We were delighted to support BBC Bitesize Scotland in the creation of these 21 videos for primary schools. Here are some of our best resources to support the teaching of these topics:
[all images on this page are copyright of the BBC]
This case study or ‘mystery’ is taken from the afternoon of the 2nd September 2013. It focusses on Scotland and N. England. It can be used for two different purposes – either to identify orographic/ relief rain (use images 2-6 below), or to go on to identify a case study of the Foehn Effect (use all images).
We recommend that teachers present students in groups with a series of images, sequentially, to allow them to work out what the weather is doing and why.
Students should be
Make a cloud in a bottle or watch the video.
Students can be helped, where appropriate to identify some of these points
Image 1 – temperatures in degrees Centigrade at 15Z (1500GMT) (copyright Met Office)
The temperatures show that it is considerably warmer on the East side of Scotland than on the West. Temperatures are up to 9°C warmer on the East.
Why?
Image 2 – a synoptic chart at 12Z (1200GMT) (copyright Met Office)
Image 3 – wind speed in knots at 15Z (1500GMT) (copyright Met Office)
Image 4 – satellite image at 15Z (1500GMT) : (c) EUMETSAT / Met Office
This is a satellite image from 1500Z (1500GMT) showing visible radiation ie light. The white areas are where the Sun’s light is being reflected from clouds.
There is cloud over the west coast of Scotland and N. England
You can also see the cloud associated with the warm front to the East of the UK, and the cold front to the west.
Image 5: Rainfall on 2nd September 2013 (copyright Met Office)
It is raining over the west coast of Scotland. Why?
A relief map of Scotland clearly shows the high ground on the East coast.
As the easterly winds blow in from the west, the air is forced to rise. As it rises, it cools until the rate of condensation is faster than the rate of evaporation. Cloud droplets form, which eventually become large enough to fall as rain.
Therefore, this rain is orographic or relief rain.
As the air descends again downwind of the mountains, the air warms and the cloud droplets evaporate.
As the cloud droplets form, they emit latent energy (heat) into the air around. This heat is remains in the air if the rain reaches the ground. This means that, downwind of the mountains when the air sinks, warms, and any remaining cloud droplets evaporate, there is more heat in the air than there was upwind of the mountains.
This is why temperatures were so much warmer on the east coast than on the west on this day!
3. What is the type of land is below these rainfall patterns? (Green is forest, brown is desert).
4. In what latitude bands are these rainfall patterns?
Use these terms to fill in the blanks below for Questions 5-8: Hadley, cloud, humid, Sun, cloud, rainfall, low, ground, Ferrel, fronts, Equator, Hadley, poles, rainfall.
5. Rainfall occurs when ______ air cools down. Air cools when it rises, or when it moves away from the _______.
6. The Atmospheric Circulation is driven by the _____. In the Tropics, the Sun warms the _____ which in turn warms the air above. hot air rises, leading to _____ and _______. This drives the ________ cells.
7. Colder air sinks at the poleward edge of the _______ cells and over the __________. Sinking air has no _______ or __________.
8. In the _______ Cells, rainfall is mainly associated with ____ pressure systems (depressions). Rainfall is mainly on the ________.
9. Complete the following table (Look at the map for help):
Tropics | Sub Tropics | Poles | |
Skies | Clear/ Cloudy | Clear/ Cloudy | Clear/ Cloudy |
Rain (or snow)-fall | Dry/ Wet | Dry/ Wet | Dry/ Wet |
Pressure | High/ Low | High/ Low | High/ Low |
10. Sketch what you think the Hadley Cell looks like in December and June by the images of the Earth below. Hint: the Equator and Tropics of Cancer and Capricorn are shown on the map.
Visit https://earth.nullschool.net/#current/wind/surface/level/overlay=precip_3hr/orthographic to see today’s rainfall patterns. Click on ‘Earth’ and then choose in the Overlay settings ‘3HPA’ to see the rainfall patterns together with surface wind speeds. Change in the settings the ‘Control’ to change the date and see rainfall patterns over time. Compare January and July rainfall patterns.
11. In the Tropics, how does the latitude of highest rainfall change between January and July? ___________________________________
12. How does this relate to the sketches you drew above? ___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Extension question: Why do you think you are asked to look at rainfall in January and July, rather than December and June? ___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
We have made a cloud wheel that can be cut out and used to identify clouds. Simply download the pdf, cut out the two circles, fasten together with a split pin and use to identify clouds.
