an elevation greater than 30° (the width of three fingers at arm’s length). Other features: May produce light rain or snow. Sometimes the cloud may result from the spreading out of cumulus, giving a light shower.
Altocumulus (Ac) Height of base: 7,000-17,000 ft Colour: Grey or white, generally with some shading. Shape: Several different types, the most common being either patches or a sheet of rounded elements but may also appear as a layer without much form. When viewed from the ground, the size of individual elements will have an apparent width of 1 to 5° when at an elevation greater than 30° (the width of one to three fingers at arm’s length). Even if the elements appear smaller than this the cloud is still classified altocumulus if it shows shading. Other features: Occasionally some slight rain or snow, perhaps in the form of a shower may reach the ground. On rare occasions, a thunderstorm may occur from one type of Ac known as altocumulus castellanus – so called because in outline, the cloud tops look like a series of turrets and towers along a castle wall.
Altostratus (As) Height of base: 8,000-17,000 ft Colour: Greyish or bluish. Shape: A sheet of uniform appearance totally or partly covering the sky. Other features: Sometimes thin enough to reveal the sun or moon vaguely, as through ground glass. Objects on the ground do not cast shadows. May give generally light rain or snow, occasionally ice pellets, if the cloud base is no higher than about 10,000 ft.
Nimbostratus (Ns) Height of base: 1,500-10,000 ft Colour: Dark grey. Shape: A thick, diffuse layer covering all or most of the sky. Other features: Sun or moon always blotted out. Accompanied by moderate or heavy rain or snow, occasionally ice pellets. Although classed as a medium cloud, its base frequently descends to low cloud levels. May be partly or even totally obscured by stratus forming underneath in precipitation.
Cirrus (Ci) Height of base: 17,000-35,000 ft Colour: Composed of ice crystals, therefore white. Shape: Delicate hair-like filaments, sometimes hooked at the end; or in denser, entangled patches; or occasionally in parallel bands which appear to converge towards the horizon. Other features: The remains of the upper portion of a cumulonimbus is also classified as cirrus.
Cirrocumulus (Cc) Height of base: 17,000-35,000 ft Colour: Composed of ice crystals, therefore white. Shape: Patches or sheet of very small elements in the form of grains or ripples or a honeycomb. When viewed from the ground, the size of individual elements will have an apparent width of less than 1° when at an elevation greater than 30° (no greater than the width of a little finger at arm’s length). Other features: Sometimes its appearance in a regular pattern of ‘waves’ and small gaps may resemble the scales of a fish, thus giving rise to the popular name ‘mackerel sky’ (this name may also be attributed to high altocumulus clouds).
Cirrostratus (Cs) Height of base: 17,000-35,000 ft Colour: Composed of ice crystals, therefore white. Shape: A transparent veil of fibrous or smooth appearance totally or partly covering the sky. Other features: Thin enough to allow the sun to cast shadows on the ground unless it is low in the sky. Produces halo phenomena, the most frequent being the small (22°) halo around the sun or moon ≬ a little more than the distance between the top of the thumb and the little finger spread wide apart at arm’s length.
Condensation trails (contrails) These are thin trails of condensation, formed by the water vapour rushing out from the engines of jet aircraft flying at high altitudes. They are not true clouds, but can remain in the sky for a long time, and grow into cirrus clouds.
What influences the colour of clouds?
Light from both the sky and from clouds is sunlight which has been scattered. In the case of the sky, the molecules of air (nitrogen and oxygen) undertake the scattering, but the molecules are so small that the blue part of the spectrum is scattered more strongly than other colours.
The water droplets in the cloud are much larger, and these larger particles scatter all of the colours of the spectrum by about the same amount, so white light from the sun emerges from the clouds still white.
Sometimes, clouds have a yellowish or brownish tinge – this is a sign of air pollution.
Why do clouds stop growing upwards?
Condensation involves the release of latent heat. This is the ‘invisible’ heat which a water droplet ‘stores’ when it changes from a liquid into a vapour. Its subsequent change of form again releases enough latent heat to make the damp parcel of air warmer than the air surrounding it. This allows the parcel of air to rise until all of the ‘surplus’ water vapour has condensed and all the latent heat has been released.
Therefore, the main reason which stops clouds growing upwards is the end of the release of latent heat through the condensation process. There are two other factors which also play a role. Faster upper atmospheric winds can plane off the tops of tall clouds, whilst in very high clouds, the cloud might cross the tropopause, and enter the stratosphere where temperatures rise, rather than decrease, with altitude. This thermal change will prevent further condensation.
Why are there no clouds on some days?
