Excellent!\u00a0 So I don’t need to worry about my flight on Friday, then?<\/h3>\n
Hopefully not.\u00a0 The eruption was really big, though.\u00a0 This animation<\/a> demonstrates the power of the plume.\u00a0 Comparing it with other eruptions that had similar-sized plumes<\/a>, it is probable that around 0.5 cubic kilometres of material were ejected.\u00a0 Much of that material is still up in the atmosphere.\u00a0 The Met Office models show the huge area that it covers.\u00a0 Some further disruption is not impossible while the remaining ash disperses.<\/p>\n The Met Office prediction of the ash cloud distribution for Wednesday and Thursday.<\/p><\/div>\n No.\u00a0 They are wrong.\u00a0 Unfortunately, the speed of the scientific process is nothing in comparison to that of a certain Irishman’s tongue, but the evidence is beginning to come together that the plume crossed the northern part of the UK yesterday as predicted.\u00a0 So far, we have indications from satellite data, observations by the public, and air quality monitoring data.\u00a0 All of these will be scrutinised and double-checked in the future, but the case is already very compelling.\u00a0 [Note: the purpose of this section is to demonstrate that the ash cloud is real.\u00a0 It doesn’t give sufficient information to say whether or not the concentrations of ash are safe to fly through.]<\/em><\/p>\n Satellite data<\/em><\/strong><\/p>\n The figure below shows data from the SEVERI infrared detector on the European Meteosat-9<\/a>, processed by the Norwegian Institute for Air Research<\/a>.\u00a0 The image shows patches of ash over the Atlantic and the North Sea.\u00a0 But the image is just a taster; the animation is a must-see!\u00a0 Click here to watch it<\/a>.\u00a0 Remember that the scale on the image is in grams-per-square-metre, the thresholds for aircraft safety are measured in milligrams-per-square-metre.<\/p>\n Map of Gr\u00edmsv\u00f6tn ash as detected by the SEVERI instrument.<\/p><\/div>\n Observations by the public<\/strong><\/em><\/p>\n The British Geological Survey<\/a> set up a webpage where people could report ash fall<\/a> in their area, similar to one that they use for reporting earthquakes.\u00a0 The results are below.\u00a0 How much you trust them depends on how much faith you have in the British public.\u00a0 The high density of points in Scotland, where the population is much lower, and the large number of negative findings in the South East are certainly consistent with the ash cloud crossing northern Britain.<\/p>\n Map of reported ash fall from the BGS website. Red markers have ash, white ones don't.<\/p><\/div>\n Furthermore, volunteers from across the country were collecting samples<\/a> of this ash.\u00a0 These are being sent to the BGS<\/a> for analysis.\u00a0 Results should begin to come out in the next few days.<\/p>\n Air quality monitoring data<\/em><\/strong><\/p>\n There is a network of air quality monitoring stations across the UK.\u00a0 Many of them publish their data online, so that they can be accessed by the public.\u00a0 One of the parameters measured is PM10 (particles less than 10 microns in diameter).\u00a0 Many of the ash particles in the plume will be less then 10 microns in diameter.\u00a0 The International Volcanic Health Hazard Network<\/a> have information on the health risks of ash on their website.\u00a0 The following plot comes from the Scottish Air Quality<\/a> website, and shows data for all of their sites in Aberdeen.\u00a0 There is a clear spike in PM10 levels at every site on Tuesday 24 May.\u00a0 Aberdeen airport was closed on Tuesday.<\/p>\n Air quality data for sites in Aberdeen from the Scottish Air Quality website. While there is a fair amount of background noise, the sharp peak in every station on Tuesday is clear.<\/p><\/div> Really?\u00a0 Well that is a truly incredible coincidence.\u00a0 Only this afternoon, I took part in a live Q&A session on the Guardian website about his very subject!\u00a0 I’ll summarise the most relevant excerpts for you below.\u00a0 You should also check out the original site<\/a> for more, and to read answers from Dr Colin Brown from the Institute of Mechanical Engineers about how much ash a jet engine can tolerate.<\/p>\n Why are the forecasts so vague?<\/em><\/strong><\/p>\n Because tracking an ash cloud is an extremely difficult thing to do.\u00a0 The distances are huge, and the concentrations that correspond to the safe flying threshold are tiny.\u00a0 Both empirical measurements (e.g. satellite data, direct sampling) and computer models (e.g. the Met Office’s NAME model) are used.\u00a0 Neither strategy is perfect, and each has it’s own advantage and disadvantages.\u00a0 In reality, both need to be used to make decent maps of where the ash cloud is present.<\/p>\n OK.\u00a0 Tell me about measurements first.<\/em><\/strong><\/p>\n Satellite data can be used to track the plume across the Atlantic. Visible light images (e.g. photographs) could show areas of high ash concentration, but only during daylight hours and if the ash was above the meteoric (weather) clouds. Ultraviolet images are able to pick out sulphur dioxide, but it is unclear whether it can travel separately from the ash. Thermal infrared (TIR) images are useful, because they contain information about both the extent and the altitude of the ash cloud. If you assume that the ash has the same temperature as the surrounding air (which is reasonable given the small size of the grains), and we know how the temperature of air changes with altitude, then you can work out its altitude from its temperature. This method falls down, however, if there are meteoric clouds at similar altitudes in the area, as they will also emit thermal infrared.