Or, if you’d like a simpler version, use our Cloud bookmark.
Or, you can buy a laminated cloud identification key, produced in conjunction with the Field Studies Council, from our shop.
Download a Cloud Wheel or bookmark as a cloud identification chart.
Experiments demonstrate clouds forming in the Classroom from Physics Education, 2012, Catalyst article on Cloud Seeding, Physics Review article on Clouds, or have a look at our Experiments and Demonstrations page for experiments which demonstrate how clouds can look dark from below but white from above, or how to make a hygrometer to measure air humidity.
For a deeper understanding of how and where clouds form, have a look at our exercise using height/ temperature graphs to investigate atmospheric stability, lapse rates and cloud formation with a worksheet for students with an introductory PowerPoint or this paper.
What causes clouds
Types of clouds
Low clouds
Medium clouds
High clouds
What influences the colour of clouds?
Why do clouds stop growing upwards?
Why are there no clouds on some days?
Measuring clouds
The formation of precipitation
The nature of clouds
Types of cloud
Cumulus
Cumulonimbus
Stratus
Stratocumulus
Altocumulus
Altostratus
Nimbostratus
Cirriform clouds
Short-answer questions
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.
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.
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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.
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.
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.
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.
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) |
Cloud | 0.001 0.005 0.01 0.5 | 0.0001 0.0025 0.01 0.25 |
Drizzle | 0.1 0.25 | 0.7 2.0 |
Rain | 0.5 1.0 1.5 2.0 2.5 | 3.9 6.5 8.1 8.8 9.1 |
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.
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
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.
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).
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).
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).
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.
© Steve Jebson
© Steve Jebson
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.
© Jim Galvin
© A. Bushell
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:
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).
© C. S. Broomfield
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.
17. What is cloud seeding?
Web page reproduced with the kind permission of the Met Office
Pathway: Basic weather
Climate Zones – Air Masses – Pressure and Wind – Water in the Atmosphere
Lesson overview: In this lesson, we focus on cloud formation due to convection, orography (relief) and frontal uplift.
The atmosphere is one of the smallest reservoirs of water in the hydrosphere. Clouds form when air is cooled. Air can cool due to convection, when air is heated from below and rises, or when air is forced to rise at a front between two air masses. When air is forced to rise over hills and mountains, cloud formation is enhanced. Climate change will intensify the water cycle, increasing the amount of water vapour in the atmosphere. As water vapour is a greenhouse gas this creates a positive feedback loop, amplifying climate change.
Learning objectives:
To understand why clouds form in the atmosphere.
To be able to explain two ways in which clouds form
Key Teaching Resources
Water in the Atmosphere PowerPoint
Water in the Atmosphere PowerPoint (easier)
Water in the Atmosphere Worksheet
Water in the Atmosphere Worksheet (easier)
Back-to-back image
Teacher CPD/ Extended Reading
Water in the Atmosphere – More for Teachers
Alternative or Extension Resources
Global Atmospheric Circulation and Global Precipitation Patterns
A ‘mystery’ – a case study of orographic rainfall in Scotland (with optional extension looking at the Foehn Effect)
Air masses are parcels of air that bring distinctive weather features to the country. An air mass is a body or ‘mass’ of air in which changes in temperature and humidity within them are relatively slight. That is to say the air making up the mass is very uniform. in temperature and humidity.
An air mass is separated from an adjacent body of air by a weather front. An air mass may cover several millions of square kilometres and extend vertically throughout the troposphere.
A thin layer of mixed gases which covers the Earth and helps it from becoming too hot or too cold. Its circulation, the heat (terrestrial radiation) and light (solar radiation) which pass through it, and the processes which go on in it, all affect the climate. The atmosphere is about 800 km (500 miles) deep and is made up of 21% oxygen, 78% nitrogen, 0.037% carbon dioxide, and other gases including hydrogen, helium, neon, argon, krypton, xenon, and water vapour.
A classification of clouds was introduced by Luke Howard (1772-1864) who used Latin words to describe their characteristics.
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 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 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).
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).
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).
The tropical Pacific Ocean has a warming and cooling cycle. This cycle is a completely natural event and usually lasts between three to seven years. When the waters become warmer it is called El Niño, and when they become cooler it is called La Niña. During the cycle, the temperature of the ocean can change by around 3 °C between the warmest and coolest times.
Fishermen off the South American coast have known about this natural event for hundreds of years. When it happens, they see a huge fall in the numbers of fish caught. But scientists are only just beginning to understand how the event affects Earth’s weather and climate.
