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#126428 - 15/08/2005 13:43 Re: Stevenson Screen Design
Stephen-wx Offline
Weatherzone Addict

Registered: 15/07/2002
Posts: 1313
Loc: Oak Flats, Illawarra, NSW
Well the fan unit is in and its doing it's job perfectly. The readings are close enough to my AWS to attribute any small variations to local factors ie 1. The AWS is in a large open space at the Airport and I am in a Gully.

The temps are alot more stable and are all reading the same as my other 2 manual thermometers.

Now the laughable part, all it took was the $13 solar education kit from Dick Smith, a hole was drilled in the base and the wiring run through a hole next to it. None of these holes are open so heat entering from below is not a worry.

The wiring runs up the bolts and out through the side of the lid which in turn is hooked up to the solar panel. Now whenever the sun shines the fan starts and sufficient air circulation is not a problem.

I'm one happy person to say the least laugh

Stephen
_________________________
Stephen Jarrett

my home AWS site: http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=INSWOAKF3


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#126429 - 15/08/2005 13:56 Re: Stevenson Screen Design
Rime Offline
Weatherzone Addict

Registered: 07/06/2001
Posts: 1444
Loc: Perth,WA
It maybe a point that there could be problems with having holes at the bottom of the screen, however, the BoM standard screens all have this. I have always placed holes on the bottom of my screens as it seems to be the standard in Australia. The argument is that without the holes the screen can become a water trap and therefore you may experience a cooling due to evaporation. Just a note though, the BoM screens all have holes drilled in the bottom, but they also have them drilled in the ceiling.

Some years ago I built a wooden screen and left out the holes. Sure enough it it did become a water trap after heavy rain.

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#126430 - 15/08/2005 14:36 Re: Stevenson Screen Design
Rocky Raccoon Offline
Weatherzone Addict

Registered: 07/02/2003
Posts: 2062
Loc: Redan, Victoria
I haven't got holes on the bottom of mine but the I do have a triple ceiling, the bottom ceiling is louvred and part of the screen box and air can flow through freely between upper ceilings and no sunlight can possibly get in.
Ventilation is excellent.
I think one problem with holes on the floors is not to have huge ones or birds may enter it easily and damage your instuments.
I have seen them trying to break into my letterbox.
_________________________
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Rainfall total for 2012: 654.7mm
Rainfall total for 2013: 493.4mm
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#126431 - 15/08/2005 16:10 Re: Stevenson Screen Design
Rime Offline
Weatherzone Addict

Registered: 07/06/2001
Posts: 1444
Loc: Perth,WA
Quote:
Originally posted by Rocky Raccoon:

I think one problem with holes on the floors is not to have huge ones or birds may enter it easily and damage your instuments.
I have seen them trying to break into my letterbox.
LOL. I have not had any problems with birds as I think the holes are still a little too small. But from time to time I have problems with frogs getting into the screen. When it happens I have to go through the screen and collect the frogs. I put them into a bucket and then dump them down in the nearby creek. I have found that little ones are okay, but when the big tree frogs get in, they make a mess.

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#126432 - 06/09/2005 11:51 Re: Stevenson Screen Design
Surly Bond Offline
Weatherzone Addict

Registered: 23/08/2003
Posts: 2193
Loc: Manilla, near Tamworth NSW
Giant Mixing Bowl Thermometer Screen (Gimiboths)

In April I made a thermometer screen based on the Logan Village flower-pot saucer screen, described here:
http://weather.overflow.net.au/station/stevenson.html

I described my version in the "Remote Sensor" thread on 26-04-2005, and expressed my concerns about the way it worked in this thread (Stevenson Screen Design) on 23-05-2005.

I was worried that the maximum and minimum daily readings were too extreme. The observed temperature range was higher than usual in this region of high temperature ranges, and the values were more extreme than both my own back porch readings and the Tamworth METAR readings. Although the METAR is 38 km SSE, the climate controls are similar.

I built a new screen made of Giant Mixing Bowls. The defects I fixed were too little air-flow, too much mass, and too much trapping of water.

