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pslct028

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<title>Precipitation</title></titleStmt>

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<idno>pslct028</idno>

<availability><p>The British Academic Spoken English (BASE) corpus was developed at the

Universities of Warwick and Reading, under the directorship of Hilary Nesi

(Centre for English Language Teacher Education, Warwick) and Paul Thompson

(Department of Applied Linguistics, Reading), with funding from BALEAP,

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<p>3. The recordings and transcriptions should not be reproduced in full for

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<p>4. The corpus developers should be informed of all presentations or

publications arising from analysis of the corpus</p><p>

Researchers should acknowledge their use of the corpus using the following

form of words:

The recordings and transcriptions used in this study come from the British

Academic Spoken English (BASE) corpus, which was developed at the

Universities of Warwick and Reading under the directorship of Hilary Nesi

(Warwick) and Paul Thompson (Reading). Corpus development was assisted by

funding from the Universities of Warwick and Reading, BALEAP, EURALEX, the

British Academy and the Arts and Humanities Research Board. </p></availability>

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<date>30/01/2001</date><equipment><p>video</p></equipment>

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<item n="speechevent">Lecture</item>

<item n="acaddept">Meteorology</item>

<item n="acaddiv">ps</item>

<item n="partlevel">UG2</item>

<item n="module">Hydrological cycle</item>

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<u who="nm0881"> the point we got to <pause dur="0.3"/> last time <pause dur="0.5"/> was that <pause dur="0.2"/> we we'd shown that using these observations of <pause dur="0.5"/> atmospheric water vapour transport <pause dur="3.0"/> we could deduce the <pause dur="0.3"/> net evaporation or net precipitation at the surface so <pause dur="0.5"/> again those of you who've now done <pause dur="0.2"/> done problem sheet one <pause dur="0.7"/> should should be well familiar with this that if we <pause dur="1.0"/> if we <pause dur="0.2"/> know the atmospheric transports of water vapour we can deduce <pause dur="0.4"/> E-minus-P as a function of latitude <pause dur="1.2"/> and so we talked about <pause dur="0.5"/> doing this for a <trunc>particul</trunc> # <trunc>pa</trunc> <pause dur="0.3"/> particular latitude bands <pause dur="0.5"/> and what i said at the end of last lecture is that we can <pause dur="0.4"/> we can carry on this procedure to consider <pause dur="0.5"/> a particular latitude <pause dur="0.6"/> longitude box <pause dur="0.6"/> so there's no reason why this technique should be restricted to particular latitudes <pause dur="0.4"/> just latitudes <pause dur="0.7"/> so <pause dur="1.1"/> the principle's exactly the same if we know <pause dur="0.3"/> the divergence of water vapour coming into the box <pause dur="0.6"/> # we can deduce the evaporation minus precipitation so there's # <pause dur="0.5"/> exactly the same technique <pause dur="1.4"/> so we can derive <pause dur="2.9"/> # E-minus-P <pause dur="2.6"/> # on a <pause dur="1.8"/> on a

latitude times longitude grid <pause dur="4.7"/> and <pause dur="1.0"/> the one example that I've got here then is from <pause dur="1.7"/> again from these assimilated data sets <pause dur="3.5"/> so <pause dur="0.3"/> these two plots here are the annual mean <pause dur="0.6"/> # <pause dur="0.3"/> E-minus-P in millimetres per day <pause dur="0.6"/> # <pause dur="0.8"/> so that these are from two two assimilated data sets <pause dur="7.0"/> and that these are <pause dur="0.6"/> one of them just happens to be European centre up the road and the other is the <pause dur="0.4"/> U-S <pause dur="0.2"/> # <pause dur="0.5"/> National Meteorological Centre <pause dur="0.4"/> and <pause dur="1.5"/> again these are giving us <pause dur="0.8"/> firstly giving us some of the <pause dur="0.2"/> patterns that we would expect to see <pause dur="0.7"/> we can see <pause dur="0.3"/> that there's a <pause dur="0.4"/> E-minus-P <pause dur="1.9"/> # is negative <pause dur="3.2"/> and over the I-T-C-Z where we would <pause dur="0.3"/> again there's nothing <pause dur="0.7"/> suprising about that we've got this band <pause dur="0.9"/> very narrow band it's striking even in the annual mean how narrow this band is <pause dur="0.4"/> across the Pacific and across the Atlantic where <pause dur="0.5"/> the precipitation is exceeding the evaporation <pause dur="0.8"/> and there's other features that we're not going to talk about much for example this <pause dur="0.4"/> this one going <pause dur="1.1"/> # <pause dur="0.4"/> south south-east from the <pause dur="0.2"/> Indonesia the South Pacific

convergence zone <pause dur="0.6"/> which is a feature i don't terribly well understand in the atmosphere <pause dur="0.4"/> we can see that <pause dur="0.4"/> # <pause dur="0.9"/> both marked in these plots <pause dur="0.5"/> and we can also see <pause dur="0.6"/> the positive regions <pause dur="0.5"/> # <pause dur="3.3"/> and again we'd expect these to me <pause dur="0.3"/> be most positive where there's <pause dur="0.4"/> loads of sunlight but loads of water <pause dur="0.4"/> so <pause dur="0.4"/> to evaporate so particularly the the subtropical anticyclones over the oceans <pause dur="0.5"/> we expect to see <pause dur="0.5"/> large quantities and they're they're in <pause dur="2.9"/> they're in both of the plots <pause dur="16.6"/> assuming there's abundant energy available over the desert regions but of course there's no water availability <pause dur="0.4"/> so we don't see these strong peaks over <pause dur="0.5"/> over the land regions <pause dur="10.3"/> and i won't write it down but again what we expect to see is E-minus-P becoming positive <pause dur="2.2"/> # <pause dur="0.4"/> sorry negative <pause dur="0.3"/> as we go into the storm tracks in the northern hemisphere so we can see <pause dur="1.5"/> we can see those there too so <pause dur="0.5"/> they're in general qualitative agreement but there's <pause dur="0.5"/> # <pause dur="0.6"/> there's some interesting <pause dur="0.8"/> interesting differences <pause dur="0.2"/> if we think about the headwaters of a big river <pause dur="0.2"/>