In an anticyclone (also referred to as a ‘high’) the winds tend to be light and blow in a clockwise direction. Also the air is descending, which inhibits the formation of cloud. The light winds and clear skies can lead to overnight fog or frost. If an anticyclone persists over northern Europe in winter, then much of the British Isles can be affected by very cold east winds from Siberia. However, in summer an anticyclone in the vicinity of the British Isles often brings fine, warm weather.
In a depression (also referred to as a ‘low’), air is rising. As it rises and cools, water vapour condenses to form clouds and perhaps precipitation. Consequently, the weather in a depression is often cloudy, wet and windy (with winds blowing in an anticlockwise direction around the depression). There are usually frontal systems associated with depressions.
Temperature affects other weather elements including air pressure, cloud formation, humidity and precipitation.
Factors affecting temperature:
A weather front is simply the boundary between two air masses.
There are three different types of weather front. These are:
Cold front
This is the boundary between warm air and cold air and is indicative of cold air replacing warm air at a point on the Earth’s surface
On a synoptic chart a cold front appears blue
The presence of a cold front means cold air is advancing and pushing underneath warmer air. This is because the cold air is ‘heavier’ or denser, than the warmer air. Cold air is thus replacing warm air at the surface. The symbols on the front indicate the direction the front is moving.
The passage of a cold front is normally marked at the earth’s surface by a rise of pressure, a fall of temperature and dew-point, and a veer of wind (in the northern hemisphere). Rain occurs in association with most cold fronts and may extend some 100 to 200 km ahead of or behind the front. Some cold fronts give only a shower at the front, while others give no precipitation. Thunder may occur at a cold front.
Warm front
This is the boundary between cold air and warm air and is indicative of warm air replacing cold air at a point on the Earth’s surface
On a synoptic chart a warm front appears red
The presence of a warm front means warm air is advancing and rising up over cold air. This is because the warm air is ‘lighter’ or less dense, than the colder air. Warm air is thus replacing cold air at the surface. The symbols on the front indicate the direction the front is moving.
As a warm front approaches, temperature and dew-point within the cold air gradually rise and pressure falls at an increasing rate. Precipitation usually occurs within a wide belt some 400 km in advance of the front. Passage of the front is usually marked by a steadying of the barometer, a jump in temperature and dew-point, a veer of wind (in the northern hemisphere), and a cessation or near cessation of precipitation.
Occluded front
These are more complex than cold or warm fronts. An occlusion is formed when a cold front catches up with a warm front
When a cold front catches up with a warm front the warm air in the warm sector is forced up from the surface
On a synoptic chart an occluded front appears purple
Weather can change on a daily basis especially at middle to high latitudes where it is controlled by weather systems, depressions and anticyclones.
On a weather chart, lines joining places with equal sea-level pressures are called isobars. Charts showing isobars are useful because they identify features such as anticyclones (areas of high pressure), depressions (areas of low pressure), troughs and ridges which are associated with particular kinds of weather.
The movement of air around the earth from high pressure to low pressure is what brings about winds. The direction given for the wind refers to the direction from which it comes. For example, a westerly wind is blowing from the west towards the east.
Measurements of wind strength are made at 10 metres (33 feet) above the ground. A specified height has to be used because the wind speed decreases towards the ground. In this country winds are measured in knots (nautical miles per hour). However, forecast winds are often given in miles per hour (where 1 knot is equivalent to 1.15 mph) or in terms of the Beaufort Scale.
There are rapid variations in the wind – these are referred to as gusts. Gusts are higher inland than over the sea or windward coasts, although the mean wind speeds tend to be lower inland. Typically, gusts can be 60% higher than the mean speed, although in the middle of cities this can reach 100%. Northerly winds tend to be gustier than southerly ones. In general, the weather is strongly influenced by the wind direction, so information about the wind provides an indication of the type of weather likely to be experienced.
Web page reproduced with the kind permission of the Met Office
Sailing is one of the most weather-dependent sports. Unfortunately, wind is not just a useful source of power for sailing craft but also a hazard. Strong winds can capsize boats, either directly or in combination with the waves they may produce.
The wind is never steady. It always fluctuates between gusts of higher wind speed and lulls that may be so light as to be near-calm. However, sudden increases of wind on a larger scale can sometimes occur. These are called squalls and are often associated not only with strong, gusty wind but also with heavy rain.
