Collect data and analyse mode, mean and median, range, interquartile range and standard deviation
Introduction: There are many words and many descriptions for different types of rain: fine rain, heavy rain, pelting down, mizzling. In fact the BBC news magazine has an article entitled “Fifty words for rain”. But how big is a rain drop? Does the size vary depending upon the time of year or the type of rain?
Aim: To collect data, manipulate data and analyse data to calculate and compare the size of raindrops.
A platform of area of about 0.5m2 with edges.
Enough flour to cover the platform to a depth of about 3cm
An accurate measuring device, e.g. electronic sliding callipers.
Collecting the data
Cover the platform with the flour.
Place the platform in the rain for about 90 seconds, long enough for about 200 raindrops to hit the platform.
Use your measuring device to measure the diameter of the raindrops and record the data.
Manipulating, analysing, displaying and interpreting the data
There follows a number of suggestions of how the data can be used depending upon the ability of the students.
1. Calculate the mode, mean and median diameter of raindrop. Which is the most appropriate measure to use? Compare results from different groups.
2. Group the data into appropriate groups. Represent the data using histograms. Discuss whether it is appropriate to have all the groups the same size of vary the size of the groups. Compare the results from different groups. Compare data collected at different times of year if possible.
3. Calculate the spread of the data using range, interquartile range and standard deviation.
4. Discuss different methods of displaying the data. Is the data discrete or continuous? Should a bar chart or a histogram be used? This activity is ideal for discussing when a histogram should be used and the reasons for using a histogram.
5. Draw box plots to show the distribution of the data. Compare the spread of different data sets. What does this information tell us?
6. Write a report comparing the size of raindrops.
A microclimate is the distinctive climate of a small-scale area, such as a garden, park, valley or part of a city. The weather variables in a microclimate, such as temperature, rainfall, wind or humidity, may be subtly different from the conditions prevailing over the area as a whole and from those that might be reasonably expected under certain types of pressure or cloud cover. Indeed, it is the amalgam of many, slightly different local microclimates that actually makes up the microclimate for a town, city or wood.
It is these subtle differences and exceptions to the rule that make microclimates so fascinating to study, and these notes help to identify and explain the key differences which can be noticed by ground-level observations.
What are the different types of microclimates?
In truth, there is a distinctive microclimate for every type of environment on the Earth’s surface, and as far as the UK is concerned they include the following:
Upland areas have a specific type of climate that is notably different from the surrounding lower levels. Temperature usually falls with height at a rate of between 5 and 10 °C per 1000 m, depending on the humidity of the air. This means that even quite modest upland regions, such as The Cotswolds, can be significantly colder on average than somewhere like the nearby Severn Valley in Gloucestershire.
Occasionally, a temperature inversion can make it warmer above, but such conditions rarely last for long. With higher hills and mountains, the average temperatures can be so much lower that winters are longer and summers much shorter. Higher ground also tends to be windier, which makes for harsher winter weather. The effect of this is that plants and animals are often different from those at low levels.
Hills often cause cloud to form over them by forcing air to rise, either when winds have to go over them or they become heated by the sun. When winds blow against a hill-side and the air is moist, the base of the cloud that forms may be low enough to cover the summit. As the air descends on the other (lee) side, it dries and warms, sometimes enough to create a föhn effect. Consequently, the leeward side of hills and mountain ranges is much drier than the windward side. The clouds that form due to the sun’s heating sometimes grow large enough to produce showers, or even thunderstorms. This rising air can also create an anabatic wind on the sunny side of the hill. Sunshine-facing slopes (south-facing in the Northern Hemisphere, north-facing in the Southern Hemisphere) are warmer than the opposite slopes.
Apart from temperature inversions, another occasion when hills can be warmer than valleys is during clear nights with little wind, particularly in winter. As air cools, it begins to flow downhill and gathers on the valley floor or in pockets where there are dips in the ground. This can sometimes lead to fog and/or frost forming lower down. The flow of cold air can also create what is known as a katabatic wind.
