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#863990 - 19/05/2010 13:57 How is the Minimum Temperature determined?
Eugene Offline
Cloud Gazer

Registered: 19/05/2010
Posts: 5
It doesn't make sense. confused

Take the readings for Cooma AP NSW for example. On Tue, May 18 it says in the "Past 5 Days" that the Minimum was -4.5°C, yet the 24-hour graph shows that the minimum was actually -6.6°C. And again for Mon May 17 it has -1.6°C as the Minimum when it was actually -2.9°C...every single day the real Minimum Temperature is lower than the "official" readings. What's going on?

One could think that the Global Warming Climate Change Conspiracy nuts are trying to fudge the numbers so the weather appears to be warmer than it actually is...


Edited by Eugene (19/05/2010 14:04)

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#863998 - 19/05/2010 14:44 Re: How is the Minimum Temperature determined? [Re: Eugene]
Andrew Miskelly Offline
Weatherzone Webmaster

Registered: 15/11/2001
Posts: 3149
Loc: Mittagong, NSW
Hi Eugene,

There are two weather stations at Cooma which you may be mixing up. One is entitled 'Cooma' (Cooma Visitors Centre) and the other is entitled 'Cooma Ap' (Cooma Airport). Note also that a day's minimum temperature is recorded over the 24 hours to 9am on that day.

Cooma Visitors Centre [daily]
Cooma Airport [daily] [24 hour graph]

Cooma Visitors Centre has no graph available as it is a manual station and only reports twice daily.


Edited by Andrew Miskelly (19/05/2010 14:45)

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#864017 - 19/05/2010 17:17 Re: How is the Minimum Temperature determined? [Re: Andrew Miskelly]
Eugene Offline
Cloud Gazer

Registered: 19/05/2010
Posts: 5
Originally Posted By: Andrew Miskelly
Hi Eugene,

Note also that a day's minimum temperature is recorded over the 24 hours to 9am on that day.


What? So Wednesdays minimim is actually calulated over 9 hours of Wednesday morning and 15 hours of the previous Tuesday? I dont get this. How does that work?

How can you give an accurate minimum temp for any day when you dont actually use the 24 hours in the same day? Doesn't make sense to me.

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#864022 - 19/05/2010 18:22 Re: How is the Minimum Temperature determined? [Re: Eugene]
Surly Bond Offline
Weatherzone Addict

Registered: 23/08/2003
Posts: 2190
Loc: Manilla, near Tamworth NSW
Eugene
I had some trouble locating the definitions of maximum and minimum temperatures for a given day on the BoM site, as they are not in the glossary. However, here they are:
http://www.bom.gov.au/climate/dwo/IDCJDW0000.shtml

Minimum temperature in the 24 hours to 9am.
Sometimes only known to the nearest whole degree.

Maximum temperaure in the 24 hours from 9am.
Sometimes only known to the nearest whole degree.

I share your frustration. Sometimes morning max/min readings show a phenomenal fall of temperature from the afternoon maximum to the morning minimum but it does not count. Only temperature rises count, and they are usually smaller.

There is still a mystery about what figures BoM publishes. Like you, I have found that what gets published is different to what you can see on the METAR plot for the same station and the same day.
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#864116 - 20/05/2010 10:59 Re: How is the Minimum Temperature determined? [Re: Surly Bond]
Andrew Miskelly Offline
Weatherzone Webmaster

Registered: 15/11/2001
Posts: 3149
Loc: Mittagong, NSW
Traditionally (going back to the early 1800s) in the Australian context, daily weather reports have been made by the staff of such places as post offices at the beginning of each day. This of course makes the beginning of each day 9am rather than midnight (when all sensible postmasters are tucked up in bed).

Most of the time this works fine as a day's minimum temperature generally occurs near dawn (before 9am) and a day's maximum temperature generally occurs in the mid to late afternoon (after 9am). Of course there are occasional exceptions though.