I will describe the new Gimiboths here. Separately I will compare data from the two screens and the METAR, and discuss factors causing the differences.

Gimiboths description

The Gimiboths looks like the Logan Village screen and is much the same size. However each element is a simple inverted bowl. The eight bowls comprise a roof, a ceiling, five wall units, and a floor. The bowls are mounted above a plywood base-board by three threaded rods.
The clear space for the thermometer inside is 50 mm radius, but the radius of the contained airspace is 120 mm. The height from floor to ceiling being 208 mm, the volume is 9.4 litres.
The bowls are separated by spacers 32.5 mm long, making the gap between bowls at the edge 7 mm. This is over three times the gap in my earlier screen. The smooth curve must also allow easier airflow. There is no line of sight to the thermometer from outside.
This screen weighs only 1.4 kg compared to the earlier screen's 4 kg. The thickness of the material is 1.8 mm rather than 2.7 mm, and I used no filler.

Materials

8x Poly Industries K032 Giant Mixing Bowls
3x 5/16" threaded rods
1 m 16 mm electrical conduit
1 m 10 mm or 12 mm PVC tube
3x 5/16 dome nuts
6x 5/16 nuts
3x 5/16 flat washers
plywood for base
(plus mounting bracket materials)
250 mm aluminium cooking foil
1 litre Prepsol or Prepwash
Sandpaper
Aerosol 400 g cans Motospray paint:
1x "Fast Drying Flexi Prime" ("...suitable for use on ...polypropylene...Adhesion Promoter")
2x "Acrylic Gloss White" ("Automotive Acrylic Top Coat Lacquer")
1 m binding wire

Method

The bowls must be cut and drilled. I am sure there are modern tools using heat or lasers, but I used ordinary drills, a hole saw,a coping saw and sanding disc. The polypropylene misbehaves when worked: it melts and chips.
The bowls start off 270 mm in diameter and 110 mm deep. Trim the four little feet off with a knife. Put each bowl upside down on a table and mark a cutting line about 30 mm up, clear of the pouring spouts. Drill a hole near the line to enter the coping saw blade, and saw away the top of each bowl. If necessary, sand the edge even.
On the base of the bowl, find the centre mark where the bowl was spun. Use a compass to draw a circle 62 mm in radius and mark for three holes on the circle. They should be 108 mm apart. Drill small pilot holes at these points and (for five bowls only) at the centre point.
Using the centre point holes to guide the hole saw, cut 100 mm diameter holes in five of the bowls. Enlarge the other holes to accept the mounting rods.
The bowls are not made completely convex but the ones for the roof, ceiling and floor need to be, so as to shed water. I "improved" them by heating over an electric element and rubbing with a teflon spoon.

Mark the conduit at intervals of 33.3 mm and saw off 24 pieces. Allowing for the saw cuts, this will space the bowls 32.5 mm apart. Cut 24 pieces of PVC tube to the same length, as spacers between the threaded rods and the conduit.
I made a triangular base board of heavy plywood drilled to take the threaded rods. I mounted the base board on a metal bracket, ensuring that the plywood formed an insulating barrier.
Assembly on the threaded rods, in my case, was: two nuts, base board, conduit, floor, conduit, wall, conduit,...wall, conduit, ceiling, conduit, roof, dome nut.

Painting

The screen must be painted for three reasons: 1. polypropylene is unstable in the sun; 2. the bowls let too much light through; 3. the screen must be white to reject as much radiation as possible.
Polypropylene is cheap but very hard to paint. The products mentioned above will do the job. (Getting an auto repairer to do it is another option.)
Dismantle the screen and arrange a set of hooks to hang the parts on for painting.
For each part, wash, sand, prepsol, sand, and prepsol, handling with gloves only. Paint with many light coats. One can of lacquer is not enough to make an opaque cover.
Finally, polish with car polish.

Final assembly

During assembly, I made a support for an Oregon and an Ewig wireless thermometer from binding wire.
I also glued a piece of aluminium cooking foil under the roof, shiny side down.