Even when it is very warm and sunny, there might not be any clouds and the sky is a clear blue. The usual reason for the absence of clouds will be the type of pressure, with the area being under the influence of a high pressure or anticyclone. Air would be sinking slowly, rather than rising and cooling. As the air sinks into the lower part of the atmosphere, the pressure rises, it becomes compressed and warms up, so that no condensation takes place. In simple terms, there are no mechanisms for clouds to form under these pressure conditions.
The cloud amount is defined as ‘the proportion of the celestial dome which is covered by cloud.’ The scale used is eighths, or oktas, with observers standing in an open space or on a rooftop to get a good view or panorama of the sky.
Complete cloud cover is reported as 8 oktas, half cover as 4 oktas, and a completely clear sky as zero oktas. If there is low-lying mist or fog, the observer will report sky obscured.
The reporter will also report the amount of each cloud level – 2 oktas of cumulus and 3 oktas of cirrus, etc.
The frequent passage of depressions across the United Kingdom means that the most commonly reported cloud amount is, not surprisingly, 8 oktas. A clear blue sky, i.e. zero oktas, is less common, as often on hot, sunny days, there are small wispy layers of cirrostratus or fine tufts of thin cirrus at high altitudes.
The formation of precipitation
Cooling, condensation and cloud formation is the start of the process which results in precipitation. But not all clouds will produce raindrops or snowflakes – many are so short-lived and small that there are no opportunities for precipitation mechanisms to start.
There are two theories that explain how minute cloud droplets develop into precipitation.
10.1 The Bergeron Findeisen ice-crystal mechanism
If parcels of air are uplifted to a sufficient height in the troposphere, the dew-point temperature will be very low, and minute ice crystals will start to form. The supercooled water droplets will also freeze on contact with these ice nuclei.
The ice crystals subsequently combine to form larger flakes which attract more supercooled droplets. This process continues until the flakes fall back towards the ground. As they fall through the warmer layers of air, the ice particles melt to form raindrops. However, some ice pellets or snowflakes might be carried down to ground level by cold downdraughts.
10.2 Longmuir’s collision and coalescence theory
This applies to ‘warm’ clouds, i.e. those without large numbers of ice crystals. Instead they contain water droplets of many differing sizes, which are swept upwards at different velocities so that they collide and combine with other droplets.
It is thought that when the droplets have a radius of 3 mm, their movement causes them to splinter and disintegrate, forming a fresh supply of water droplets.
This theory allows droplets of varying sizes to be produced, and as shown in the table below, each will have a different terminal (or falling) velocity.
Particle radius (mm)
Terminal velocity (m/s)
Table 2: The terminal velocities of different particle sizes
10.3 Man-made rain
In recent years, experiments have taken place, chiefly in the USA, China and the former USSR, adding particles into clouds that act as condensation or freezing nuclei. This cloud seeding involves the addition into the atmosphere from aircraft of dry ice, silver iodide or other hygroscopic substances. These experiments have largely taken place on the margins of farming areas where rainfall is needed for crop growth, or to divert rain from major events such as the 2008 Beijing Olympics.
The nature of clouds
A classification of clouds was introduced by Luke Howard (1772-1864) who used Latin words to describe their characteristics.
Cirrus – a tuft or filament (e.g. of hair)
Cumulus – a heap or pile
Stratus – a layer
Nimbus – rain bearing
There are now ten basic cloud types with names based on combinations of these words (the word ‘alto’, meaning high but now used to denote medium-level cloud, is also used).
Clouds form when moist air is cooled to such an extent that it becomes saturated. The main mechanism for cooling air is to force it to rise. As air rises it expands – because the pressure decreases with height in the atmosphere – and this causes it to cool. Eventually it may become saturated and the water vapour then condenses into tiny water droplets, similar in size to those found in fog, and forms cloud. If the temperature falls below about minus 20 °C, many of the cloud droplets will have frozen so that the cloud is mainly composed of ice crystals.
The main ways in which air rises to form cloud
Rapid local ascent when heated air at the earth’s surface rises in the form of thermal currents (convection).
Slow, widespread, mass ascent where warm moist air is forced to rise above cold air. The region between warm and cold air is called a ‘front’.
Upward motion associated with turbulent eddies resulting from the frictional effect of the earth’s surface.
Air forced to rise over a barrier of mountains or hills.
The first of these tends to produce cumulus-type clouds, whereas the next two usually produce layered clouds. The last can produce either cumulus-type cloud or layered cloud depending upon the state of the atmosphere. The range of ways in which clouds can be formed and the variable nature of the atmosphere give rise to the enormous variety of shapes, sizes and textures of clouds.