Despite the fact that we now have digital technology which doesn't mind being up at midnight it's important to maintain the 9am-9am tradition for the consistency of the records and the integrity of those sciences which use them.

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#864150 - 20/05/2010 14:56 Re: How is the Minimum Temperature determined? [Re: Andrew Miskelly]
Eugene Offline
Cloud Gazer

Registered: 19/05/2010
Posts: 5
Originally Posted By: Andrew Miskelly
...it's important to maintain the 9am-9am tradition for the consistency of the records and the integrity of those sciences which use them.


Pff, traditions are changed all the time, as are the way records are kept. And scientists are used to interpreting and manipulating data all the time too. It wouldn't be hard to put an asterisk on the date when it is changed to a midnight-midnight recording, everyone would get used to it almost immediately and everything would carry on as usual. It would be a win for common sense. Plus it would make for more accurate science and isnt that what all scientist want?

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#864171 - 20/05/2010 19:33 Re: How is the Minimum Temperature determined? [Re: Eugene]
Wet Wet Wet Offline
Weatherzone Addict

Registered: 08/01/2008
Posts: 2332
Loc: Mackay, Qld
Eugene, would you be prepared to get up at midnight EVERY night to record the data for the previous 24hrs?? And remember as already mentioned, numerous sites are located at post offices and government locations – and the observers don’t actually live there.
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#864183 - 20/05/2010 21:43 Re: How is the Minimum Temperature determined? [Re: Wet Wet Wet]
kizz Offline
Weatherzone Addict

Registered: 11/02/2002
Posts: 2202
Loc: Beacon Hill, Sydney 152m asl
Blair has mentioned that comparing these different diurnal cut-off times is on his list of things to do.

The consistency argument is not so rock solid when you consider that daylight savings effectively makes Summer readings an 8am-8am measurement. Some quite rapid heating occurs at that time. Makes a lot of difference for sites that take longer to warm up than others. Sydney's Observatory Hill takes a while to heat up in the morning and many minimums are taken from 8am the day before.

Daylight savings has not always been around so it can be like comparing apples with pears year on year on year.
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#864279 - 21/05/2010 17:20 Re: How is the Minimum Temperature determined? [Re: Wet Wet Wet]
Eugene Offline
Cloud Gazer

Registered: 19/05/2010
Posts: 5
Originally Posted By: Wet Wet Wet
Eugene, would you be prepared to get up at midnight EVERY night to record the data for the previous 24hrs?? And remember as already mentioned, numerous sites are located at post offices and government locations – and the observers don’t actually live there.


We have, you know, Computers to do that.

Everyone adjusted fine when we brought in the decimal system. If as Andrew Miskelly has said they are so concerned about keep the "Tradition", why isnt everything still officially kept in °F/Knots/Inches/Feet and Miles instead of the decimal equivalent? Or is it?

I think weather observation needs to be brought up to modern times.

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#864359 - 22/05/2010 07:28 Re: How is the Minimum Temperature determined? [Re: Eugene]
Wet Wet Wet Offline
Weatherzone Addict

Registered: 08/01/2008
Posts: 2332
Loc: Mackay, Qld
Eugene, on paper your theory makes sense, however the dilemma is that all daily measurements are taken at 9am – this includes rainfall. If the time of measurement were to change for temperature, it would be logical to change the time of measurement for rainfall as well; otherwise it defeats the purpose of changing the time in the first place. From memory there are currently about 8,000 sites in Australia that record rainfall each day, with the majority of these sites located on rural properties that voluntarily measure rainfall each day and forward the data on to the BoM.

The proposition of automatic gauges is valid, however in my opinion they are inaccurate and very much unreliable, not to mention the initial cost of implementation and ongoing maintenance. If one were to only rely on automatic gauges to take the midnight measurements, unfortunately at some stage the equipment would malfunction and consequently 1 or more days measurement would be lost (the BoM currently operate a flood warning network that uses automatic gauges - which when they break down it sometimes takes longer than a month, and in some cases over a year before they become operational again). At present there are some manual rainfall sites in Australia that have over 100 years of data with not one day missed.