Supply of bowls

The K032 Giant Mixing Bowls are made by Poly Industries in Sydney and marketed through IGA stores. Only some stores have them, usually in small numbers, and prices vary widely: anything over $3.00 is a rip-off. I can offer a spare set of ten at cost.
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#126433 - 13/09/2005 23:52 Re: Stevenson Screen Design
Paul (Stargazer) Offline
Weatherzone Addict

Registered: 16/05/2003
Posts: 1262
Loc: Salisbury North, SA
Quote:
Originally posted by Surly Bond:
In April I made a thermometer screen based on the Logan Village flower-pot saucer screen, described here:
http://weather.overflow.net.au/station/stevenson.html
I also used the same plans to build my screen ... I posted details in this thread on Weatherzone.
_________________________
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(Stargazer)
Weather Station
Wunderground

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#126434 - 08/06/2007 07:59 Re: Stevenson Screen Design
Citizen Bob Offline
Junior Member

Registered: 07/06/2007
Posts: 2
Loc: Houston
How about a temperature shield based on an aluminum foil externally wrapped PVC pipe lined internally with longitudinally split styrofoam pipe insulation and aspirated by a fan?

A 4" ID PVC pipe with a 2" ID insulation would accommodate small wireless sensors like the La Crosse TX7U.

Regarding the effectiveness of PVC pipes as screens,

http://www.geo.uni.lodz.pl/~icuc5/text/P_6_5.pdf

where the authors compare various unaspirated screens with the Stevenson Screen.

With forced air circulation and nearly perfect thermal and radiation shielding, this pipe system should out-perform the simple pipes in that article.

You can use a computer fan and adjustable AC to DC converter brick with a built-in voltage adjustment (WalMart has them) to set the fan speed. Use wire mesh fan filters on both ends to keep critters out. The rest of the design is up to your imagination.

For remote applications a battery-backed solar power supply would keep the fan running 24x7 which is essential for a cramped design like this.

For larger applications I see no reason in principle why this design can't be scaled up to the largest available components which are at least as large on the inside as Gill shields made out of dishes.

Comments, please.

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#126435 - 08/06/2007 13:24 Re: Stevenson Screen Design
Surly Bond Offline
Weatherzone Addict

Registered: 23/08/2003
Posts: 2193
Loc: Manilla, near Tamworth NSW
That paper you link by Erell, Leal and Maldonado is very interesting, Citizen Bob:
http://www.geo.uni.lodz.pl/~icuc5/text/P_6_5.pdf

By trimming the url I find that the paper was presented at the Fifth International Conference on Urban Climate at Lodz (Trivia: it's pronounced "Wooj"!), Poland, Sept 2003:
http://www.geo.uni.lodz.pl/~icuc5/

A PhD student recommended the Israeli horizontal tube screens to me years ago, but I decided to go for a Gill type (my "Gimiboths"). My screen must be very similar to the authors' Type 13. For a high-radiation site (as I also have), Type 13 turns out to be the best approach to a Stevenson Screen of the 16 screens that the authors tested. Like all the others it over-reads maxima and under-reads minima, if the Stevenson screen is taken to be correct: maxima were 1.76 deg over; minima were 1.40 under.

I seem to be getting maxima about a degree over, and minima about a degree under, readings in the Stevenson Screen at Tamworth Airport METAR. That could be due to other factors: it is nearly 40 km away, and a much windier site.

I still think that ventilated thermometer screens, though they give a better estimate of true air temperature, do not relate well to published temperatures recorded at standard met stations.
_________________________
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#126436 - 10/06/2007 03:00 Re: Stevenson Screen Design
Citizen Bob Offline
Junior Member

Registered: 07/06/2007
Posts: 2
Loc: Houston
Quote:
Originally posted by Surly Bond:
I seem to be getting maxima about a degree over, and minima about a degree under, readings in the Stevenson Screen at Tamworth Airport METAR. That could be due to other factors: it is nearly 40 km away, and a much windier site.
I still think that ventilated thermometer screens, though they give a better estimate of true air temperature, do not relate well to published temperatures recorded at standard met stations.
I do not believe you can use temperatures measured outside your microclimatic environment, which is the one next to your thermometer. I have 4 amateur stations nearby my house and they are all a couple degrees different from one another.