say the Amazon <pause dur="0.4"/> then what sign would we expect E-minus-P to be <pause dur="0.7"/> over the headwaters of the Amazon <pause dur="4.2"/> if you've got a great big river flowing out of it what sign would you expect E-minus-P to be <pause dur="3.8"/> yeah so we'd expect <pause dur="0.3"/> strongly negative and if we look at the <pause dur="0.3"/> European centre one <pause dur="1.9"/> then <pause dur="0.8"/> we can <trunc>f</trunc> we find that E-minus-P is indeed negative we'd expect the precipitation to exceed rainfall where you've got a great big river flowing out of <pause dur="0.5"/> but interestingly over this region in the N-M-C analyses <pause dur="0.6"/> it's not negative so it's indicating that there's some <pause dur="0.5"/> # some perplexing differences <pause dur="5.9"/> and i've just picked out one example here <pause dur="3.6"/> over the headwaters of the Amazon <pause dur="1.1"/> where we think we know the answer because we've <pause dur="0.7"/> we can measure the river flow coming out of the region <pause dur="1.7"/> i don't <pause dur="0.3"/> i <trunc>ha</trunc> i haven't done that to try and say E-C-M-W-F is better than N-M-C i've just picked out one i'm sure we could pick out other regions of the world where <pause dur="0.5"/> # <pause dur="0.3"/> E-C-M-W-F would be worse <pause dur="0.5"/> than than N-M-C so <pause dur="0.7"/> # <pause dur="1.6"/> it'd be

nice to <pause dur="0.2"/> having said that it would be nice to know how <pause dur="0.3"/> how accurate are <pause dur="0.9"/> <trunc>ho</trunc> <pause dur="0.6"/> how accurate is the <pause dur="3.9"/> # is the E-minus-P data <pause dur="10.9"/> what what we're going to do just to finish off this section is just look at a <pause dur="0.5"/> rather nice <pause dur="1.7"/> analyses <pause dur="2.4"/> that i've taken from a <pause dur="1.8"/> so it's at the bottom of this sheet again the <pause dur="0.8"/> quite a lot of the <pause dur="2.3"/> quite a lot of the papers i'm going to refer to were from the Bulletin of the American Meteorological Society which we have in the library in the main library <pause dur="0.5"/> and they're often <pause dur="0.2"/> quite readable articles and this is <pause dur="0.5"/> one where the instead of looking over the Amazon <pause dur="0.3"/> headwaters <pause dur="0.4"/> where we haven't got much data there's not many radiosonde the sense over <pause dur="0.5"/> that part of the world <pause dur="0.7"/> what they've done is to look <pause dur="0.4"/> in two regions where <pause dur="1.5"/> # <pause dur="1.1"/> where there are <pause dur="0.2"/> good instrumental records <pause dur="1.2"/> in in the United States so good <pause dur="0.6"/> you'd expect a lot of radiosonde offence in these regions <pause dur="0.5"/> and they've taken two <pause dur="0.2"/> two basins <pause dur="0.5"/> so this this again is just very much an example but # <pause dur="0.6"/> quite an illustrative one <pause dur="1.7"/> is that we'll

look over <pause dur="3.4"/> # to <pause dur="1.9"/> # U-S river basins <pause dur="5.1"/> so the Ohio Tennessee and the upper Mississippi <pause dur="0.5"/> and for both these we've got # good <pause dur="3.1"/> radiosonde coverage <pause dur="3.8"/> and so the <pause dur="0.5"/> the data going into our assimilated data sets should be <pause dur="0.6"/> should be pretty good <pause dur="0.6"/> and we've also got # stream flow measurements <pause dur="11.2"/> so the situation here <pause dur="0.2"/> what do <trunc>w</trunc> got here is if this is the <pause dur="0.4"/> if this is the catchment of the river and this is this is a river flowing out without with all its <pause dur="0.7"/> tributaries instead of <pause dur="0.4"/> calculating the E-minus-P on a nice square box what we're doing is calculating the <pause dur="0.7"/> the convergence of water vapour into this <pause dur="0.5"/> # <pause dur="0.5"/> so this is the <pause dur="0.8"/> the boundary of the catchment <pause dur="4.2"/> we <trunc>ca</trunc> we calculate <pause dur="1.8"/> and the <pause dur="0.3"/> divergence or its convergence is written on these slides but doesn't matter which <pause dur="4.9"/> calculate the divergence of the water vapour transport <pause dur="0.5"/> and <pause dur="1.2"/> we also then measure the stream flow <pause dur="8.4"/> the stream flow leaving the catchment <pause dur="26.9"/> so if our system is perfect <pause dur="0.2"/> the amount of rainfall <pause dur="0.7"/> the the amount of <pause dur="1.5"/> the amount of water

leaving the catchment assuming this is the only way that water can get out <pause dur="0.5"/> ought to be the same as the amount being <pause dur="0.5"/> the convergence into the catchment <pause dur="1.0"/> so we can ask how <pause dur="0.4"/> how how good it is and these are <pause dur="1.5"/> two diagrams which <pause dur="0.8"/> # <pause dur="1.6"/> illustrate that so <pause dur="0.4"/> if we just for a moment concentrate on the upper Mississippi <pause dur="0.8"/> and the this is the <pause dur="0.5"/> variation of the function of time of year from January <pause dur="0.2"/> back through to January <pause dur="0.5"/> and this dashed line is a measured stream flow <pause dur="1.1"/> and this solid line is how much <pause dur="0.6"/> how much water is essentially being <pause dur="0.6"/> deposited into the <pause dur="1.5"/> into the catchment by <pause dur="0.2"/> by convergence <pause dur="2.4"/> and <pause dur="2.3"/> and this is for the <pause dur="0.5"/> Ohio Tennessee and i've just put in the annual averages and what we can see is that <pause dur="0.5"/> # <pause dur="0.9"/> is that these numbers don't agree wonderfully well i mean these are <pause dur="0.3"/> errors of maybe thirty per cent between the <pause dur="0.6"/> # <pause dur="0.3"/> convergence and the stream flow <pause dur="1.4"/> so even in one of these <pause dur="0.3"/> well instrumented areas <pause dur="0.8"/> where we think we've got good data <pause dur="0.4"/> # the the difference between <pause dur="4.1"/> and the deduced <pause dur="1.4"/> # convergence of water

vapour by the atmosphere <pause dur="6.3"/> and stream flow is <pause dur="0.7"/> is only around thirty per cent <pause dur="0.8"/> it turns out that the sign differs between this in one case <pause dur="0.5"/> one's getting more <pause dur="0.7"/> more convergence and stream flow <pause dur="0.4"/> so where's the water going <pause dur="0.6"/> and in the other case <pause dur="0.6"/> # it's the opposite way round <pause dur="16.4"/> now <pause dur="0.2"/> of course there's <pause dur="0.2"/> when you've got two sets of measurement there's <pause dur="0.3"/> two sets of measurements could be an error so is <pause dur="0.3"/> is this telling us that the stream flow measurement is wrong <pause dur="0.5"/> or is it that the convergence measurement were wrong or is there some other way that water can <pause dur="0.4"/> get out this catchment the <pause dur="0.4"/> the authors i'm not a <pause dur="0.4"/> obviously not an expert in <pause dur="0.4"/> in measuring stream flow the authors conclude <pause dur="1.5"/> # that most of the error <pause dur="3.4"/> # <pause dur="1.0"/> is in the E-minus-P <pause dur="1.2"/> so <pause dur="1.0"/> we reckon we can measure stream flow with <pause dur="0.4"/> reasonable accuracy <pause dur="0.2"/> so we know how much water was leaving the catchment <pause dur="0.7"/> but we can't measure the <pause dur="1.6"/> and that we <trunc>c</trunc> the implication is that # <pause dur="0.4"/> even our best systems <pause dur="0.5"/> say from <pause dur="0.2"/> the European centre <pause dur="1.9"/> aren't getting this <pause dur="0.4"/> # <pause dur="4.3"/> aren't allowing us