The coastal climate is influenced by both the land and sea between which the coast forms a boundary. The thermal properties of water are such that the sea maintains a relatively constant day to day temperature compared with the land. The sea also takes a long time to heat up during the summer months and, conversely, a long time to cool down during the winter. In the tropics, sea temperatures change little and the coastal climate depends on the effects caused by the daytime heating and night-time cooling of the land. This involves the development of a breeze from off the sea (sea breeze) from late morning and from off the land (land breeze) during the night. The tropical climate is dominated by convective showers and thunderstorms that continue to form over the sea but only develop over land during the day. As a consequence, showers are less likely to fall on coasts than either the sea or the land.
Around the Poles, sea temperatures remain low due to the presence of ice, and the position of the coast itself can change as ice thaws and the sea re-freezes. One characteristic feature is the development of powerful katabatic winds that can sweep down off the ice caps and out to sea.
In temperate latitudes, the coastal climate owes more to the influence of the sea than of the land and coasts are usually milder than inland during the winter and cooler in the summer. However, short-term variations in temperature and weather can be considerable. The temperature near a windward shore is similar to that over the sea whereas near a leeward shore, it varies much more. During autumn and winter, a windward shore is prone to showers while during spring and summer, showers tend to develop inland. On the other hand, a sea fog can be brought ashore and may persist for some time, while daytime heating causes fog to clear inland. A lee shore is almost always drier, since it is often not affected by showers or sea mist and even frontal rain can be significantly reduced. When there is little wind during the summer, land and sea breezes predominate, keeping showers away from the coast but maintaining any mist or fog from off the sea.
Tropical rainforests cover only about 6% of the earth’s land surface, but it is believed they have a significant effect on the transfer of water vapour to the atmosphere. This is due to a process known as evapotranspiration from the leaves of the forest trees. Woodland areas in more temperate latitudes can be cooler and less windy than surrounding grassland areas, with the trees acting as a windbreak and the incoming solar radiation being ‘filtered’ by the leaves and branches. However, these differences vary depending on the season, i.e. whether the trees are in leaf, and the type of vegetation, i.e. deciduous or evergreen. Certain types of tree are particularly suitable for use as windbreaks and are planted as barriers around fields or houses.
These are perhaps the most complex of all microclimates. With over 75% of the British population being classed as urban, it is no surprise that they are also the most heavily studied by students of geography and meteorology. Therefore, the rest of these notes focus on the various elements that constitute an urban microclimate.
What is an urban microclimate?
The table below summarises some of the differences in various weather elements in urban areas compared with rural locations.
5 to 15% less
Annual mean temperature
0.5-1.0 °C higher
Winter maximum temperatures
1 to 2 °C higher
Occurrence of frosts
2 to 3 weeks fewer
Relative humidity in winter
Relative humidity in summer
8 to 10% lower
5 to 10% more
Number of rain days
Number of days with snow
5 to 10% more
Occurrence of fog in winter
Amount of condensation nuclei
10 times more
Urban heat islands
Marked differences in air temperature are some of the most important contrasts between urban and rural areas shown in the table above. For instance, Chandler (1965) found that, under clear skies and light winds, temperatures in central London during the spring reached a minimum of 11 °C, whereas in the suburbs they dropped to 5 °C.
Indeed, the term urban heat island is used to describe the dome of warm air that frequently builds up over towns and cities.
The formation of a heat island is the result of the interaction of the following factors:
the release (and reflection) of heat from industrial and domestic buildings;
the absorption by concrete, brick and tarmac of heat during the day, and its release into the lower atmosphere at night;
the reflection of solar radiation by glass buildings and windows. The central business districts of some urban areas can therefore have quite high albedo rates (proportion of light reflected);
the emission of hygroscopic pollutants from cars and heavy industry act as condensation nuclei, leading to the formation of cloud and smog, which can trap radiation. In some cases, a pollution dome can also build up;
recent research on London’s heat island has shown that the pollution domes can also filter incoming solar radiation, thereby reducing the build up of heat during the day. At night, the dome may trap some of the heat from the day, so these domes might be reducing the sharp differences between urban and rural areas;
the relative absence of water in urban areas means that less energy is used for evapotranspiration and more is available to heat the lower atmosphere;
the absence of strong winds to both disperse the heat and bring in cooler air from rural and suburban areas. Indeed, urban heat islands are often most clearly defined on calm summer evenings, often under blocking anticyclones.