Types of cloud
The ten main types of cloud can be separated into three broad categories according to the height of their base above the ground: high clouds, medium clouds and low clouds.
High clouds are usually composed solely of ice crystals and have a base between 18,000 and 45,000 feet (5,500 and 14,000 metres).
Cirrus – white filaments
Cirrocumulus – small rippled elements
Cirrostratus – transparent sheet, often with a halo
Medium clouds are usually composed of water droplets or a mixture of water droplets and ice crystals, and have a base between 6,500 and 18,000 feet (2,000 and 5,500 metres).
Altocumulus – layered, rippled elements, generally white with some shading
Altostratus – thin layer, grey, allows sun to appear as if through ground glass
Nimbostratus – thick layer, low base, dark. Rain or snow falling from it may sometimes be heavy
Low clouds are usually composed of water droplets – though cumulonimbus clouds include ice crystals – and have a base below 6,500 feet (2,000 metres).
Stratocumulus – layered, series of rounded rolls, generally white with some shading
Stratus – layered, uniform base, grey
Cumulus – individual cells, vertical rolls or towers, flat base
Cumulonimbus – large cauliflower-shaped towers, often ‘anvil tops’, sometimes giving thunderstorms or showers of rain or snow
Most of the main cloud types can be subdivided further on the basis of shape, structure and degree of transparency.
Cumulus clouds are often said to look like lumps of cotton wool. With a stiff breeze, they march steadily across the sky; their speed of movement gives a clue to their low altitude. Cumulus clouds occasionally produce light showers of rain or snow.
Typically, the base of cumulus clouds will be about 2,000 feet (600 metres) above ground in winter, and perhaps 4,000 feet (1,200 metres) or more on a summer afternoon. Individual clouds are often short-lived, lasting only about 15 minutes. They tend to form as the ground heats up during the day and become less frequent as the sun’s heat wanes towards evening.
The cause of small cumulus clouds is usually convection. Heat from the sun warms the ground, which in turn warms the air above. If a ‘parcel’ of warm air is less dense than the cooler air around it or above it, the ‘parcel’ of air starts to rise – this is known as a ‘thermal’. As it rises it expands and cools, and, if cooled sufficiently, the water vapour condenses out as tiny cloud droplets. A cumulus cloud is born.
The air within the cloud will continue to rise until it ceases to be buoyant. On some sunny days there is insufficient moisture or instability for moisture to form.
In hilly regions, a high, south-facing slope acts as a good source of thermals, and therefore of cumulus. Occasionally, a power station or factory will produce a cloud of its own.
When air rises in thermals there must be compensating downdraughts nearby. These create the clear areas between cumulus clouds and make it easier for glider pilots to find the thermals that they can use to gain height.
Just as cumulus is heaped cloud, so cumulonimbus is a heaped rain cloud (nimbus means rain).
In many ways the rain-bearing variety can be considered as a bigger, better-organised version of the cumulus. A cumulonimbus may be 10 km across and extend 10 km above the ground. This compares with a cumulus cloud which is typically a few hundred metres across and reaches a height of only a few kilometres. Instead of a ball of cotton wool, a cumulonimbus will resemble a huge cauliflower of sprouting towers and bulging turrets.
But there is one important structural difference in that the uppermost levels of the cumulonimbus have turned to ice and become fibrous in appearance, whereas cumulus clouds are composed entirely of water droplets. This icy section at the top may flatten out into an ‘anvil’ shape when the cloud is fully developed. When it reaches this stage, the base is usually dark, and there will be showers of rain or, sometimes, hail. In winter, the showers may be of sleet or snow. The showers are often quite heavy and may be accompanied by lightning and thunder.
Sometimes cumulonimbus will be ’embedded’ or half hidden among other clouds. On other occasions they will be well separated and the ‘anvil’ may well be visible many miles away. Cumulonimbus clouds may be seen at any time of the day, but are most common inland during the afternoon in spring and summer, and frequently occur in the tropics. They develop where convection is at its strongest and most organised.
The lifetime of a cumulonimbus is usually less than one hour.
There are exceptions though. The ‘Hampstead storm’ of 14 August 1975 was an example of a cumulonimbus cloud that managed to keep regenerating itself over one small area of London. About 170 mm of rain fell in three hours, causing severe flooding.
Stratus is a low-level layer cloud (not to be confused with altostratus and cirrostratus, which are much higher). In appearance, it is usually a featureless grey layer. Sometimes, when a sheet of stratus is affecting an area, the cloud base will be right down to the ground and will cause fog. However, the usual base is between the ground and 1,000 feet (300 metres), which means that hilltops may be obscured by cloud. Sometimes stratus will produce drizzle or light snow, particularly over hills.