I just mounted my LC TX7U on the covered patio away from the sun. I do not have any shield - it's just hanging naked. It is currently recording as much as 5 degrees F higher than the average of those 4 stations, and about 3F higher than the nearest station which is about 1/2 mile away. Clearly the patio is heating up from the sun.

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#126437 - 10/06/2007 10:12 Re: Stevenson Screen Design
Surly Bond Offline
Weatherzone Addict

Registered: 23/08/2003
Posts: 2193
Loc: Manilla, near Tamworth NSW
Hi Bob

Each person has to think through what they want to get data on.

I, for example, want to compare the climate inside my house with the climate outside it. My figures on the outdoor climate must be plausible to people who know where I live. However, the fact that there are a lot of trees around the house, and I am on a narrow ridge about 5 metres high is part of my environment. Figures I get will reflect these facts,and will differ from those at a typical met site.

I think a lot of people, when they first unpack their met station thermometers, will hang them in some shady area (like your covered patio). This is unlikely to be the best location once you have a proper screen. I think the thermometer should be as far away from other objects as you can get it. This is generally not very far: the size of the block of land, and the cost of wires and trenching, or reliability of a radio link limit the options.

Your horizontal pipe screen is very like the ones used by my student friend. He used 4 inch PVC pipe lined with plumber's insulation surrounding a smaller PVC pipe. I found I would have to buy quite a lot of materials and throw a lot away. You need access to a builders scrap heap.

May I suggest the mesh screens should not be metallic? A metal mesh in sunshine on the "air-in" end of the screen would heat the thermometer rather efficiently I think.

Cheers.
_________________________
Data are cheap; information is expensive!

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#126438 - 10/06/2007 11:47 Re: Stevenson Screen Design
ROM Offline
Meteorological Motor Mouth

Registered: 29/01/2007
Posts: 6628
Interesting reading this thread although I am not into accurate weather recording equipment but I think SB would agree with me here.
There are significant short term temperature peaks lasting only a couple of minutes when thermals move through an area.
Recording equipment will register these peaks as higher temperatures even though they may only cover a small area of say a few hundred metres across and are a couple or more degrees above the overall temperature at the time.
Anybody who has ridden in the back of a ute on a still summer's evening will have felt the quite big changes in air temperature every few hundred metres as the ute drives through these warmer and cooler patches of air.
A good strong breeze or wind would have the effect of better mixing of the airmass but even here, strong thermals, ie; significant increases in the temperature of the local air mass, are still very obvious.
Thermal air mass temperatures and the temperature changes that occur when they drift through would be one of a number of reasons for large differences in temperatures between adjoining sites

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#126439 - 10/06/2007 16:01 Re: Stevenson Screen Design
Surly Bond Offline
Weatherzone Addict

Registered: 23/08/2003
Posts: 2193
Loc: Manilla, near Tamworth NSW
This almost calls for its own thread,ROM.

I don't know much, but I can make a few points.

Most thermometers I know have rather long time constants, more than a minute, I think. They may not pick up some of these phenomena.

The big lumps of warm and cool air that you feel in the evening probably do not occur in the heat of the day, when we record the maximum.
When convection is active, air from a hot spot is rapidly mixed with air that is cooler. When the evening inversion has formed, the air a few metres above the ground is degrees warmer than air on the ground. Then air from a hot spot has nowhere to go; it just sits there, waiting for you to drive through.
_________________________
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#126440 - 10/06/2007 17:26 Re: Stevenson Screen Design
ROM Offline
Meteorological Motor Mouth

Registered: 29/01/2007
Posts: 6628
I wouldn't argue with your assessment and I think you are probably right SB.
I thought the above might possibly be an explanation for different temps between closely situated recorders.