The precise nature of the heat island varies from urban area to urban area, and it depends on the presence of large areas of open space, rivers, the distribution of industries and the density and height of buildings. In general, the temperatures are highest in the central areas and gradually decline towards the suburbs. In some cities, a temperature cliff occurs on the edge of town. This can be clearly seen on the heat profile below for Chester.
As noted previously, the greater presence of condensation nuclei over urban areas can lead to cities being wetter and having more rain days than surrounding rural areas. Indeed, it was often said that Rochdale, the famous mill town, had significantly smaller amounts of rain on Sundays when the town’s factories were closed.
However, other factors play a major role, especially the heat islands. These can enhance convectional uplift, and the strong thermals that are generated during the summer months may serve to generate or intensify thunderstorms over or downwind of urban areas. Storms cells passing over cities can be ‘refuelled’ by contact with the warm surfaces and the addition of hygroscopic particles. Both can lead to enhanced rainfall, but this usually occurs downwind of the urban area.
Smogs were common in many British cities in the late 19th and early 20th centuries, when domestic fires, industrial furnaces and steam trains were all emitting smoke and other hygroscopic pollutants by burning fossil fuels. The smogs were particularly bad during the winter months and when temperature inversions built up under high pressure, causing the pollutants to become trapped in the lower atmosphere and for water vapour to condense around these particles.
One of the worst of these ‘pea-soup fogs’ was the London smog of the winter of 1952/53. Approximately 4,000 people died during the smog itself, but it is estimated that 12,000 people may have died due to its effects. As a result, the Clean Air Act of 1956 was introduced to reduce these emissions into the lower atmosphere. Taller chimney stacks and the banning of heavy industry from urban areas were just two of the measures introduced and, consequently, fewer smogs were recorded in the UK during the 1960s and 1970s.
Research in the 1990s has shown, however, that another type of smog – photochemical – is now occurring in some urban areas as a result of fumes from car exhausts and the build up of other pollutants in the lower atmosphere which react with incoming solar radiation. The presence of a brown-coloured haze over urban areas is an indication of photochemical smog, and among its side effects are people experiencing breathing difficulties and asthma attacks.
Tall buildings can significantly disturb airflows over urban areas, and even a building 100 metres or so high can deflect and slow down the faster upper-atmosphere winds. The net result is that urban areas, in general, are less windy than surrounding rural areas.
However, the ‘office quarter’ of larger conurbations can be windier, with quite marked gusts. This is the result of the increased surface roughness that the urban skyline creates, leading to strong vortices and eddies. In some cases, these faster, turbulent winds are funnelled in between buildings, producing a venturi effect, swirling up litter and making walking along the pavements quite difficult.
How to measure wind speed and direction using bubbles
Bubble blowing kit
Stopwatch or phone
Compass or compass app
Bubble Chase to Measure Wind Direction
Place a marker at your start location. Choose an area of open ground which is safe and away from roads.
Blow some bubbles then pick one to follow.
Chase your chosen bubble, without getting in its way, until it pops or floats somewhere you cannot follow.
Blow another bubble from where you end up and follow that one.
Wherever you end up, look back at where you have come from. Now use your compass to work out the direction back to the starting point. This will give you the wind direction, because wind direction refers to where the wind is blowing from. Repeat a few times if possible.
Remember: wind direction is the direction wind is blowing from. Turbulence: If you are in an area where there is a lot of turbulence the bubbles may not move very far from the release point, or may go all over the place!
Bubble Race to Measure Wind Speed
Two people are needed for this: a ‘blower’ to blow bubbles and a ‘timer’ to time them using a stopwatch or watch.
Place a marker at your start location and another marker 10 metres away (about 10 adult paces) in the general direction the wind will carry the bubbles. This is your finishing line. If the wind is very light you can use a shorter distance.
The blower blows some bubbles and the timer picks one to follow.
The timer uses the stopwatch or watch to measure how long it takes the bubble to reach the finishing line.
Calculate the wind speed by dividing 10 metres by the time the bubble takes to cover that distance. So if the time it takes is 5 seconds, then the windspeed is 10 ÷ 5 = 2 metres per second.
There are two ways to measure temperature and humidity in your garden; either using individual instruments or using a weather station.