Perhaps the most important indication of its low altitude is its apparent rapid movement across the sky in any wind stronger than a flat calm. For example, a stratus cloud at 500 feet (150 metres) moving at 20 miles per hour will appear to move much faster than altostratus with its base at 10,000 feet (3,000 metres) moving at 60 miles per hour.
An approximate guide to the height of stratus may be gained by measuring the relative humidity and subtracting it from 100. The resulting number gives some idea of the height of the low cloud in hundreds of feet. For example, 94% relative humidity would indicate that the stratus is about 600 feet (180 metres) above the ground.
Stratus forms as the result of condensation in moist air at low levels due to cooling. The cooling may be caused in a number of ways:
lifting of air over land due to hills or ‘bumping’ over rough ground;
warm air moving over a cold sea. If the cloud moves in over the land, it will readily cover any relatively high ground. In some cases, the base of the cloud falls to the sea surface, causing fog. This may drift in over the coast and is called sea fog, though it goes by the name of haar in the north and east of Scotland and fret in the east of England;
temperature falling over land at night. The air may have been brought inland during the day on a sea breeze. There needs to be some wind, otherwise the cooling may lead to radiation fog.
Stratocumulus clouds usually form between 1,000 and 6,500 feet (300 and 2,000 metres).
Stratocumulus will often give a sheet of almost total cloud cover, with perhaps one or two breaks. The cloud elements are rounded and almost join up. Occasionally, the sheet is composed of a series of more or less parallel rolls, which often, but not always, lie ‘across the wind’. Stratocumulus sometimes produces light falls of rain or snow.
Stratocumulus is formed by weak convection currents, perhaps triggered by turbulent airflows aloft. The convection affects a shallow zone because dry, stable air above the cloud sheet prevents further upward development.
Sometimes there are huge sheets of stratocumulus covering thousands of square kilometres around the flanks of a high pressure system, especially over the oceans. The weather below such sheets tends to be dry, but it may be rather dull if the cloud is two or three thousand feet thick.
Altocumulus clouds usually form between 6,500 and 17,000 feet (2,000 and 5,000 metres) and are referred to as medium level clouds.
In most cases, there is little difference between the properties of stratocumulus and altocumulus, since both are composed of water droplets and are normally limited in vertical extent. The deciding factor between stratocumulus and altocumulus normally comes down to height as both types are formed in the same way.
Altocumulus also provides a sort of dappled pattern, but, since it is at a greater altitude, the cloud elements look smaller. One significantly different form is altocumulus castellanus, which is like a vigorous medium-level cumulus , sometimes with rain falling from their base, known as trailing virga. This type of cloud is sometimes an indication that thunderstorms will follow
Altostratus clouds normally have a base between 8,000 and 17,000 feet (2,500 and 5,000 metres).
Altostratus appears as a uniform sheet either totally or partially covering the sky. Sometimes it is thin enough to just reveal the sun or moon. The sun appears as if through ground glass but shadows are not visible on the ground. Sometimes, if the base is below 10,000 feet (3,000 metres) it may give light rain or snow.
Nimbostratus clouds are found between 1,500 and 10,000ft (450 and 3,000 metres).
Nimbostratus forms a thick, diffuse layer of dark grey cloud covering all or most of the sky, which always obscures the sun or moon. It is accompanied by moderate or heavy rain or snow, occasionally ice pellets. Although classed as a medium cloud, its base frequently descends to low cloud levels. Nimbostratus may be partly or even totally obscured by stratus forming underneath in precipitation.
Cirriform clouds (i.e. clouds from the cirrus family) are found at high altitude, usually above 20,000 feet (6,000 metres). They are composed of ice crystals. Three types of cloud make up the group: cirrus, cirrostratus and cirrocumulus.
Cirrus itself is very common in the British Isles and throughout most of the world. It is thin, wispy and white in appearance, and its name, coming from the Latin word for ‘tuft of hair’, gives a good description of the cloud. Another name for the cloud, ‘mares tails’, also conjures up an accurate image. Cirrus may be hooked or straight depending on the airflow aloft. Sometimes it comes as a very dense patch which is left over from the ‘anvil’ cloud of a cumulonimbus that has disappeared. On other occasions, cirrus may be quite extensive when associated with a jet stream – the cloud can then be seen moving across the sky, despite its great altitude. Aircraft condensation trails are a form of man-made cirrus. They can sometimes be seen in ‘historical’ films, to the delight of film buffs who enjoy spotting technical inaccuracies.