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#126441 - 13/06/2007 14:09 Re: Stevenson Screen Design
Carl Smith Offline
Member

Registered: 21/12/2001
Posts: 1042
Loc: Gold Coast
Some of those interested in Stevenson screen design may be interested in an experiment by Anthony Watts underway in the US to determine the thermal qualities of paints used on Stevenson screens.

To quote Anthony:

Quote:
Since they mentioned paint, I thought Iíd share this experiment with everyone. You may recall that Iím conducting an experiment measuring the differences in temperature imparted by different coatins on the Stevenson Screens. Early screens were specíd to have whitewash. I made up a batch of whitewash based on a known historical formula and setup a test rig:


Here is one dayís worth of plotted data related to whitewash -vs- latex -vs-
bare wood I have three weeks of this logged data and they all look pretty
much the same. Russ Steele was kind enough to plot the data for me for a day, May 21st.


These are temps recorded in a 1/4" borehole in the center of the wood slat
along with air temperature from an aspirated stacked plate IR shield. The
datalogger I used is NIST calibrated, and the probes are NIST calibrated.
Error is 0.1 degree F or less, and they are quite linear.

I have made it available on
http://www.surfacestations.org/downloads/paint_test1.zip These are all comma
demilited text files, suitable for import into R or Excel. This is three weeks worth of data.

The columnar format is:
date, time, Air Temperature, Bare Wood Temp(control), Latex Paint Temp, Whitewash Temp all in degrees F

I have my three new Stevenson Screens in, so Iím going to switch now from
wood borehole tests of the painted slats to actual air temperature tests in
three standard Stevenson screens. I expect the temperature differences to be
smaller, but still measureable.

I have aquired the exact formula for lime based whitewash and the exact
materials from a chemist at the National Lime Company. They are watching
closely too becuase whitewash appears to be a superior coating for buildings
to create energy savings for air conditioning.

Will advise when the new experiment is up and running, it will be live online.
Positioning a screen is important too - Anthony has also got the ball rolling a photographic survey of all the screens used in the US climate network:

Some examples of what he has found so far:

A screen above a wood chips over weedmat surface that was hot to walk on surrounded by an asphalt car park.


A site listed as 'rural' and 'light=0' that is in fact mounted on a metal pylon next to a caretakers shack and car park:

Here is where the site above was located before someone had the bright idea to move it:


Another interesting position:

- I wonder how often they burn off the trash there!

Here is another 'rural' one in an irrigated ploughed field used for growing crops:

- note the steps needed to read the instruments because the screen has been elevated to keep it above crop height!

And perhaps my favorite:

- one wonders what effects jet exhaust has on the daily max!

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#126442 - 13/06/2007 14:25 Re: Stevenson Screen Design
Rob G Offline
Weather Freak

Registered: 31/07/2003
Posts: 792
Loc: Porters Retreat NSW
An interesting experiment Carl. However, the temperature of the outside surface of a screen must be just one parameter that determines inside temperature. For example, I read that screens painted black inside read lower than screens painted white inside. On the other hand, under very still conditions, the inside temperature would approach the screen surface temperature, while in windy conditions, or forced ventilation the surface temperature may only have a minor effect.

I wonder how a highly polished reflective surface would compare to white paint?

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#126443 - 26/06/2007 13:50 Re: Stevenson Screen Design
Surly Bond Offline
Weatherzone Addict

Registered: 23/08/2003
Posts: 2193
Loc: Manilla, near Tamworth NSW
Related threads

By chance, this thread has become the survivor of the various threads about thermometer screens (or shields, or shelters). There is a 3-page thread, "Stevenson Screen" now on p.6 of this index:

http://forum.weatherzone.com.au/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=006158

The thread "Remote Sensor" (2 pages), now on p.35, is also mainly about screens:

http://forum.weatherzone.com.au/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=005694
_________________________
Data are cheap; information is expensive!