to deduce this convergence of water vapour and hence the E-minus-P with an accuracy of better than thirty per cent <pause dur="0.4"/> so this is kind of a limit of how well we can do <pause dur="0.5"/> with these techniques <pause dur="4.7"/> okay so <pause dur="0.6"/> # <pause dur="2.5"/> that <pause dur="0.2"/> then concludes this first section where we've looked at <pause dur="0.5"/> atmospheric water vapour and its transport and what we can deduce from it <pause dur="0.8"/> so we've deduced we've shown that one of the most <pause dur="0.3"/> powerful if if somewhat flawed <pause dur="1.2"/> outputs is that we can actually work out what the net <pause dur="0.3"/> surface water balance is which is very powerful <pause dur="0.6"/> but obviously we need to know how much precipitation's <pause dur="0.5"/> going on so <pause dur="1.1"/> now let's go to the next big section <pause dur="5.7"/> this is section <pause dur="1.2"/> section four <pause dur="6.7"/>

precipitation <pause dur="0.9"/> and i hope a section like this i don't need to motivate why we're <pause dur="0.4"/> why we're doing it <pause dur="1.2"/> but what we're going to be concerned with is <pause dur="0.4"/> # <pause dur="2.0"/> is essentially how how is rainfall measured <pause dur="0.6"/> which again you'll have touched on in first year courses <pause dur="0.7"/> the problems in making those measurements <pause dur="0.5"/> and <pause dur="1.1"/> one of the problems we're going to come across which is a major one is the sampling <pause dur="0.4"/> if you've got one little rain gauge here trying to represent a huge area around it <pause dur="0.5"/> # <pause dur="0.3"/> how how reliable is that <pause dur="0.2"/> tiny little sample <pause dur="0.7"/> and then we'll go on to talk about <pause dur="0.4"/> observed distributions of rainfall <pause dur="0.7"/> so let's <pause dur="0.2"/> # <pause dur="0.9"/> spend the first <pause dur="1.1"/> first section <pause dur="0.2"/> thinking about techniques <pause dur="1.2"/> for measuring precipitation <pause dur="6.0"/> and of course there's many <pause dur="1.0"/> many different techniques we're going to touch on we're going to touch on <pause dur="0.4"/> obviously rain gauges <pause dur="0.8"/> radars satellites <pause dur="0.8"/>

but let's start <pause dur="0.2"/> with the <pause dur="1.2"/> in many ways the simplest which is <pause dur="3.3"/> rain gauges <pause dur="3.4"/> and so this <pause dur="0.8"/> this hopefully will build on what you learned in Dr Pedder's course <pause dur="1.1"/> # measuring the atmosphere <pause dur="0.5"/> in the first year <pause dur="4.7"/> now we're going to as i said we're going to look at <pause dur="0.2"/> loads of different techniques for measuring rainfall <pause dur="1.0"/> but there's one distinguishing feature about <pause dur="0.4"/> rain gauges <pause dur="0.5"/> compared to say satellite systems and radar <pause dur="0.6"/> does anyone know <pause dur="0.4"/> what it is <pause dur="6.5"/> i mean you might not you might think that radars measure rainfall <pause dur="0.5"/> but they don't rain gauges are the only technique that actually measure <pause dur="0.4"/> how much water is reaching the surface <pause dur="0.4"/> so we're going to look at lots of other techniques <pause dur="0.5"/> where people talk about <pause dur="0.3"/> measuring rainfall but they're not actually measuring rainfall they're deducing it <pause dur="0.4"/> indirectly so these are the <pause dur="0.7"/> the only technique <pause dur="6.8"/> that actually measures the rainfall reaching the surface <pause dur="7.1"/> and <pause dur="1.0"/> don't be tricked into thinking otherwise when you read <pause dur="0.4"/> read about some other techniques <pause dur="0.4"/> in in books so it's

# <pause dur="1.9"/> and the other thing that we'll find is that every other technique radars and satellite techniques they are absolutely rely <pause dur="0.2"/> on rain gauges <pause dur="0.4"/> so # <pause dur="0.4"/> so that's the other important point <pause dur="0.9"/> no other techniques <pause dur="4.2"/> can rely on them <pause dur="3.6"/> so we can go to some fancy high tech solutions with radars and satellites but at the end of the day <pause dur="0.7"/> # <pause dur="1.3"/> they can't do without <pause dur="1.5"/> # <pause dur="0.6"/> our old Victorian <pause dur="0.6"/> technology <pause dur="12.5"/> # <pause dur="0.2"/> types again a lot of this you'll <pause dur="0.2"/> hopefully know <pause dur="1.1"/> know very <pause dur="2.6"/> very well <pause dur="0.2"/> the <pause dur="0.4"/> the main type of rain gauge are storage gauges <pause dur="1.6"/> where <pause dur="0.3"/> these are generally read <pause dur="1.0"/> these are <trunc>s</trunc> <pause dur="0.2"/> simple <pause dur="2.5"/> # collectors <pause dur="0.3"/> of rainfall <pause dur="1.2"/> and they're normally just read <pause dur="0.9"/> # once per day <pause dur="7.3"/> matters up to even at operational <pause dur="0.5"/> weather sites </u><pause dur="1.0"/> <u who="sm0882" trans="pause"> sorry i </u><u who="nm0881" trans="overlap"> that that you that you measure only once per day from a rain gauge <pause dur="0.4"/> i mean certainly on a site like ours <pause dur="0.2"/> they're only read once a day </u><pause dur="0.5"/> <u who="sm0882" trans="pause"> yeah you you <pause dur="0.2"/> you <gap reason="inaudible" extent="1 sec"/> then for the standard five inch gauges but the </u><u who="nm0881" trans="overlap"> mm </u><u who="sm0882" trans="overlap"> there are <pause dur="0.9"/> automatic loggers which actually do what the <gap reason="inaudible" extent="1 sec"/></u><u who="nm0881" trans="overlap"> yeah well i understand you <pause dur="0.4"/> so these these are what so so at some sites they would be measured twice a day but say on our <pause dur="0.3"/> our climatological web <pause dur="0.6"/> climatological site <pause dur="0.4"/>