Temperature can be measured with a simple glass thermometer (below left, £3-5) filled with alcohol, which expands up a thin tube when the temperature increases. (Older thermometers may use mercury). Use the Celsius (C) scale, and estimate temperature to the nearest degree, or half degree if it is in between.
Digital thermometers (above right, £5) use electronics to measure temperature and are easier to read. The probe at the end of the cable that senses the temperature could be put outside in the shade with the readout in a garage, for example. They usually show temperature to a tenth of a degree, for example 28.9ºC, so enter this number in your report. (NB: When using digital thermometers (or indeed digital instruments of any type) remember that, although the display may have a precision of 0.1°C, its accuracy is likely to be much poorer than that – maybe a degree or two).
Instruments for measuring humidity are known as hygrometers. We will be measuring relative humidity in percent (%). A dial hygrometer (above left, around £10) uses hair, which expands when the atmosphere is moist. An electronic hygrometer uses electronics and has a clear display – devices which read both temperature and humidity are popular. Some of them (above right, £16-20) transmit data from outside sensors to a display indoors.
Humidity can also be measured with a wet and dry bulb hygrometer. This gives a more accurate reading, but involves the use of tables, so is a lot more complicated. Instructions will come with the hygrometer.
All types of thermometer and hygrometer must be kept out of direct sunlight at the time of reading (15:00-16:00) and for half an hour beforehand, either using some form of white louvered screen (below, £90), or by placing it in a position where sunlight doesn’t reach when you are observing, for example sheltered north facing location. As a last resort the thermometer can be hung on a north-facing wall or fence, but stood off as far away from the wall as possible to allow air to circulate all around it.
An Automatic Weather Station (AWS) measures temperature and humidity (and other quantities) with outdoor instruments which radio the data to an indoor display console. They can be bought for as little as £100 (below left), but for better accuracy you will have to pay £300 or more (below, right). They are generally mounted on top of fences or garages, to put them out in the open as much as possible. AWS can be mounted in direct sunlight, but in light winds and strong sunlight the budget versions can be up to 4 degrees in error. Temperature and humidity can be read direct from the indoor display and entered in your report.
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.
An urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas. The temperature difference is usually larger at night than during the day, and is most obvious when winds are weak. One of the main causes of the urban heat island is the fact that there is little bare earth and vegetation in urban areas. This means that less energy is used up evaporating water, that less of the Sun’s energy is reflected and that more heat is stored by buildings and the ground in urban than in rural areas. The heat generated by heating, cooling, transport and other energy uses also contributes, particularly in winter, as does the complex three dimensional structure of the urban landscape.
What effects do urban heat islands have on measurements of climate change?
Although most of the really long temperature records available to meteorologists come from in or near urban areas, the weather stations tend to be found in parks and open spaces which are less affected by changes in urbanisation. One study has attempted to see how much the urban heat island effect has affected long temperature records, by comparing the temperatures recorded on calm nighst (big urban heat island effect) with those recorded on windy nights (less urban heat island effect) – this suggested that the long temperature records were not affected by the urban heat island effect. In other words, any long term trends in temperature seen in the records were probably the same as if they had been recorded in a rural area. In the last few decades, data from satellites has been added to the records available to meteorologists. The IPCC concluded in their most recent (2007) climate change report:
“Recent studies confirm that effects of urbanisation and land use change on the global temperature record are negligible (less than 0.006°C per decade over land and zero over the ocean) as far as hemispheric and continental-scale averages are concerned. All observations are subject to data quality and consistency checks to correct for potential biases. The real but local effects of urban areas are accounted for in the land temperature data sets used [both by excluding as many of the affected sites as possible from the global temperature data and by increasing the error range]. Urbanisation and land use effects are not relevant to the widespread oceanic warming that has been observed. Increasing evidence suggests that urban heat island effects also affect precipitation, cloud and diurnal temperature range.”
As the climate changes, what impacts will there be on urban areas?