Cirrostratus is a fairly uniform sheet of thin cloud through which the sun or moon can be seen. Sometimes, if the cloud is thin, a bright ring of light (called a halo) surrounds the sun or moon. A layer of cirrostratus is often an indication of a deterioration in the weather.
Cirrocumulus is often present in small amounts along with cirrus, but rarely does it dominate the sky. On those occasions when it is widespread, a beautiful spectacle is created, especially at sunset. The individual clouds appear very small – often tiny rows of roughly spherical pear-like cloud elements. Sometimes they occur in undulating patterns like tiny ripples.
This information sheet is based on a series of articles written by Dick File that appeared in The Guardian. Web page reproduced with the kind permission of the Met Office
1. Make concise definitions of the following terms. (a) Condensation. (b) Dew point. (c) Supercooled. (d) Humidity.
2. Explain the two ways by which parcels of air can reach saturation.
3. Outline the five factors that will cause parcels of air to rise and cool.
4. Match up the descriptions in list B with the correct term in list A: List A: Cumulus; Cirrus; Stratus; Nimbus. List B: Rain bearer; Heaped; Thread-like or hairy; Sheets or layers.
5. Which of the following are correct statements? (i) Low clouds form up to 10,000 feet above the surface. (ii) High clouds form between 17,000 and 35,000 feet above the surface. (iii) Altocumulus and altostratus are two types of high cloud. (iv) Nimbostratus is a medium-level cloud. (v) Cumulonimbus is a low cloud.
6. Describe the likely characteristics of the following cloud types. (a) Cumulus (b) Stratus (c) Cirrus
7. With which cloud formations would you associate the phrase ‘mackerel sky’?
8. What weather conditions might follow the appearance of altocumulus castellanus?
9. What are contrails? What clouds might they produce over time?
10. Why do most clouds appear white?
11. What prevents clouds from building up to very high levels in the troposphere?
12. Under what conditions might you find warm, sunny weather, but no clouds forming?
13. Outline how clouds are measured by observers.
14. Which amount of cloud cover is most commonly observed in the British Isles? Explain why?
15. Why is it quite rare to observe zero oktas of cloud cover?
16. Explain the two theories that explain how cloud droplets turn into precipitation.
It was reported in The Times newspaper on 15 April 1986 that a hailstorm lashing Dhaka, the capital of Bangladesh, had killed nearly 50 people and injured more than 400. The storm had brought winds of about 60 mph and hailstones weighing up to 2 lb (nearly 1 kg). Houses had been flattened, communications disrupted and the windscreens of more than 700 cars shattered. In such conditions, an umbrella was no use whatsoever; even a riot shield may not have provided adequate protection! According to Dick File, in Weather Facts (Oxford University Press, 1991), this storm (which struck on 14 April 1986) killed 92 people and produced hailstones that weighed 1.02 kg.
The heaviest hailstones to fall on the United Kingdom did so at Horsham, West Sussex, on 5 September 1958 and weighed 140 g. They were almost the size of a tennis ball. When they hit the ground, they were travelling at speeds in excess of 100 mph (50 m/s). If you find this surprising, do a little calculation, using the formula: V2 = u2 + 2as where u is the initial speed, v the terminal speed, a the acceleration (in this case, due to gravity) and s the distance travelled. For a hailstone falling from a height of 500 m through still air, v = 100 m/s! The impact of a missile the size of a tennis ball travelling this fast is much more serious than that of a cricket ball hit for six.
Should you ever get the chance, collect some large hailstones and cut them in half. You may find a layered structure, with alternate layers of clear and opaque ice (as in the picture on the right, which shows a section of a hailstone viewed by transmitted light). The layers are acquired in different parts of the storm clouds. As hailstones fall, they collect tiny water droplets, which flow around them and freeze. If no air is trapped, the ice is clear.
The storm which struck the Wokingham area of Berkshire on 9 July 1959 produced hailstones more than 2.5 cm in diameter. This storm was studied in detail by Professor Frank Ludlam of Imperial College and his team of co-workers, who produced a striking three-dimensional model of the airflow with-in the storm and explained how large multi-layered hailstones may form in such weather systems.
In the diagram on the right, streamlines of air in which condensation occurred are shaded. The surface areas affected by rain and hail are shown by, respectively, grey and black shading. Heights are shown in thousands of feet. Precipitation formed in air which entered the storm near position H. As shown, the precipitation was carried across relative to the storm to around 13-15,000 feet, whereupon it fell and re-entered the strong updraught near posit-ion O. Some precipitation particles reached altitu-des of 30,000 feet or more and grew into large hail-stones before falling again, forward of the strong updraught, near position H’. The storm moved from left to right, with rain on its left flank and a squally ‘gust front’ (shown as a cold front) on its right flank. Behind the storm, chilled air reached the ground.