As for the decimal system you mentioned, I assume you are referring to the transition from imperial to metric? If so, I cannot see how it would have been a problem as all data is easily convertible with a simple equation. i.e. °F to °C conversion is simply…… (x-32 ) ÷9×5.
Also 1kt is equivalent to 1.852km
As 25.4mm = 1 inch
So, if on February 7th 1941 Sydney recorded a maximum temperature of 79°F and 2 inches of rain, we know that in today’s standards this is equivalent to 26.1°C and 50.8mm of rain.

However, it is not possible to go back in time and re-measure daily rainfall totals or temperatures at midnight. Therefore if the time of measurement were to change from 9am to midnight, comparisons could not be made between the new and old data as they would not be compatible with each other - making it (in my opinion) quite useless.
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#864418 - 22/05/2010 17:55 Re: How is the Minimum Temperature determined? [Re: Wet Wet Wet]
Eugene Offline
Cloud Gazer

Registered: 19/05/2010
Posts: 5
But it seems most things are measured 24/7, all it requires is "moving the goalpost" so that the daily max/min/rainfall/wind/etc are in line with the 24 hour cycle of midnight-midnight and not the current 9am-9am cycle.

To me the current system seems very archaic and antiquated. I bet if you told the person on the street they would be flabbergasted with the current reading system, I know I was when I found out. Guess I will just have to put up with it.

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#864663 - 24/05/2010 11:31 Re: How is the Minimum Temperature determined? [Re: Eugene]
Arnost Offline
Weatherzone Addict

Registered: 10/02/2007
Posts: 3909
"Guess I will just have to put up with it"

And that's what we all do... I think that everyone has the same reaction like you had - but in the end, we realise that the 9am - 9am time made a lot of sense (in the past certainly so), and it is just too difficult to reconcile the change in the time of observation (say midnight - midnight to 9am - 9am) without causing serious inconsistencies in the historical record.

The 9am time is only an hour or two past what typically is the coldest part of the day - and if it is "really" cold around that time on a given day, then it may count twice - i.e. 8.59am for Day 1 and 9.01am for Day 2 records. If you go midnight to midnight - this will introduce a spurious warming trend into the record as you will potentially eliminate these double counts of cold minima - i.e. that the discontinuity / inconsistency. And adjusting for it will be simply guess work.

So we put up with it.

However - saying that - in an environment, where temps are recorded automatically (to the minute in a lot of cases).. there is no reason why you can't have two records (so long as it's understood that they should not be mixed).
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“No. Not even in the face of Armageddon. Never compromise” ...

And this of course applies to scientific principles. Never compromise these. Never! [Follow the science and you will be shown correct in the end...]

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#864938 - 25/05/2010 00:21 Re: How is the Minimum Temperature determined? [Re: Arnost]
ROM Offline
Meteorological Motor Mouth

Registered: 29/01/2007
Posts: 6628
From;
World Meteorological Organisation

Manual on the Global Observing System

Volume 1

Part III

2.3.1.3 The main standard times for surface
synoptic observations shall be 0000, 0600, 1200
and 1800 UTC.
2.3.1.4 The intermediate standard times for
surface synoptic observations shall be 0300, 0900,
1500 and 2100 UTC.

Volume II

Regional Aspects

Instruments and Observing Methods
Report No 81

Instruments and Observing Methods
Report No 66

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#865028 - 25/05/2010 13:45 Re: How is the Minimum Temperature determined? [Re: ROM]
AaronD Offline
Weatherzone Addict

Registered: 27/12/2007
Posts: 1017
Loc: Tokyo, Japan
Yes what arnost said.