Urban areas are particularly vulnerable to changes in the climate, and, as the world becomes increasingly urbanised, more and more people will become vulnerable to changes in climate and extreme weather events. The 2003 heat wave was considered responsible for 14,802 and 2,045 excess deaths in France and England & Wales respectively. Many of these deaths occurred in urban areas because of the combined effect of the heat wave with the urban micro-climate. Heat waves like this are expected to become more common in the future. The summer of 2003, for example, is expected to become `typical’ by the 2050s. A lot of research is currently going into understanding the urban micro-climate, and into finding ways of designing cities and building to minimise the effect of a changing climate – both by reducing the urban heat island, and by finding ways to cool the insides of buildings.
Conversely, urban heat islands can have important consequences for which areas are most badly affected by fuel poverty.
Manchester Mapping Manchester’s Urban Heat Island a 2010 Weather paper by Knight, Smith and Roberts, looking at the results of the Royal Meteorological Society’s urban heat island experiment in Manchester.
The rainbow is a familiar sight when the sun is shining and rain is falling. It can also be seen in the spray from sprinklers and car-washes and in the spray above waterfalls. Sunshine and showers are the ideal condition for rainbows to be seen as they are formed by sun shining through raindrops.
Download rainbow fact file for printing
Where should you look for a rainbow, and when?
To see a rainbow you should look opposite from the sun, against a showers or thunderstorms. In the UK, rainbows tend to be most common in the late afternoon and early evening period when the sun is in the west. Remember: the sun rises in the east and sets in the west.
When does this type of fog form?
Rainbows have been talked about for many years. The ancient Greeks wrote about rainbows as a path made by Iris (the messenger of the Gods) between heaven and earth. Chinese mythology speaks of a slit in the sky sealed by the Goddess Nüwa using stones of five different colours. The Bible in the story of Noah talks about the rainbow of a sign from God that life would never again be destroyed by floods. But perhaps the most famous is that the Leprechauns keep their pot of gold at the end of the rainbow.
Rainbows were first explained by the infamous scientists Sir Isaac Newton and Rene Descartes in the 1600’s. Descartes explained that rainbows were caused by the reflection of light from raindrops, but couldn’t explain why. However, Sir Isaac Newton (shown in the picture) explained with the use of a glass prism experiment in 1666 how raindrops separate light into the colours of the rainbow we see.
What colours do we normally see?
The colours we normally see in a rainbow are red, orange, yellow, green, blue, indigo and violet, but really a rainbow has an unlimited number of colours! There are a few different mnemonics that help you remember the seven colours of the rainbow, but one of the favourites is ‘Richard Of York Gave Battle In Vain’ – that’s Red, Orange, Yellow, Green, Blue, Indigo, Violet.
What is the science behind rainbows?
Sunlight is refracted in raindrops and is split into the different colours that make up the sunlight. The refracted light is then reflected off the back of the raindrop at an angle of around 42 degrees, which defines the angle in the sky that we see the rainbow. The blue light is a shorter wavelength and so is refracted at a bigger angle than the longer wavelength red light, which means that in the bow you see the red at the top and the blue near the bottom. The spreading out of light at different wavelengths is called dispersion. Because we see only one colour from each raindrop, a great many drops must be present for us to see a rainbow.
Why can we never reach the end of the rainbow and find the pot of gold?
Unfortunately you will never reach the end of the rainbow for two reasons. The first is that because it’s an optical effect then it moves as you move and so you can never reach the bottom. Secondly, and perhaps more importantly, a rainbow is really a circle, it’s just that we see half of it.
Sometimes you can see a secondary rainbow. The secondary rainbow occurs when the light undergoes a double reflection in the raindrop. Because this is a second reflection the colours occur upside down compared to the primary rainbow, and they are dimmer. We call the area in between the two bows Alexander’s band after the ancient Greek Alexander of Aphrodisias who wrote about it. It is possible on very rare occasions to see a third bow, but as by this stage the light is very dim and it appears in the direction of the Sun it is extremely difficult to spot.
Have you ever noticed clouds like those in the upper part of the picture on the right? They are often seen when a warm or occluded front is approaching, generally 12 hours or more before the front’s precipitation arrives.
The clouds are high in the troposphere, the layer of the atmosphere where precipitation and most clouds occur. They are composed of ice crystals and their height is typically 8 to 10 km. They are called cirrus uncinus (from the Latin cirrus, meaning ‘a curl or tuft of hair’, and uncinus, meaning ‘hooked’). At cloud level, the temperature is typically -40 to -50°C.