we'll only use them once a day <pause dur="0.5"/> so <pause dur="0.9"/> and then obviously the <pause dur="0.4"/> the second type of gauge <pause dur="0.6"/> which i'm not going to <pause dur="0.2"/> talk about much in this <pause dur="0.8"/> these lecture courses the <pause dur="2.2"/> the automatic gauge <pause dur="0.2"/> gauges <pause dur="1.4"/> for example the <pause dur="0.5"/> there's various different models but the <pause dur="1.6"/> the <trunc>c</trunc> whoop <pause dur="0.3"/> let's get it right the tilting syphon <pause dur="4.0"/> # gauge that we have on our <pause dur="2.2"/> on our met site <pause dur="0.6"/> which i hope you've all <pause dur="0.6"/> all seen <pause dur="2.5"/> and the these of course give <pause dur="0.8"/> give the amount <pause dur="2.2"/> # plus the timing <pause dur="1.7"/> of the rainfall <pause dur="0.7"/> so they give us <pause dur="0.2"/> extra information <pause dur="7.6"/> as i say i'm not going to talk too much about these these have real advantages also in <trunc>t</trunc> <pause dur="0.2"/> in terms of <pause dur="0.4"/> and when we get to talk about radars is that these can be set up so they can actually <pause dur="0.3"/> transmit the data they're recording so they'll <pause dur="0.4"/> you'll get data in real time coming back for a <pause dur="0.5"/> from an automatic rain gauge if they're <pause dur="0.3"/> properly set up <pause dur="1.1"/> but the other thing about them is that they're rather complex pieces of equipment there's <trunc>lo</trunc> <pause dur="0.5"/> many more things that can go wrong with a <pause dur="0.4"/> an automatic rain gauge

than a <pause dur="0.2"/> than a storage gauge so they're <pause dur="0.6"/> less robust in many ways <pause dur="0.2"/> but we'll going to <pause dur="2.0"/> over the world as a whole <pause dur="0.5"/> it's these these kind of rain gauges <pause dur="0.4"/> that are dominant and the ones that are used most in <pause dur="0.5"/> in trying to understand the hydrological cycle <pause dur="2.9"/> so let's just <pause dur="0.4"/> sit back and think about rain gauges <pause dur="0.6"/> a little bit <pause dur="35.5"/> sorry did i <pause dur="0.8"/> failed to count <unclear>properly</unclear> <pause dur="0.2"/> sorry <pause dur="7.8"/> so <pause dur="0.9"/> so the first point is that rain gauges are <pause dur="0.2"/> the storage gauges <pause dur="4.9"/> # are the most common <pause dur="3.1"/> i think Strangeways in his book estimates something like two-hundred-thousand of them <pause dur="0.5"/> across the world <pause dur="0.5"/> but there's <pause dur="0.3"/> there's a <pause dur="0.4"/> lot of problems we have to be aware about with rain gauges and again some you will have touched on the first is that there's a <pause dur="1.7"/> there's a whole <pause dur="0.2"/> zoo of different rain gauges which are routinely <pause dur="0.5"/> used <pause dur="1.2"/> and <pause dur="1.1"/> so <pause dur="0.5"/> so this is potentially <pause dur="1.8"/> problems is that there are about fifty <pause dur="1.6"/> in routine use <pause dur="2.2"/> # around the world <pause dur="9.9"/> and <pause dur="0.3"/> the these <pause dur="0.3"/> what i've <pause dur="0.3"/> reproduced here is just a <pause dur="0.4"/> a subset of <pause dur="0.7"/> # <pause dur="0.2"/> nine of these <pause dur="0.4"/> nine of these different ones and i'm not going

to <pause dur="1.1"/> look at them <pause dur="0.4"/> <trunc>i</trunc> in any great detail <pause dur="0.5"/> and the <pause dur="0.3"/><vocal desc="sneeze" iterated="n"/><pause dur="0.3"/> the U-K one happens to be this one <pause dur="0.6"/> and <pause dur="0.7"/> and these are ones used in <pause dur="0.2"/> in <trunc>d</trunc> <pause dur="0.2"/> <trunc>b</trunc> various other countries as # <pause dur="1.3"/> is indicated there <pause dur="0.4"/> and they all differ in <pause dur="0.2"/> in in characteristics they differ in <pause dur="2.4"/> # <pause dur="2.3"/> in size <pause dur="1.4"/> they differ in <pause dur="1.0"/> they can differ in <pause dur="0.3"/> in the shape of the <pause dur="0.5"/> collector <pause dur="1.8"/> things that also <pause dur="0.4"/> matter # <pause dur="0.3"/> # <pause dur="0.7"/> is the material they're made out of and that can have an impact on <pause dur="1.0"/> whether the <pause dur="1.1"/> surface tension forces cause little droplets just to sit in the gauge or run down into the <pause dur="0.4"/> collecting bottle <pause dur="0.4"/> and so some of those things are are indicated on this diagram <pause dur="0.7"/> # <pause dur="2.7"/> i should say one i don't believe this figure if it one of the <pause dur="0.2"/> figures it <pause dur="0.4"/> it tells us that the first number <pause dur="0.4"/> indicates the code <pause dur="1.1"/> gives the orifice of the <pause dur="0.2"/> gauge area and i don't think it's correct for this <pause dur="0.3"/> <trunc>s</trunc> one so i'm not sure quite what the units are <pause dur="0.5"/> but nevertheless the main point about this is that we've got this whole <pause dur="0.6"/> zoo of <pause dur="0.2"/> of different gauges <pause dur="2.3"/> and so <pause dur="0.4"/> # <pause dur="1.0"/> <trunc>th</trunc> <pause dur="0.2"/> the reason why this is a

problem is that is that they're <pause dur="0.2"/> they're not not all <pause dur="1.5"/> # easily compared if you're getting rainfall measured by one gauge <pause dur="0.4"/> you can't immediately <pause dur="0.2"/> compare that to <pause dur="1.0"/> measurements from another gauge <pause dur="2.1"/> another problem can come if you're looking at trends in rainfall <pause dur="0.5"/> is if <pause dur="0.2"/> there's a slow changeover from one of these gauge types to another <pause dur="0.7"/> # you have to be <pause dur="0.2"/> damn sure you know that if you're going to not <pause dur="0.4"/> if the trends you see are are real trends in rainfall rather than <pause dur="0.5"/> a a trend <pause dur="0.3"/> in use from one type to another <pause dur="1.1"/> so that's <pause dur="0.2"/> # <pause dur="0.8"/> big <pause dur="0.6"/> big potential problem <pause dur="3.1"/> and the second problem about rain gauges i said that they're <pause dur="0.2"/> they're the only technique that actually measure rainfall but they're <pause dur="0.4"/> they're they're imperfect collectors of rainfall <pause dur="19.3"/> and <pause dur="0.6"/> what this second diagram <pause dur="0.2"/> does is just sort of <pause dur="0.4"/> shows some <pause dur="2.1"/> some of the different things that we have to <pause dur="0.5"/> worry about <pause dur="0.8"/> # <pause dur="3.2"/> when when we're thinking about rain getting into a rain gauge so one of the things that can happen <pause dur="0.4"/> is that rain falls into the <pause dur="0.6"/> into the mouth

