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<publicationStmt><distributor>BASE and Oxford Text Archive</distributor>


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

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

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<langUsage><language id="en">English</language>



<person id="nm0881" role="main speaker" n="n" sex="m"><p>nm0881, main speaker, non-student, male</p></person>

<person id="sm0882" role="participant" n="s" sex="m"><p>sm0882, participant, student, male</p></person>

<person id="sm0883" role="participant" n="s" sex="m"><p>sm0883, participant, student, male</p></person>

<person id="sm0884" role="participant" n="s" sex="m"><p>sm0884, participant, student, male</p></person>

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<personGrp role="speakers" size="6"><p>number of speakers: 6</p></personGrp>





<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>




<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