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Rainfall

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Tweefo
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« on: January 08, 2012, 11:08:32 AM »

At the start of the season we we warned that it was going to be a very wet one. Lot of rain and heavy falls. I know it happened in Natal but here on the Highvelt it is dry. The blue is the long term average, red last season in green this one so far. One thing I learned from my farming years is that you have to take the forecast with a very generous dose of salt.
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brianvds
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« Reply #1 on: January 08, 2012, 14:43:29 PM »

Alas, I don't have MS Office on my computer, and when I try to open the document with OpenOffice, I just get a blank page with nothing on it.

But I am rather skeptical of long-range forecasts too. Perhaps climate change plays a role? Purely intuitively, it seems to me we are long overdue for a drought here in Pretoria; we used to get one every few years. I can't even remember when last we have had a summer drought though. We seem to be among the global warming winners.

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BoogieMonster
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« Reply #2 on: January 09, 2012, 09:27:31 AM »

Ditto on the blank page, but I have to say in joburg it seems we've not gone a day without some kind of thunder-shower in the last month or so.
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Tweefo
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« Reply #3 on: January 09, 2012, 09:37:43 AM »

How do I change a Ms Office graph into a picture? This rainfall is measured at my house in Middelburg and if you can see it you will understand why I feel sorry for the farmers. On the other hand maybe you are right that a dry year is overdue. The farmers can mostly all afford Land Cruisers anyway, so maybe this won't harm them too much.The weather service still got it wrong though.
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Faerie
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« Reply #4 on: January 09, 2012, 10:33:28 AM »

Ditto on the blank page, but I have to say in joburg it seems we've not gone a day without some kind of thunder-shower in the last month or so.

Accompanied by some spectacular electrical storms. Its normal Jo'burg weather though, bleeding hot until mid/later afternoon when it thunders and rains.
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Mefiante
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« Reply #5 on: January 09, 2012, 10:34:08 AM »

How do I change a Ms Office graph into a picture?
  • Select the Excel graph by clicking on its border.
  • Click the “Copy” option on Excel’s “Home” tab.
  • Open MS Paint (Start —> Programs —> Accessories —> Paint).
  • Click the “Paste” option on MS Paint’s “Home” tab.
  • Save the picture (I recommend in PNG format).

'Luthon64
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Tweefo
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« Reply #6 on: January 09, 2012, 12:23:28 PM »

How do I change a Ms Office graph into a picture?
  • Select the Excel graph by clicking on its border.
  • Click the “Copy” option on Excel’s “Home” tab.
  • Open MS Paint (Start —> Programs —> Accessories —> Paint).
  • Click the “Paste” option on MS Paint’s “Home” tab.
  • Save the picture (I recommend in PNG format).

'Luthon64
Thanks M. Let's try.
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Rigil Kent
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« Reply #7 on: January 15, 2012, 08:46:46 AM »

http://www.telegraph.co.uk/topics/weather/9010030/South-Africa-weather-forecasters-threatened-with-jail-if-predictions-wrong.html

Well, well!
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brianvds
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« Reply #8 on: January 15, 2012, 09:25:11 AM »



Typical of the somewhat confused way in which the media report things. The heading implies that any weather forecaster who gets it wrong will be imprisoned, which is simply not true, but of course, the heading is what most people will read and brainlessly repeat.

In the local press I have seen it breathlessly reported that if your husband is on his way home and you phone him to warn him that there's a big storm brewing and he should be careful, you can be sent to prison for it. Predictably, a, er, big storm ensued over it and the weather bureau had to hastily backtrack and issue statements and who knows what else, all over media reports that anyone should have been able to see were probably at best half true.

It is probably perfectly sensible to regulate weather reporting, in order to prevent needless panic or doomsday prophets freely predicting the world will end next week when a superstorm hits Jo'burg. Perhaps the law is even written in such a way as to imply you're not allowed to phone your husband to tell him there's a big hailstorm breaking over the neighbourhood, but I think it is pretty obvious that no one will ever be prosecuted for that.
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Rigil Kent
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« Reply #9 on: January 15, 2012, 11:08:57 AM »

Maybe a possible way around this is already being employed when predicting rainfall. Rather than saying that it will rain or that it won't, the weather service estimates the chance of precipitation. So, if the chance that a devastating storm with flooding will occur is, likewise, reported as a percentage, I don't see how any weather service can be held accountable when the day in question turns out bone dry. Undecided

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Rigil Kent
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« Reply #10 on: January 15, 2012, 11:27:02 AM »

It is probably perfectly sensible to regulate weather reporting,

Nah, don't like this at all, simply because it is yet another onslaught on my rights to say what I please. Rather, people should be taught how to discover for themselves the difference between good and suspect sources of information.