You need for the max/min to be over a whole day/night otherwise its stupid and the science doesn't work. 9am is before the day heats up but after the night time min (in most cases).
i mean fiar enough it is confusing to go oh wait its 850am but these readings are still from yesterday but deal with it.


i give you a hypothetical example (assuming the mins occurred around midnight).

say you have an cold night Saturday night (8deg) then a warm night on Sunday night (15deg) and another cold night Monday (8deg). if the data was taken from midnight - midnight you would get Saturdays min around 8deg and also Sundays min around 8deg but Mondays min would also be about 8deg...

how is there any record of the warm night on sunday night???

9-9 makes total sense to me.

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#894148 - 28/10/2010 17:32 Re: How is the Minimum Temperature determined? [Re: AaronD]
Surly Bond Offline
Weatherzone Addict

Registered: 23/08/2003
Posts: 2190
Loc: Manilla, near Tamworth NSW
I am still puzzled about the source of official records of daily maximum and minimum temperatures.

It is not just the confusion of having more than one local station, or anomalies due to the convention of using 9 am observations. Neither of these cause any ambiguity at the Tamworth Airport Automatic Weather Station.
I have been meticulously copying the maximum and minimum figures displayed at the peaks and troughs of the data traces for the AWS for several years.
The figures I copy are HARDLY EVER the same as those published by the BoM or by weatherzone.

Why is it so?
Where do the official figures come from?

Minimum figures beginning 10/10/10:
(Date, My reading, Official reading, Discrepancy)
10, 8.2, 7.9, -0.3;
11, 14.2, 13.7, -0.5;
12, 11.5, 11.3, -0.2;
13, 11.9, 11.4, -0.5;
14, 14.6, 14.2, -0.4;
15, 14.8, 14.0, -0.8;
16, 6.5, 6.5, -0.0;
17, 1.4, 1.0, -0.3;
18, 3.6, 3.4, -0.2....
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#894198 - 28/10/2010 22:08 Re: How is the Minimum Temperature determined? [Re: Surly Bond]
Dave-Wx Offline
Weatherzone Addict

Registered: 27/08/2001
Posts: 4950
Loc: Heritage Park, Brisbane, QLD
That would be because the graph generated by WZ is from the half hourly obs reported by the AWS as we go throughout the day...and the official figure is the lowest temperature that the AWS measured during the entire day. It reports it either in realtime on the right hand side of the BoM observations (mostly capital city areas), or just reports it at 9am the next day. So in any half hour period, the temperature may drop a little bit lower (up to 0.8 of a degree on some occasions it seems) than the temperatures at the start and end of that 30 minute period.

When I am looking at the coldest temps of the year...I will see the lowest recorded temp on the AWS of say, 5.7, and think to myself "oooh it might have got down to 5.5 or 5.4 which would just break the September record of 5.6"

Hope that helps smile

Edit: I just looked at the BoM observations page for NWSP, and I see Tamworth is one of the AWS's that report the running max and min from midnight to midnight each day:

http://www.bom.gov.au/nsw/observations/nswall.shtml#NWS


Edited by Dave-Wx (28/10/2010 22:13)

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#894211 - 28/10/2010 23:15 Re: How is the Minimum Temperature determined? [Re: Dave-Wx]
Surly Bond Offline
Weatherzone Addict

Registered: 23/08/2003
Posts: 2190
Loc: Manilla, near Tamworth NSW
Thanks, Dave.
That adds another layer to the confusion.
I found that the BoM "Daily Summaries: October", which apparently records midnight to midnight, had the same minimum values as I copied from the weatherzone "Daily Summaries: October" (minimum to 9 am) that I cited above, except that the 15th had 14.8 (like my reading from the graph) rather than 14.0. Looking again at the wz Summary, I see it is now 14.8, too. I am sure it was 14.0 when I first looked.

So it seems to me that the daily minimums published by both BoM and weatherzone in their monthly summaries are taken from half-hourly readings, not from instantaneous lowest values, which is what a re-settable minimum-reading thermometer would show.

Is that right?
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