The importance of any hazard varies with the skill level of the crew, the type of boat and the kind of sailing being undertaken. For example, a novice crew in a small boat may underestimate wind strength before setting off. This is an easy mistake to make, especially if they are launching from a relatively sheltered location. Sailors who are more experienced are unlikely to be caught out like this but are still vulnerable in other ways. Hard sailing, especially in colder conditions, can tire a crew very quickly. Exhaustion or exposure can creep up on them before they know what is happening. For example, a dinghy crew might be having great fun practising sailing across the wind, only to find when they feel they have had enough that they do not have the reserves of energy for a long struggle upwind or a tricky run downwind.
These days, yachtsmen do not have to rely on old folklore or gamble on good weather. All sailors should pay attention to weather forecasts. These are available through radio, television, the internet and other means of broadcasting.
The particular forecast that is most appropriate depends on the kind of sailing being planned. The shipping forecast broadcast on the radio is perhaps the most useful to offshore sailors. The terms used in it are precisely defined, and the information that is included in it on pressure changes and movements of weather systems is very useful to anyone with a deeper than average understanding of meteorology.
When yachtsmen study meteorology as part of a training course, either at sea or ashore, they are often asked to create a weather map from a recorded shipping forecast as an exercise. The Royal Yachting Association can provide forms called ‘Metmaps’ that make recording and interpreting the shipping forecast a lot easier. Completing one of these is a good exercise for anyone who wants to go into meteorology seriously. The ‘Metmap’ is a two-sided A4 form. On one side, a shipping bulletin broadcast by the BBC can be taken down. On the other, a simple up-to-date weather map can be drawn from the information contained in the bulletin.
The shipping forecast gives a lot of information about visibility at sea. This is because poor visibility can sometimes be a greater hazard than strong winds. The forecast will not only give a guide to overall visibility in terms of ‘good’, ‘moderate’ or ‘poor’ but will often indicate if visibility is poor for a specific reason, such as ‘visibility poor in showers’. This particular occurrence can give a sailor a real fright, as views of nearby vessels or navigation marks can be lost suddenly when showers occur.
Two possible hazards are not often mentioned in weather forecasts for sailors but usually are in forecasts for land areas:
One of these is lightning, though the possibility of its occurrence may be indicated indirectly in a forecast or station report as ‘thundery showers’. In fact, lightning is not such a risk to sailors as it might at first appear. Boats are surrounded by a very good conductor of electricity – water – and unless the boat suffers a direct hit, which is unlikely, the current is dissipated much more quickly than on land.
The other neglected hazard is exposure to sunlight and sunburn. Sailors are at particular risk for two reasons. First of all, yachtsmen can get an increased dose of sunlight because of reflections from the water. Secondly, they may not notice this because the wind will make them feel cool and unaware they are ‘cooking’. Many a sailor has returned to work on a Monday morning with a ‘yachtsman’s tan’ (from the neck up!). This might seem a nice problem to have, but all sailors should note the example of the America’s Cup crews, who often display extremely colourful suncream to protect against harmful solar radiation.
Despite the apparently long list of hazards described, sailing is, in fact, a very safe sport. It is also a sport where knowledge of meteorology can increase a participant’s enjoyment and even give a competitive advantage!
Acapulco in 1968, sailors competing in the Olympic Games had an unusual surprise from the weather. While not actually hazardous, it was certainly not pleasant. One day, following a sudden squall, the covers of the boats were covered with maggots, which had, presumably, been drawn up into clouds by a whirlwind or waterspout, only to fall out in a downpour of rain.
The year 1979 is famous to yachtsmen for the worst possible reason. In August of that year, during the Fastnet Race (from Cowes on the Isle of Wight to the Fastnet Rock [51°24’N 9°35’W] off south-west Ireland and back again to Plymouth), the fleet of yachts ran into severe storms and rough seas. Fifteen lives were lost. Despite ‘survival conditions’, many crews kept records of the severe conditions, based on their barometers and wind instruments. To the meteorologists who have analysed the Fastnet Storm and its structure, these records have proved invaluable. For a recent analysis of the storm, see the article by D.E.Pedgley in the August 1997 issue of Weather (Volume 52, pp.230-242).
Shipping forecasts are currently broadcast four times a day on BBC Radio 4 Long Wave. They are also available via the websites of the Met Office and the BBC Weather Centre. In shipping forecasts, the Beaufort Scale is used for describing wind strength. This scale originated in the days of sail but is now defined precisely in terms of the wind at a height of ten metres averaged over a ten-minute period.
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