of the gauge <pause dur="0.2"/> forms a little <pause dur="0.3"/> droplet and <pause dur="0.4"/> during the <pause dur="0.3"/> during the day that water is evaporated up into the <pause dur="0.6"/> atmosphere and that obviously depends on the input of the sun and the wind <pause dur="0.6"/> rather than going into the gauge <pause dur="0.4"/> we've got other things if we're going to <pause dur="0.3"/> accurately know how much rainfall <pause dur="0.6"/> has gone on we need to know what size <pause dur="0.6"/> our gauge is <pause dur="0.4"/> now if someone <pause dur="0.7"/> if someone when they're mowing the lawn on the <trunc>ga</trunc> on the <pause dur="0.3"/> on the <pause dur="0.4"/> on the site manages to bash their rain gauge with a <pause dur="0.6"/> # <pause dur="0.2"/> lawnmower and i gather that's not # uncommon <pause dur="0.5"/> then you you end up with your <vocal desc="laugh" iterated="n"/><pause dur="0.4"/> with your rain gauge being something less than <pause dur="0.5"/> less than circular <pause dur="0.5"/> so there's there's <pause dur="0.2"/> there's various different # <pause dur="1.4"/> different <pause dur="0.8"/> # <pause dur="0.7"/> sources of error <pause dur="6.2"/>

and let's just # <pause dur="2.1"/> note note some of them <pause dur="1.8"/> potential errors <pause dur="1.0"/> include things like precision of manufacture <pause dur="6.3"/> do we really know accurately what the area is <pause dur="0.3"/> course that's <trunc>c</trunc> if you've got a a bottle full of water <pause dur="0.4"/> and we need to convert that to rainfall we need to know <pause dur="0.5"/> precisely how big the <pause dur="0.6"/> the collecting <pause dur="0.4"/> # orifice is <pause dur="2.4"/> we've got <pause dur="0.2"/> # <pause dur="1.3"/> evaporation <pause dur="3.3"/> from the gauge and again this isn't <pause dur="0.2"/> insignificant it's about <pause dur="0.6"/> # <pause dur="1.4"/> in some gauge it <pause dur="0.2"/> the # it's reckoned at about point-<pause dur="0.2"/>two millimetres per <pause dur="0.5"/> rainfall event <pause dur="0.2"/> there's a kind of a rough <pause dur="0.4"/> figure is lost just through <pause dur="0.2"/> evaporation <pause dur="5.8"/> another other ones that can be important depending on the gauge type is water <pause dur="0.2"/> either falling into the gauge <pause dur="0.4"/> and splashing out again or the opposite falling out of the gauge and splashing <pause dur="0.5"/> splashing in so those

are <pause dur="0.8"/> referred to as outsplash <pause dur="3.5"/> and insplash <pause dur="9.8"/> and <pause dur="1.8"/> well all these <pause dur="0.2"/> and <pause dur="0.8"/> various ones i've listed there are <pause dur="0.4"/> # <pause dur="0.2"/> all <pause dur="2.4"/> of of order <pause dur="0.3"/> cause an error of order of about one per cent <pause dur="0.7"/> and it's reckoned <pause dur="2.5"/> reckoned that # the kind of random error <pause dur="1.6"/> due to a the collection of all these things a random error <pause dur="0.8"/> # <pause dur="4.4"/> to all all these things is about point <pause dur="1.0"/> about point-six to one millimetres <pause dur="0.6"/> per day <pause dur="0.9"/> so the random error in daily rainfall is what i'm trying to say <pause dur="3.9"/> is around point-six to one <pause dur="0.3"/> millimetres <pause dur="1.1"/> so <pause dur="1.8"/> very small rainfall amounts we have to be quite careful about <pause dur="0.3"/> how big the error is <pause dur="13.4"/> now the biggest error <pause dur="0.6"/> in terms of measuring rainfall is <pause dur="0.4"/> is is windspeed <pause dur="0.2"/> biggest by far <pause dur="1.1"/> so the biggest error source <pause dur="6.6"/> is wind <pause dur="1.4"/> and so we're going to spend a few minutes just <pause dur="0.6"/> # <pause dur="0.2"/> thinking about this <pause dur="2.4"/> and i've got to have a <pause dur="2.4"/> # and the kind of error that we're talking about so that <trunc>s</trunc> the U-K rain gauge <pause dur="1.2"/> the standard Met Office one has a <pause dur="0.4"/> has a has it's top at <pause dur="1.5"/> # <pause dur="0.2"/> three-hundred millimetres <pause dur="1.3"/> again as you <pause dur="0.4"/> all very familiar with <pause dur="0.6"/> and the

kind of error we have <pause dur="0.7"/> <trunc>ha</trunc> <pause dur="0.3"/> have <pause dur="0.3"/> for this one <pause dur="0.5"/> is <pause dur="1.3"/> is about ten per cent at <pause dur="1.6"/> at four metres <pause dur="0.2"/> per second and is # <pause dur="0.7"/> and the error <pause dur="0.3"/> sorry the error is <pause dur="1.3"/> and that this error increasingly linearly with <pause dur="1.6"/> with wind speed <pause dur="3.1"/> so generally it's a <pause dur="1.1"/> unless someone has clouted their rain gauge with a <pause dur="1.6"/> with a lawnmower it's generally <pause dur="0.7"/> larger than <pause dur="0.2"/> than these <pause dur="0.3"/> error sources here <pause dur="5.0"/> and <pause dur="1.5"/> this this is kind of for a typical lowland site in <pause dur="0.3"/> in mountainous regions <pause dur="0.6"/> of course you've got more <pause dur="0.2"/> # <pause dur="0.4"/> wind <pause dur="1.6"/> but but more particularly 'cause in mountainous areas a lot of the rainfall is from <pause dur="0.4"/> is from drizzle <pause dur="0.6"/> which is <pause dur="0.5"/> small raindrops we can find errors <pause dur="0.6"/> # <pause dur="2.4"/> of about fifty per cent <pause dur="9.1"/> and so why <pause dur="0.7"/> where do these errors <pause dur="0.2"/> come about from <pause dur="2.7"/> second handout <pause dur="23.2"/> ooh sorry <pause dur="27.5"/> so again as <pause dur="0.2"/> many of you will be familiar that the reason why this error comes about is that the <pause dur="0.8"/> gauge itself <pause dur="1.1"/> provides a <pause dur="2.7"/> a block on the flow <pause dur="0.6"/> and the <pause dur="0.9"/> the air which is blocked by the <pause dur="0.3"/> the # <pause dur="0.4"/> rain gauge has to go somewhere and so <pause dur="0.4"/> in general there'll be an acceleration of of of the air <pause dur="0.4"/>