Rigil
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Mefiante
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« Reply #11 on: January 15, 2012, 12:28:53 PM »

…the weather service estimates the chance of precipitation.
However, that probability is quite precisely defined and, in point of fact, fairly reliable.  When the Weather Bureau cites a “probability P of rain,” then that figure is relevant to a certain area or region A for a certain time period T, and means that it will rain over fraction P of area A within time period T.  These are the limits of accuracy/reliability of our current meteorological models, which are known to be of a class called “chaotic dynamical systems.”  These models are extremely sensitive to tiny variations in their input parameters, and typically produce vastly different results for very slightly different inputs.

It therefore has little to do with hedging bets on the part of the Weather Bureau.

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Rigil Kent
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« Reply #12 on: January 15, 2012, 16:26:02 PM »

... means that it will rain over fraction P of area A within time period T.

It does  Shocked?

I thought that it meant that there is a P chance that rain will occur at any given point in area A during T.

Please see http://www.srh.noaa.gov/ffc/?n=pop

(Which I took to imply that it is at least possible that no precipitation could occur anywhere in the area).

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Mefiante
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« Reply #13 on: January 15, 2012, 16:33:35 PM »

If you think about it, the two are saying the same thing.  That’s because within A and T, there is no preferred point and time at which it will rain.  That is, any point and time within A and T is statistically equivalent to any other, and so the probability of having rain at any randomly chosen point within A and T is P.

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Rigil Kent
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« Reply #14 on: January 15, 2012, 16:44:49 PM »

No, I don't immediately see the equivalence of the two interpretations,

The first claims that it will rain over some fraction of the area, the second that it needn't.

.... but i'll go away and and try and work it out first before becoming too confident. Undecided

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Mefiante
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« Reply #15 on: January 15, 2012, 17:00:42 PM »

Take a simple shape like a square or a circle.  If the chances of rain are, say, 30% then 30% of that area are shaded blue and the remainder red.  Pick another simple shape to represent the blue area.  Pin your drawing to a dartboard.  Put on a blindfold and throw a dart at your drawing.  Assuming your dart hits anywhere within the total area, what is the probability that you hit blue?

There is, of course, a small residual possibility that there is no rain anywhere within A during T, but if the Weather Bureau cites a chance of 20% or more, I wouldn’t put any money on it not raining anywhere over the relevant area and period.  That is the nature of probabilities.

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Rigil Kent
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« Reply #16 on: January 15, 2012, 19:53:24 PM »

I've checked this in a simulation - attached, hope it opens - and it turns out that if the probability of rain at each point in the forcast area is equal to P, then the percentage of points that experienced rain after the simulation is also jolly close to P!  Cool Unfortunately the math as to why it should be so is beyond me. Lips Sealed

Rigil


« Last Edit: January 15, 2012, 20:29:40 PM by Rigil Kent » Logged
Tweefo
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« Reply #17 on: January 15, 2012, 20:06:15 PM »

This is short term forecast. My original stance was that this was supposed to a a El Nina year but as you can see on my graph this (here in M'burg at least) is definitely not the case.

I became very skeptical about the SA Weather when they forecast a 80% chance of heavy falls and there was not even a cloud in the sky on the day. I was still farming then and we needed the rain.
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Mefiante
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« Reply #18 on: January 15, 2012, 20:17:20 PM »

Good job Rigil (you star), but does it convince you that the two views are equivalent?  More importantly, has it refined your understanding of what those rain probabilities mean?

The maths, as in most real-world problems, go back to calculus:  Consider an area A and shrink it progressively so that it gets closer and closer to zero without ever quite getting there.  The assumption is that a sub-area is representative of the whole.  (Meanwhile, forget that the process is an idealisation!)

Tweefo, long-term weather prediction is, with today’s technology, a pipe-dream.  Meteorological models are continuously being refined with advances in mathematics, computation and empirical understanding.  It’s not an easy problem.

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Rigil Kent
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« Reply #19 on: January 15, 2012, 20:25:41 PM »

.. does it convince you that the two views are equivalent?  More importantly, has it refined your understanding of what those rain probabilities mean?
Yes and yes. Smiley

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« Reply #20 on: January 16, 2012, 01:30:50 AM »

I attended a lecture a few years back at my university by someone(cant remember who) from Weather SA. He pointed out that the 30% rain fall area they show on the news is not as random as most people think. That 30% means they have a 99% confidence based on the model that it would rain over 30% of the given area. But it is not random, because of the time window they give. He showed a simulation with a storm moving from right to left over the area. He said the models could predict that the storm would reach critical mass and start raining anytime in the next 10 hours. If it started to rain in the first hour or so the most right hand side of the given area would receive  rain whereas if it reached critical mass at 9 hours the most left hand side of the area would receive rain, based on the size and speed they know the storm would only last long enough to precipitate about 30% of the area.   
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