The diagram on the right shows a vertical section through a typical severe hailstorm (moving from right to left) and is also taken from Ludlam’s 1961 article in Weather. Compare this diagram with the three-dimensional model above
The paths of the air are drawn as if the storm was stationary. They are, therefore, relative streamlines. The dashed lines are trajectories of small hailstones. The thick full line shows the trajectory of a large hailstone.
To some extent, the features shown on this vertical section occur also in vigorous cumulo-nimbus systems which do not produce large hail. Students can look out for mamma, the udder-like cloud feature that hangs under the anvil and other parts of the cloud. How are mamma formed? Students can also observe gust fronts and measure the temperature drop that occurs when a storm passes. It is often several degrees Celsius. Perhaps, with the help of someone who has a car, they can map areas of rain and hail relative to moving storms.
In severe storms, downdraughts may be as strong as 30-40 m/s and reach the ground as ‘down-bursts’. These are dangerous, as the diagram above shows. Downbursts spread out near the ground. An aeroplane that flies into such an outflow first encounters an increasing head-wind (at 1 and 2), which adds to the speed of the speed of the air flowing over the aircraft’s wings and thus increases lift. At 3, however, the strength of the downdraught begins to reduce the altitude of the aircraft; and at 4 and 5 the aircraft experiences both a tail-wind (which reduces air speed and lift) and a downward force from the downdraught. Over the years, there have been many air disasters caused this way, especially in North America.
On 15 August 1952, the village of Lynmouth in North Devon was devastated by a torrent of water which poured off Exmoor; 34 people died. On 29 May 1920, in and around the Lincolnshire town of Louth, 22 people died when water from a storm over the Wolds caused the River Lud, normally a small stream, to rise 5 m above its normal level. In Dorset and Somerset, there have been similar occurrences; and in all cases, severe storms caused the havoc. When such storms occur in the British Isles, the wind in the upper troposphere is typically from the south-west, with the wind in the lower troposphere from a north-easterly point (and pressure low to the south and south-west). If this flow is lifted orographically, the storm may become stationary and deposit several inches of rain in a short time. Thus, it is places below slopes that face northwards or north-eastwards that are most at risk.
To frighten away the evil spirits that caused hail, primitive tribes used to shoot arrows into storm clouds; and Christians have tried to exorcise these spirits by ringing church bells (a dangerous practice because of lightning strikes on bell towers). Not only arrows, but also cannon-balls, artillery shells and rockets have been fired into storm clouds, but all to no avail. Though there is some evidence that cloud seeding may help to reduce the size of hailstones, there is nothing we can yet do to prevent the formation of severe storms.
The word monsoon is derived from the Arabic word ‘mausim’, meaning season. It was first used by Arabic navigators to describe the seasonal winds of the Arabian Sea. These generally blow from the north-east for one half of the year, and from the south-west for the other half. Although the term monsoon actually means a seasonal wind, it is often used to refer to the torrential rainfall associated with these winds.
Monsoons occur mainly in tropical regions – northern Australia, Africa, South America and the USA. However, the best known area affected by monsoons is south-east Asia, particularly India. During the winter, air over the Siberian plateau becomes colder than air over the surrounding seas, producing a large anticyclone with winds circulating clockwise, thus causing cool north-easterly winds to blow across India and its neighbouring countries. This brings dry, pleasant weather, and has a marked drying effect on the land. During April and May the winds abate, causing temperatures to rise rapidly to over 35 °C.
In the summer the process reverses. The Siberian plateau is now warmer than the seas, and low pressure develops over these seas. The winds circulate anticlockwise and approach India from the south-west, bringing very moist air. These south-westerly winds bring a drop in temperature and heavy downpours of rain. In fact, during this monsoon, which generally lasts from June to September, India receives virtually all its rainfall for each year.
The mountains of southern India split the summer winds. The western arm of the monsoon is deflected northwards, by the western Ghats, to Bombay and then on to Pakistan. The eastern arm travels up through the Bay of Bengal to Calcutta and Assam, and is deflected north-westwards by the Himalayas. On average, the winds arrive in southern India about six weeks before they arrive in north-west India.