Even at temperatures as low as this, the first products of condensation are believed to be water droplets, but the droplets are supercooled and freeze very rapidly. Indeed, they freeze too quickly for clouds of water droplets to become observable. The upcurrents responsible for the cloud formation are slow (typically 5 to 10 cm/s). The fall-speeds of ice crystals are, however, greater than this (50 cm/s or more), so the crystals descend.
Air that is saturated with respect to water is supersaturated with respect to ice. Newly-formed crystals can grow, therefore, even when descending, because they find themselves in air that is supersaturated. Once in air that is not saturated with respect to ice, however, they evaporate as they fall. They then become smaller and smaller and eventually disappear, about a kilometre below the level at which they formed initially. As they become smaller, their descent speed decreases.
When the ice crystals grow, latent heat is released. This warms the air sufficiently (about 0.5°C) for buoyant thermals to form. Thus, cloud regeneration takes place, because additional condensation, and therefore additional ice-crystal formation, occurs in these rising thermals. The small tufts of cloud that are often observed near the heads of cirrus uncinus are formed in this way.
The ice crystals are blown along by the wind, and in the layer where they exist (a kilometre or so deep) wind increases with height. Thus, the tops of the clouds blow ahead of the lower parts, so that hook-shaped streaks of cloud are created. The smallest crystals are blown almost horizontally.
In the upper troposphere, narrow currents of fast-moving air called ‘jet streams’ accompany warm and occluded fronts. They are usually many hundreds of kilometres in length and a few hundreds of kilometres in width, and they contain strong vertical and horizontal wind shears. Vertical shears are typically 5 to 10 m/s per km but can be much more. In the jet streams of middle latitudes, winds are strongest at a height of 9 or 10 km and often exceed 50 m/s, especially in winter. They blow from a westerly point. As the tops of cirrus uncinus clouds blow ahead of the lower parts, the tails of the clouds point towards the west.
Vertical Section through a Warm Front of an Active Depression
When an active warm or occluded front approaches a given point, the following sequence of clouds is usually observed: Cirrus uncinus, Cirrostratus, Altostratus, Nimbostratus. Sometimes, the sequence also includes Cirrocumulus and Altocumulus clouds. As the slope of the warm or occluded front is typically 1 in 100 to 1 in 150, Cirrus uncinus clouds normally begin to appear twelve hours or more before the precipitation arrives. The passage of a warm front is marked by a change of cloud type to Stratus or Stratocumulus. The passage of an occluded front is marked by a change from Nimbostratus to Cumulonimbus clouds.
Demonstrations and experiments of supersaturation and supercooling
A supersaturated solution contains more than the normal saturation quantity of a solute. Air is therefore supersaturated if it contains more than enough water vapour to saturate it at its current temperature. Because the atmosphere contains condensation nuclei, significant amounts of supersaturation with respect to water rarely occur. As the example overleaf suggests, however, supersaturation with respect to ice occurs quite often.
To demonstrate supersaturation, try the following:
Clean an area of a laboratory bench;
Place a few small crystals of sodium acetate on this area;
Slowly drip a solution of sodium acetate over the crystals.
This should cause a column of crystals to form. The supersaturated solution may be prepared as follows:
Place 50g of sodium acetate trihydrate in a small flask;
Add 5ml of water and slowly warm the flask;
Make sure the solid is completely dissolved;
Remove the flask from the heat, cover it with aluminium foil and allow it to cool at room temperature.
Alternatively, a supersaturated solution may be seeded by addition of a small crystal. This will create a solid mass of the chemical.
To demonstrate this:
Half fill a glass with crystals of sodium thiosulphate (Na2S2O3);
Heat the glass in a bath of hot water until the crystals melt to form a transparent liquid;
Filter out any impurities using a funnel and cotton;
Cover the glass and allow the liquid to cool to room temperature;
Shake the glass, whereupon the liquid freezes immediately
Notice that the glass feels warm.
Sodium thiosulphate freezes at a temperature of 48°C. Thus, a solution of this chemical is supercooled when at room temperature (around 20°C). The shaking of the glass, or the addition of a small crystal, triggers the freezing process. When freezing occurs, latent heat is released and the glass is therefore warmed.
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