both around the gauge and <pause dur="0.2"/> for our purposes most importantly is <pause dur="0.4"/> over the gauge so <pause dur="0.5"/> what this means is that droplets <pause dur="0.3"/> drops raindrops that would otherwise <pause dur="0.4"/> be falling into the gauge <pause dur="0.4"/> are swept <pause dur="0.3"/> swept from it <pause dur="0.6"/> and the <pause dur="0.2"/> situation is worse in mountainous areas because there's <pause dur="0.4"/> they if <pause dur="0.2"/> if you've got a large droplet with a not lot of inertia it doesn't care too much about the wind speed <pause dur="0.5"/> but if you've got a small droplet with <pause dur="0.5"/> # <pause dur="0.7"/> with not much <trunc>i</trunc> inertia so a drizzle droplet will tend to be more <pause dur="0.3"/> susceptible to the effects of this acceleration <pause dur="0.6"/> so that's the <pause dur="0.3"/> the physical cause then is the <pause dur="1.2"/> # <pause dur="0.3"/> the acceleration <pause dur="2.8"/> of the flow <pause dur="1.3"/> # <pause dur="0.3"/> due to the <pause dur="2.7"/> # <pause dur="1.2"/> due to the effect of the gauge itself <pause dur="5.5"/> so this is a classic example <pause dur="0.9"/> in physics of the measurement actually perturbing what we want to measure <pause dur="0.5"/> so the presence of a rain gauge is <pause dur="0.4"/> is disrupting the measurement <pause dur="10.7"/> so so what are the <pause dur="4.4"/> possible solutions to this <pause dur="0.7"/> # <pause dur="0.4"/> well there are <pause dur="0.2"/> there are various ones # <pause dur="0.9"/> one is to derive is simply <pause dur="1.0"/> do lots of <pause dur="0.3"/> measurements and derive

correction factors <pause dur="5.2"/> so <pause dur="0.9"/> if you know the wind speed at your meteorological site and the rainfall you can use the wind speed to <pause dur="0.4"/> # <pause dur="0.4"/> to make a correction so that's actually <pause dur="0.4"/> done <pause dur="1.7"/> # <pause dur="0.3"/> quite routinely in the big <pause dur="0.4"/> big analyses of global rainfall <pause dur="0.5"/> <trunc>i</trunc> in some but not all of them <pause dur="5.5"/> the other technique is to put some kind of <pause dur="0.4"/> shield <pause dur="0.4"/> around the rain gauge which is <pause dur="0.6"/> indicated by <pause dur="0.5"/> # <pause dur="1.7"/> by some of the gauges <pause dur="0.4"/> here <pause dur="0.5"/> and so the idea of the gauge <pause dur="0.6"/> oh sorry the idea of the shield <pause dur="3.8"/> is <pause dur="0.7"/> is that it # <pause dur="0.2"/> doesn't cause so much distortion of the air flow <pause dur="1.2"/> over the <pause dur="6.2"/> over the gauge <pause dur="10.6"/> another one that's sometimes used but you need <pause dur="0.5"/> loads of space is something called a <pause dur="0.3"/> a turf wall so wall so what <pause dur="0.2"/> this is <pause dur="0.6"/> # <pause dur="0.2"/> kind of a cross section through the turf wall what you do is have a standard rain gauge <pause dur="0.7"/> # <pause dur="0.2"/> still <trunc>th</trunc> <pause dur="0.8"/> three-hundred millimetres above ground level but you surround it by a <pause dur="0.4"/> a turf wall some distance away from it and again <pause dur="0.3"/> that's supposed to <pause dur="0.5"/> reduce the <trunc>editi</trunc> # <trunc>ed</trunc> <pause dur="0.3"/> <trunc>th</trunc> the <pause dur="0.8"/> eddying and the acceleration of the <pause dur="1.2"/> air over the gauges reduced

because # <pause dur="0.9"/> it it's sheltered so that's one <pause dur="0.5"/> one other technique <pause dur="18.2"/> but all <pause dur="0.2"/> all these <pause dur="0.2"/> gauge types have the problem in that they're trying to measure rainfall <pause dur="0.5"/> from <pause dur="0.5"/> from a gauge which is stuck <pause dur="0.2"/> typically thirty <pause dur="0.4"/> thirty centimetres above the ground so it's not measuring at ground level and you've got <pause dur="0.4"/> an acceleration of the flow <pause dur="0.9"/> so probably the best <pause dur="1.8"/> # solution <pause dur="3.9"/> is the one that again <pause dur="0.4"/> we can see on our own <pause dur="0.7"/> met site <pause dur="1.0"/> outside <pause dur="0.6"/> is the <pause dur="0.4"/> # <pause dur="0.8"/> is to put the gauge at ground level <pause dur="0.7"/> or the gauge opening <pause dur="5.1"/>

at ground level <pause dur="0.6"/> so flush with the surface <pause dur="0.5"/> and surround it by a <pause dur="0.5"/> a pit with a grating on <pause dur="18.8"/> so <pause dur="0.2"/> let's just think about this design for a second <pause dur="0.3"/> the <pause dur="0.9"/> obviously putting it at ground level <pause dur="0.7"/> is is a solution but if we <pause dur="0.4"/> if we just put it at ground level on normal <pause dur="0.4"/> normal ground then you'd have <pause dur="0.2"/> terrible trouble with <pause dur="0.2"/> water splashing and running into the gauge so you want to have <pause dur="0.6"/> an area around the gauge where <pause dur="0.9"/> # where the rain can't bounce off so you you sink <pause dur="0.4"/> you you put it in surrounded by a pit <pause dur="0.4"/> but if you just left that pit open <pause dur="0.6"/> then <pause dur="0.5"/> # <pause dur="0.2"/> you'd have all kinds of eddying <pause dur="0.3"/> due to the sudden change in surface from the <pause dur="0.2"/> from the grass <pause dur="0.4"/> to a <pause dur="0.8"/> # to a deep pit <pause dur="0.3"/> so the the idea of the grating is to <pause dur="0.4"/> # not really <pause dur="0.2"/> give much <pause dur="1.0"/> # <pause dur="0.2"/> insplash but it <pause dur="0.2"/> gives you a a more smooth aerodynamic surface so you don't get too much eddying <pause dur="0.6"/> so this is generally regarded as the best solution it's not always <pause dur="0.3"/> a practical solution