The heaviest monsoon rainfalls occur where the winds blow side-on to the hills. The higher the hills and more moist the air, then the greater the amount of rainfall. These factors give Cherrapunji, inWeb page reproduced with the kind permission of the Met Office Assam, one of the highest rainfalls in the world; the western monsoon winds having travelled an extra distance over the warm seas of the Bay of Bengal, then meeting the Himalayas. On average, Cherrapunji has an annual rainfall total of nearly 11 metres, the maximum monthly amount occurring in June. Bombay, in the eastern monsoon, receives about 1.8 metres with the maximum monthly total in July. In comparison, Delhi registers only 64 cm of rainfall each year (about the same as London), with the maximum monthly total occurring in both July and August. At Madras the pattern of rainfall is different because the monsoon winds blow along the coast. Here, the rainfall increases gradually through the summer months with larger amounts falling in October and November, owing to tropical cyclones travelling westwards across the Bay of Bengal.
Monsoon rainfalls are unreliable in that the amount varies considerably from year to year. Low rainfalls cause great problems for agriculture and water supplies in general. On the other hand, even moderate rainfalls can cause flood hazards. The eastern monsoon releases most of its rainfall in the Ganges plain, causing flooding to low-lying areas where the river flows into the Bay of Bengal. In the Indus river the flood problem is often made worse because the monsoon rainfalls can coincide with high river levels in its tributaries, caused by water from the melting mountain snows of the Himalayas.
Web page reproduced with the kind permission of the Met Office
A high pressure system, also known as an anticyclone occurs when the weather is dominated by stable conditions. Under an anticyclone air is descending, forming an area of higher pressure at the surface. Because of these stable conditions, cloud formation is inhibited, so the weather is usually settled with only small amounts of cloud cover. In the Northern Hemisphere winds blow in a clockwise direction around an anticyclone. As isobars are normally widely spaced around an anticyclone, winds are often quite light.
Anticyclones can be identified on weather charts as an often large area of widely spaced isobars, where pressure is higher than surrounding areas.
In winter the clear, settled conditions and light winds associated with anticyclones can lead to frost and fog. The clear skies allow heat to be lost from the surface of the earth by radiation, allowing temperatures to fall steadily overnight, leading to air or ground frosts. Light winds along with falling temperatures can encourage fog to form; this can linger well into the following morning and be slow to clear. If high pressure becomes established over Northern Europe during winter this can bring a spell of cold easterly winds to the UK.
In summer the clear settled conditions associated with anticyclones can bring long sunny days and warm temperatures. The weather is normally dry, although occasionally, very hot temperatures can trigger thunderstorms. An anticyclone situated over the UK or near continent usually brings warm, fine weather.
Low pressure systems
A low pressure system, also known as a depression occurs when the weather is dominated by unstable conditions. Under a depression air is rising, forming an area of low pressure at the surface. This rising air cools and condenses and helps encourage cloud formation, so the weather is often cloudy and wet. In the Northern Hemisphere winds blow in anticlockwise direction around a depression. Isobars are normally closely spaced around a depressions leading to strong winds.
Depressions can be identified on weather charts as an area of closely spaced isobars, often in a roughly circular shape, where pressure is lower than surrounding areas. They are often accompanied by fronts.
What to do next
Using this information on pressure systems you should now be able to complete worksheet 1.
A front is a boundary between two different types of air masses, these are normally warm moist air masses from the tropics and cooler drier air masses from polar regions. Fronts move with the wind so over the UK they normally move from west to east. The notes below provide information about the most common types of fronts. The descriptions given apply to active well developed fronts, weaker fronts may not display all the characteristics or they may be less well defined.
A warm front indicates that warm air is advancing and rising up over the colder air. This is because the warm air is ‘lighter’ or less dense, than the cold air. Therefore warm fronts occur where warmer air is replacing cooler air at the surface. As the warm front approaches there is a gradual deterioration in the weather. Clouds gradually lower from higher cirrus, through altostratus, to stratus and nimbostratus at the front. There is often a prolonged spell of rainfall which is often heavy. Behind the warm front the rain becomes lighter, turns to drizzle or ceases, but it remains cloudy. Temperatures rise behind the warm front and winds turn clockwise, also known as a wind ‘veer’. Pressure falls steadily ahead of and during the passage of the warm front, but then rises slowly after its passage.
The diagram below shows the formation of a warm front in diagrammatic form.
The diagram below shows a cross section through a warm front, with associated cloud, temperature and weather changes.
A cold front indicates that cold air is advancing and pushing underneath warmer air at the surface. This occurs because the cold air is ‘heavier’ or denser than the warm air. Therefore cold fronts occur where cooler air is replacing warmer air at the surface. The passage of weather associated with a cold front is much shorter lived than that with a warm front. As there is often a lot of cloud in the warmer air ahead of the cold front, there is often little indication of the approaching cold front. As the front passes temperatures fall and there is often a short spell of very heavy rain, sometimes with inbedded thunderstorms and cumulonimbus clouds. Behind the front the weather is much brighter with broken clouds but occasional showers. Winds veer with the passage of the cold front and are often strong and gusty, especially near showers. Pressure rises throughout the approach and passage of the cold front.