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#126444 - 26/06/2007 20:45 Re: Stevenson Screen Design
Paul (Stargazer) Offline
Weatherzone Addict

Registered: 16/05/2003
Posts: 1262
Loc: Salisbury North, SA
_________________________
Regs, Paul.
(Stargazer)
Weather Station
Wunderground

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#126445 - 04/08/2007 20:08 Re: Stevenson Screen Design
Surly Bond Offline
Weatherzone Addict

Registered: 23/08/2003
Posts: 2193
Loc: Manilla, near Tamworth NSW
Calibration of my home-made Gill-type thermometer screen.

I now have 27 months of daily maximum and minimum readings from my "Gimiboths" thermometer screen to compare with readings from the Tamworth METAR.

As I have mentioned, the Tamworth site is 38 km to the SSE, and is a "blasted heath" (short grass, actually), while my site is on a low narrow ridge with trees and houses.

I made a scatter diagram including plots of maxima and of minima separately. To the eye, both maxima and minima plot pretty close to a straight line through (zero:zero) with a slope of one to one. Of the maxima, one of my readings is about 5 degrees off the line, but nearly all are within 2 degrees from it. Of the minima, two readings are about 7 degrees off the line, but nearly all are within 4 degrees from it.

The regression line relating my maxima to Tamworth maxima is:
y=1.015x+0.67
At the highest Tamworth maximum, 41 degrees, my reading would then be 42.3 degrees: up by only 1.3.
At the lowest Tamworth maximum, 7 degrees, my reading would then be 7.8 degrees: up by only 0.8.
For this regression, R squared is 0.98.

The regression line relating my minima to Tamworth minima is:
y=1.006x-0.37
At the highest Tamworth minimum, 28 degrees, my reading would then be 27.8 degrees: down by only 0.2.
At the lowest Tamworth minimum, minus 5 degrees, my reading would then be minus 5.4 degrees: down by only 0.4.
For this regression, R squared is 0.94.

The discrepancies are rather small. In the daytime my screen reads about a degree over Tamworth. That could be because the trees and houses prevent cooling by the wind. At night my readings are, on the average, practically the same as those at Tamworth. However, there is a lot of scatter, shown by the lower value of R squared.
I believe this is to be expected. The flow of air at night will differ greatly from place to place and time to time.

I am encouraged to think that my screen is a success. I am confident that my screen readings represent the temperature (and humidity) in the environment of my house.
_________________________
Data are cheap; information is expensive!

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#876951 - 13/08/2010 12:32 Re: Stevenson Screen Design [Re: Surly Bond]
Surly Bond Offline
Weatherzone Addict

Registered: 23/08/2003
Posts: 2193
Loc: Manilla, near Tamworth NSW
There is now a photo of the Gimiboths home-made Gill-type thermometer screen in the "Indoor Climate" thread here.
_________________________
Data are cheap; information is expensive!

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#876969 - 13/08/2010 14:53 Re: Stevenson Screen Design [Re: Surly Bond]
SBT Offline
Meteorological Motor Mouth

Registered: 07/02/2007
Posts: 14286
Loc: Townsville Dry Tropics

UK Steve - This goes to explain a bit about climate extremes in Australia because of the size of the country.

Northern Australia subject to Monsoons and Cyclones seasons, Wide climatic range from wet to dry tropics - high humdity and tempratures. Houses built to dissapate heat rapidly. Insulated roofs and walls with airconditioning - Cooling only or raised off the ground to catch the cooler breezes. Entire side walls made up of louvers floor to ceiling. High roofs trap heat well above the occupants and various forms of venting for teh roof cavity.

Middle Australia Mild almost Mediterranean Climate. Insulated roofs and walls. Airconditioning is reverse cycle (Heating and cooling). Quite a bit of central heating.

Southern regions Bloody cold. Central heating - very little if any airconditioning. Tasmania thinks a day over 30C is a heat wave and requires moaning about at great length.

In WA it is common to have temprature maximums of 44C in the Pilbra and down the southern coast at Esperance 14C. A temprature diffrence of 30C in the one state.

Size comparissons:
Australia / Europe
Australia / UK
Australia / USA
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