particularly if you're <pause dur="0.5"/> # in a rocky area and need to <vocal desc="laugh" iterated="n"/><pause dur="0.4"/> dig a dig a deep pit to do this <pause dur="1.8"/> and it also has to be looked after <pause dur="2.5"/> you have to # make sure this pit is kept <pause dur="0.2"/> kept clear and weed free <pause dur="1.1"/> and if you want to which i do encourage you to do <pause dur="0.2"/> read a little more about this i i refer to <pause dur="0.6"/> this book <trunc>b</trunc> # <pause dur="0.2"/> a book by Ian Strangeways earlier in the course he's written a few <pause dur="0.4"/> nice little articles for Weather so these are <pause dur="0.4"/> just a few pages long <pause dur="0.4"/> so this one's <pause dur="0.2"/> # <pause dur="0.4"/> just a few years old now so i'd encourage you to go and <pause dur="0.4"/> read that and <pause dur="0.3"/> # he also talks about <pause dur="0.5"/> # his experiments with rather more <pause dur="0.7"/> bizarre <pause dur="0.3"/> types of gauge which might be <pause dur="0.6"/> # sort of gauges of the future <pause dur="10.6"/> so how how much <pause dur="0.2"/> difference do these <pause dur="0.3"/> these make <pause dur="0.4"/> well it it's <pause dur="0.3"/> typically <pause dur="0.2"/> so # # <pause dur="0.6"/> one of these <pause dur="1.5"/> # <pause dur="0.7"/> these pit gauges <pause dur="3.1"/> they typically measure <pause dur="4.1"/> # for a for a U-K site they'll typically measure something like <pause dur="1.4"/> three to six per cent more than a standard gauge <pause dur="10.1"/> and again in mountainous areas where we tend to have stronger winds and <pause dur="0.4"/> often smaller raindrops it

can be <pause dur="0.6"/> # <pause dur="0.9"/> can be as much as twenty per cent <pause dur="8.8"/> so they're much more <pause dur="0.4"/> much much more efficient collectors of rainfall <pause dur="12.0"/> but i should stress that this kind of rain gauge is much <pause dur="0.2"/> much less common than the <pause dur="0.4"/> the standard <pause dur="0.6"/> the more standard type that we're used to seeing <pause dur="6.9"/> now there's two other kinds of <trunc>precipito</trunc> well there's a two two other <pause dur="0.4"/> things that we have to worry about with measuring rainfall it's very easy to put a rain gauge out on the <pause dur="0.5"/> on on the land but how <pause dur="0.4"/> any ideas how you'd measure <trunc>r</trunc> rain <pause dur="0.2"/> in the <trunc>o</trunc> <pause dur="0.3"/> on the ocean <pause dur="1.9"/> so <pause dur="0.3"/> two-thirds of the planet is covered by ocean <pause dur="0.5"/> and we need to know the rainfall there </u><pause dur="1.7"/> <u who="sm0883" trans="pause"> buoys maybe </u><pause dur="0.8"/> <u who="nm0881" trans="pause"> yeah do you think that would work very well <pause dur="0.2"/> 'cause the <pause dur="0.8"/> one of the problems these things are rocky you need to keep keep your gauge <pause dur="0.6"/> level and you've got a lot of <pause dur="1.3"/> a lot of <pause dur="0.2"/> waves splashing in and things like that so <pause dur="0.4"/> they don't tend to work terribly well <pause dur="1.1"/> any other ideas <pause dur="1.9"/> measuring rain in <pause dur="0.2"/> the ocean </u><pause dur="1.7"/> <u who="sm0884" trans="pause"> put them on ships </u><pause dur="0.2"/> <u who="nm0881" trans="pause"> pardon </u><pause dur="0.2"/> <u who="sm0884" trans="pause"> put them on ships </u><u who="nm0881" trans="latching"> well you can but you've got real

problem there is keeping the ship <pause dur="0.4"/> steady and # <pause dur="0.2"/> it <pause dur="0.2"/> it tends to be many metres above the surface so it's not <pause dur="0.2"/> really regarded as very reliable <pause dur="0.9"/> i mean there's two things that tend to be done <pause dur="0.4"/> # one is just to use <pause dur="0.9"/> stations in island <trunc>re</trunc> # island stations and hope that they're somehow representative of the surrounding region which is a <pause dur="0.5"/> a <pause dur="0.2"/> a big assumption <pause dur="0.7"/> <trunc>a</trunc> another thing that's being thought about is actually just measuring <pause dur="0.2"/> which is amazing is is measuring the noise <pause dur="0.2"/> due to the raindrops hitting the surface <pause dur="0.4"/> so there are now people trying to develop acoustic techniques <pause dur="0.4"/> of actually having little microphones under the ocean <pause dur="0.4"/> literally listening to the pitter-patter of the rainfall <pause dur="0.4"/> so they're very much at the <pause dur="0.2"/> at the research level <pause dur="0.5"/> # and <pause dur="0.3"/> # <pause dur="0.6"/> they can be interfered with by <pause dur="0.2"/> by all kinds of things <pause dur="0.3"/> so that that's a problem <trunc>th</trunc> <trunc>th</trunc> the other problem i'm just going to <pause dur="0.3"/> touch on <pause dur="0.5"/> # <pause dur="0.8"/> briefly is <pause dur="0.2"/> is of course <pause dur="0.8"/> measuring snow <pause dur="0.5"/> snowfall <pause dur="1.1"/> which in certain parts of the world is a a large part of

the <pause dur="0.8"/> rainfall again we're not well large part of the precipitation <pause dur="1.1"/> is <pause dur="0.3"/> is that gauges are of <pause dur="0.3"/> of of limited use <pause dur="5.9"/> and <pause dur="0.2"/> of course one of the most severe problems in these situations is <pause dur="0.4"/> is drifting where all you're doing is redistributing snow that's already fallen <pause dur="0.7"/> which ends up in your gauge and you don't know whether it's <pause dur="0.4"/> it's just due to drifting or whether it's due to # <pause dur="2.1"/> # <pause dur="3.0"/> <trunc>whe</trunc> <pause dur="0.6"/> whether it's real precipitation or whether it's due to drifting <pause dur="0.3"/> and also it doesn't take that much snowfall in some some areas to overtop <pause dur="0.4"/> top the gauge <pause dur="0.2"/> 'cause obviously snow's a lot less dense so it doesn't take so much snow to actually <pause dur="0.4"/> fill up your <pause dur="0.7"/> fill up the top of your <pause dur="0.8"/> # rain gauge so <pause dur="0.4"/> those are some problems there's <pause dur="0.4"/> # <pause dur="1.9"/> so there's various other techniques that # <pause dur="0.2"/> try to be used <pause dur="0.4"/> one is to try and <pause dur="1.3"/> # <pause dur="4.6"/> one is that you simply forget the gauge <pause dur="0.4"/> and you just measure the depth <pause dur="0.3"/> of snow <pause dur="7.7"/> and and assume the volume <pause dur="0.9"/> oh sorry assume assume a density <pause dur="0.7"/> but even the density of snow <pause dur="0.4"/> depends very much on its form and how