The diagram below shows the formation of a cold front in diagrammatic form.
The diagram below shows a cross section through a cold front, with associated cloud, temperature and weather changes.
In a mature depression the warm front normally precedes the cold front. Cold fronts generally travel much quicker than warm fronts, and eventually it will catch up with the warm front. Where the two fronts meet, warm air is lifted from the surface and an occlusion is formed. An occlusion can be thought of as having similar characteristics to both warm and cold fronts. The weather ahead of an occlusion is similar to that ahead of a warm front, whilst the weather behind is similar to that behind a cold front.
The diagrams below depict the formation of an occlusion.
The diagram below shows the occlusion in cross section.
A Norwegian scientist called Vilhelm Bjerknes devised a simple model which described how depressions developed from the meeting of warm and cold air. The model had four stages which are detailed below.
Origin and infancy
Initially a warm air mass such as one from the tropics, meets a cooler air mass, such as one from the polar regions. Depressions which affect the UK normally originate over the Atlantic Ocean.
The warm air rises up over the colder air which is sinking. A warm sector develops between the warm and cold fronts. The mature stage of a depression often occurs over the UK.
The cold front travels at around 40 to 50 miles per hour, compared to the warm front which travels at only 20 to 30 miles per hour. Therefore the cold front eventually catches up with the warm front. When this occurs an occlusion is formed.
Eventually the frontal system dies as all the warm air has been pushed up from the surface and all that remains is cold air. The occlusion dies out as temperatures are similar on both sides. This stage normally occurs over Europe or Scandinavia.
What to do next
Now you can go on to Part D – Depression cross-section and weather sequence.
Anticyclones, Depressions and Fronts
Part D – Depression cross-section and weather sequence
Cross-section through a classic Depression
Most depressions have a warm and cold front, more mature depressions may also have an occluded front. The diagram below shows a cross-section through a depression, showing the warm and cold fronts and an indication of the associated weather.
WEATHER ASSOCIATED WITH THE PASSAGE OF A CLASSIC DEPRESSION
Ahead of the warm front
Passage of the warm front
Passage of the cold front
starts to fall steadily
continues to fall
starts to rise
continues to rise
quite cold, starts to rise
continues to rise
cloud base drops and thickens (cirrus and altostratus)
cloud base is low and thick (nimbostratus)
cloud may thin and break
clouds thicken (sometimes with large cumulonimbus)
clouds thin with some cumulus
Wind speed and direction
speeds increase and direction backs
veers and becomes blustery with strong gusts
remain steady, backs slightly
speeds increase, sometimes to gale force, sharp veer
winds are squally
none at first, rain closer to front, sometimes snow on leading edge
continues, and sometimes heavy rainfall
rain turns to drizzle or stops
heavy rain, sometimes with hail, thunder or sleet
What to do next
Using this information on the passage of depressions you should now be able to complete worksheet 3, and worksheet 4.
Trees can tell stories about past climates. Scientists can decode the pattern of a tree’s growth rings to learn which years were warm or cool, and which were wet or dry. Scientists combine the ring patterns in living trees with wood from trees that lived long ago, such as the wood found in old logs, wooden furniture, buildings like log cabins, and wooden ships, in order to build a longer historical record of climate than the lifespan of a single tree can provide.
You can decode tree ring data to learn about past climates using the simulation above. Line up tree ring patterns to reveal temperatures in the past. The simulation has two versions. The standard version is the best place to start. The custom version for schools in the United Kingdom was created to go along with a specific curriculum. It has a longer timeline and includes information about some historical events.
The process scientists use to build a climate history timeline has an extra step that, for the sake of simplicity, is not represented in this simulation. When scientists decode long climate records from tree ring patterns, they don’t physically line up the tree core samples next to each other. Instead, they make graphs called skeleton plots for each sample. They combine the skeleton plots from many samples to build a climate history timeline.
Data source for this simulation The tree ring data in this simulation is from oak trees in southern England. The data, from the UK Oak Project, was collected from living trees, logs in bogs, beams and rafters in old buildings, old wooden furniture, and wall paintings in a farmhouse dating back to 1592. One sample came from the windlass – the wooden crank used to raise and lower a castle’s gate – of the Byward Tower in the Tower of London.
Collect tree ring samples, align the samples to create a 300 year record and see what weather and climate events emerge here.