<\/p>\n Using lasers in space, the altitude of the ash cloud can be checked, and the thickness could be measured. When people went through the data from the Eyjafjallaj\u00f6kull eruption, they found that the TIR methods underestimated the altitude of the ash by up to 1.5 km. The thickness was usually <1 km. Using a combination of TIR and lasers in space, the mass of ash in the cloud was estimated, but the uncertainties were around 40-50%. Ground based lasers were also able to measure the height and thickness of the cloud, and gave confirmation of ash above a number of locations across continental Europe. Ground-based lasers (LiDAR) have the disadvantage that they can\u2019t see through cloud, and only give information at a specific point (or along a narrow track in the case of a satellite-based system). Knowing the ash content above Manchester is of little help to a 747 over the middle of the Atlantic.<\/p>\n Sounds complicated.\u00a0 So are models the answer, then?<\/strong><\/em><\/p>\n Because measurement techniques are so strongly conditions-dependent and spatially limited, models are necessary to fill in the gaps.\u00a0 The models can give you a result for any time and place, irrespective of other factors. The way that they work is quite straightforward.<\/p>\n Imagine an ash grain, falling through the sky.\u00a0 The aim of the model is to work out how it moves during each second (or other time step) of the modelled time period.\u00a0 The motion is a controlled by four factors:<\/p>\n Adding up all the movement during all the steps over hours or days will show where the particle goes. All of this processing is very computer intensive, so it is not realistic to try to simulate each of the bazillion gazillion ash grains produced by the eruption.\u00a0 Modelling a few thousand turns out to be sufficient to see how the plume spreads and to make a map of where it is.\u00a0 You can do this for any time period or location and the data are 3D, so individual layers in the plume can even be identified.<\/p>\n So what are the disadvantages?<\/em><\/strong><\/p>\n The disadvantages of the computer models is that they are only computer models.\u00a0 The results are only as good as the data that is put in<\/a>.\u00a0 This means that errors in particle size distribution, or weather data will lead to errors in where the particle ends up. There is an unavoidable degree of uncertainty in each of the input parameters.\u00a0 How can you ever expect to know the exact wind speed and direction at 100 m height intervals at a given location 200 km west of Shetland?<\/p>\n And it gets worse.\u00a0 If you just want to know where the plume is you can simply draw an envelope round all the particles.\u00a0 But now the airlines demand to know the mass concentration of ash at each point.\u00a0 You can easily calculate this by assuming that each modelled particle represents some number of kilograms of ash, but getting this number right depends on knowing the mass eruption rate of the volcano. As I wrote in a post last month, this is easier said than done<\/a>.<\/p>\n I see what you mean that neither strategy is perfect.<\/strong><\/em><\/p>\n Exactly.\u00a0 Which is why both are required.\u00a0 The models produced by the Met Office aren’t just spat out of the computer and sent out to the airlines; they are compared against satellite data and anything else that is available and adjusted accordingly by experienced weather forecasters.\u00a0 This ensures that the strengths of both measurements and models are combined.<\/p>\n The results are still full of uncertainty, but this is not because of a failing on the part of the scientists.\u00a0 It is because producing accurate maps of the ash distribution is an extremely difficult thing to do.\u00a0 This situation can only be improved by increased funding for research and equipment to pay for things like LiDAR on the Faroe Islands, or a fleet of dedicated research planes, or more studies into the relationship between eruption style and mass discharge rate.<\/p>\n Clive Oppenheimer, a volcanologist at Cambridge, wrote a good piece about how difficult it has been to secure funding<\/a> for this kind of thing.\u00a0 Realistically, a lot of the money needs to come from the airline industry.\u00a0 Uninformed, attention-seeking criticism doesn’t help anyone.<\/p>\n The Gr\u00edmsv\u00f6tn eruption ended today, but there is still plenty of ash up there.\u00a0 Despite contrasting views in the press, firm evidence that the plume crossed the UK is slowly surfacing.\u00a0 Discussion has moved on from volcanology, to detecting and … Continue reading ![\"Met](\"http:\/\/all-geo.org\/volcan01010\/wp-content\/uploads\/2011\/05\/met_office_plot.png\")
<\/a>But lots of people are saying that there was no plume at all.\u00a0 Are they right?<\/h3>\n
![\"\"](\"http:\/\/all-geo.org\/volcan01010\/wp-content\/uploads\/2011\/05\/Stats201105241200.jpg\")
<\/a>![\"Map](\"http:\/\/all-geo.org\/volcan01010\/wp-content\/uploads\/2011\/05\/bgs_map.png\")
<\/a>![\"Aberdeen](\"http:\/\/all-geo.org\/volcan01010\/wp-content\/uploads\/2011\/05\/aberdeen_air_quality_small.png\")
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\n<\/a><\/p>\nDo you know, I’ve always wondered how people can make maps of the ash cloud thousands of kilometres away from the volcano.<\/h3>\n
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EDIT: 2011-05-26 10:25.\u00a0 Added IVHHN link.
\nEDIT: 2011-06-22 10.35.\u00a0 Added note on whether ash is safe to fly through.<\/h6>\n","protected":false},"excerpt":{"rendered":"