old it is <pause dur="0.5"/> or actually <pause dur="0.4"/> # take <pause dur="2.2"/> take a core of the snow <pause dur="3.4"/> and <trunc>ju</trunc> and just melt it <pause dur="3.4"/> and so you measure the <pause dur="0.2"/> and <pause dur="0.8"/> and calculate the rainfall <pause dur="0.2"/> or calculate the <pause dur="0.9"/> the <pause dur="0.3"/> the precipitation <pause dur="6.4"/> there are more subtle techniques being <pause dur="0.4"/> being <pause dur="0.4"/> being used in some areas and one one of these is # <pause dur="0.9"/> is that we know that the earth is a natural source of radioactivity <pause dur="0.5"/> and there's <pause dur="0.4"/> for example gamma rays being emitted <pause dur="0.3"/> just by natural radioactivity in the earth and <pause dur="0.6"/> and <pause dur="0.7"/> snow is quite a good absorber of those <pause dur="0.4"/> # <pause dur="0.6"/> <trunc>o</trunc> of gamma rays <pause dur="1.7"/> and so <pause dur="0.2"/> <trunc>i</trunc> if you if you measure how much attenuation you've got of the normal gamma rays you'd expect <pause dur="0.4"/> say measured by an aircraft you get can get some idea of the volume of of snow <pause dur="0.4"/> so <pause dur="0.4"/> that's a kind of a <pause dur="0.7"/> a very <pause dur="1.5"/> modern technique <pause dur="10.6"/> and <pause dur="0.3"/> you you probably wouldn't use that just for a <pause dur="0.2"/> a little area but for getting some kind of aerial average picture you can get some idea of the <pause dur="0.5"/> <trunc>w</trunc> the # <pause dur="0.2"/> snow water content <pause dur="0.2"/> there <pause dur="0.9"/> and i'm not going to <pause dur="0.3"/> to do any more and talk about this if you're interested in them

then both <pause dur="0.3"/> # <pause dur="0.7"/> two of the books that i i've referred to <pause dur="0.2"/> Ward and Robinson <pause dur="2.0"/> and Strangeways <pause dur="2.0"/> # go into quite a lot of detail about snow measuring techniques it's really quite an interesting area <pause dur="0.7"/> and of course if in some <pause dur="0.5"/> <trunc>continelt</trunc> <trunc>ar</trunc> continental areas <pause dur="0.4"/> it's an important contribution to the # <pause dur="2.2"/> to the whole hydrological cycle <pause dur="6.3"/> okay now what we're not going to touch on here <pause dur="0.2"/> which is a <pause dur="1.2"/> which is a serious issue <pause dur="1.7"/> is <pause dur="0.6"/> is as i said before a rain gauge only <pause dur="0.5"/> only measures a a very small <pause dur="0.9"/> fraction of the total area <pause dur="0.4"/> and if say we want to know the rainfall over the U-K <pause dur="0.3"/> how many rain gauges do we need <pause dur="0.5"/> is it one <pause dur="0.2"/> ten a hundred a thousand <pause dur="0.5"/> and what we're going to do have to do later is to <trunc>c</trunc> try and come up with some quantative way of saying <pause dur="0.4"/> how <pause dur="0.7"/> how densely do we need to pack our rain gauges to get a reasonable <pause dur="0.5"/> # indication of the <pause dur="0.5"/> of the total rainfall and of course that will be <pause dur="0.8"/> # dependent on whether <pause dur="1.4"/> # on on the particular weather we have <pause dur="0.3"/> for example whether it's # frontal <pause dur="0.3"/> or <pause dur="0.6"/> or convective <pause dur="0.4"/> so

we'll come back to that because that's and that's a very important part of <pause dur="0.3"/> hydro-meteorology is <pause dur="0.5"/> how you <pause dur="0.3"/> reliably average <pause dur="1.0"/> the <pause dur="0.2"/> the next technique we're going to touch on <pause dur="0.3"/> # <pause dur="1.1"/> for measuring rainfall is one that we can see <pause dur="0.9"/> on the telly every night of the week these days <pause dur="1.5"/> is radar <pause dur="1.5"/> and <pause dur="0.3"/> i'm not going to <pause dur="0.3"/> labour the technical side of radar <pause dur="0.4"/> the <pause dur="0.9"/> Met students will get it i think in their third year from <pause dur="0.4"/> Dr <gap reason="name" extent="1 word"/> <pause dur="1.6"/> what i'm going to just going to do is is put it in the context of <pause dur="0.4"/> # <pause dur="1.8"/> as a hydrological cycle <pause dur="0.7"/> so <pause dur="0.7"/> # <pause dur="0.9"/> the real routes of of the radar growth of <trunc>c</trunc> of course radars grew out of the Second World War but it's only been since i guess the <pause dur="0.5"/> # mid-nineteen-seventies <pause dur="3.2"/> # we've seen a <pause dur="1.4"/> a massive <pause dur="1.4"/> growth

in the use of radars for rainfall growth in <pause dur="1.3"/> rainfall radars <pause dur="2.7"/> and particularly in developed countries <pause dur="0.5"/> and the U-K now is is pretty well covered by <pause dur="0.5"/> by by radars <pause dur="1.8"/> and <pause dur="0.9"/> # <pause dur="0.5"/> what we're going to do is look at <pause dur="3.1"/> look at some of the <trunc>prob</trunc> some of the advantages and disadvantages of radars one of the big problems we're going to find is that they don't actually measure rainfall <pause dur="0.5"/> they're measuring the water while it's still up in the atmosphere <pause dur="0.5"/> which isn't telling you about <pause dur="0.5"/> how much is actually hitting the ground and we have to go through a lot of assumptions to <pause dur="0.4"/> to deduce that so <pause dur="0.4"/> and that's a convenient place to leave it <pause dur="0.7"/> leave it for now <pause dur="1.3"/> and <pause dur="0.4"/> see you nine o'clock tomorrow morning

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