Cloudless skies

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RWood
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Cloudless skies

Unread post by RWood »

Many parts of NZ were sunny in November, particularly Wellington, but Perth and Adelaide would have dried out very fast - both very dry with record high sunshine. Perth had the remarkable total of 368 hrs for the month, and less than 10mm rainfall, with its dullest day getting 9.5hrs. :shock:
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David
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Re: Cloudless skies

Unread post by David »

RWood wrote:Perth had the remarkable total of 368 hrs for the month
What was the maximum possible hours Perth could have got? 368hrs seems very close to this. If it were incredibly cloudless for a whole month I assume a total of 400hrs could occur?
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Re: Cloudless skies

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If the observing site had no horizon obstruction, they could log about 415. Realistically the value is probably over 90% of the possible. The NZ record is 327 at Blenheim in Nov 1997 (about 79% of possible recordable). Interestingly, if you discard the rather doubtful calculations for the Queenstown site the record for any month in NZ is 84-85% of possible at Blenheim in July 1952 (231 hrs), followed by about 83% at Nelson in the same month (227 hrs).
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David
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Re: Cloudless skies

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Do you have the record for Auckland? (max sunshine hours in any calender month). I know there was 304hrs in January 1974 at the Auckland City site but I'm not sure if this is the record. (there was only 8.7mm during this month, at the Owairaka site) I'd think it would be lower than many other site's records around NZ, due to our lack of clear weather.

Auckland does seem to make an effort though in January, in the data I've got (which is composed of several observing sites to fill in all the missing gaps) the average for Auckland in January is 229hrs, with 10 January's 1962-2006 inclusive (1998 and most of 1962 missing) totaling over 250hours, and 5 months over 275hrs. The minimum for January over these 45 years is 160hrs at the Auckland City site in 1989.

June has twice as less sun with an average of 114 hours and 3 June's under 75hrs, the lowest being at the Whenuapai Aero site in 2002 with 62hours. There is not one incidence where over 200hrs has been recorded in any month from April to September inclusive in the whole data set.

For standard deviations, Dec-Feb have the highest, between 33 and 36hrs. August and September have the lowest standard deviation, both 20hrs. The standard deviation for annual totals is 125hrs. The skewness is interesting, for Jan, Feb, Aug, Oct and Dec the totals are skewed above the mean, with the skewness value for October being 0.55. For the other months the totals are skewed below the average, especially September (-0.64). The annual totals are skewed below the mean also (-0.18).

The data set has a lowest annual total of 1724hrs at Auckland City in 1988 and a highest annual total of 2313hrs (which I think is rather high for Auckland) at the same site in 1970. There have been 6 sunny years over 2200hrs and 2 cloudier years under 1800hrs. How does this compare with other centers of NZ?

Also see the uploaded attachment graph, why has sun in Auckland decreased since the earlier decades of recording?
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RWood
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Re: Cloudless skies

Unread post by RWood »

David wrote:Do you have the record for Auckland? (max sunshine hours in any calender month). I know there was 304hrs in January 1974 at the Auckland City site but I'm not sure if this is the record. (there was only 8.7mm during this month, at the Owairaka site) I'd think it would be lower than many other site's records around NZ, due to our lack of clear weather.

Auckland does seem to make an effort though in January, in the data I've got (which is composed of several observing sites to fill in all the missing gaps) the average for Auckland in January is 229hrs, with 10 January's 1962-2006 inclusive (1998 and most of 1962 missing) totaling over 250hours, and 5 months over 275hrs. The minimum for January over these 45 years is 160hrs at the Auckland City site in 1989.

June has twice as less sun with an average of 114 hours and 3 June's under 75hrs, the lowest being at the Whenuapai Aero site in 2002 with 62hours. There is not one incidence where over 200hrs has been recorded in any month from April to September inclusive in the whole data set.

For standard deviations, Dec-Feb have the highest, between 33 and 36hrs. August and September have the lowest standard deviation, both 20hrs. The standard deviation for annual totals is 125hrs. The skewness is interesting, for Jan, Feb, Aug, Oct and Dec the totals are skewed above the mean, with the skewness value for October being 0.55. For the other months the totals are skewed below the average, especially September (-0.64). The annual totals are skewed below the mean also (-0.18).

The data set has a lowest annual total of 1724hrs at Auckland City in 1988 and a highest annual total of 2313hrs (which I think is rather high for Auckland) at the same site in 1970. There have been 6 sunny years over 2200hrs and 2 cloudier years under 1800hrs. How does this compare with other centers of NZ?

Also see the uploaded attachment graph, why has sun in Auckland decreased since the earlier decades of recording?
You've struck the "curse" of there having been a variety of sites over the decades, with differing levels of possible sunshine, so it's difficult to assert that the values have dropped. To answer your first question before going into detail, the Mechanics Bay site recorded 316 hrs in January 1957, which was the North Island's sunniest January in reliable record.

Auckland had sun recording at Albert Park from 1909-1955. The earlier part of this period (pre-1930 roughly) was one when there were few NZ stations recording sunshine and some of the records are probably pretty dubious. The Auckland ones from that time look a little low to me. From 1930 to about 1946 the AP readings were reasonably high, especially from 1937 to 1945. It is acknowledged that the growth of trees increasingly compromised the data from about then, and during a period of overlap (1950-1955) with Mechanics Bay, the latter site was averaging about 150 hrs per year more than Albert Park. If you want to collect data from AP, ignore anything after about 1947.

Mechanics Bay was used from Jan 1950 to August 1962, and estimates by extrapolation and intercomparison with other sites gave it a 1941-1970 average of over 2100 hours. Then the city readings were switched to the NAC (as it was then) building in the city in September 1962, and these looked very similar to Mechanics Bay values for quite a long period of years. However, there were more gremlins, because intercomparison with Auckland Aero (and another site at Mangere as well) showed a sudden relative decline from 1986 onwards, till closure in late 1990. It was a case of the C-S recorder being much more shaded by changes in surrounding buildings (or it may have been vegetation, don't have the details). So if you're using them, discard records after 1985!

For sites not in the main city - Whenuapai had records from 1954-1969, averaging around 2055 hrs for that period and running a little behind the city readings. Meanwhile Mangere ran records from 1962 to 1998, and Auckland Airport from 1969 to 1994. Mangere generally was a little lower than the city, and the Airport slightly higher. After Mangere stopped, Whenuapi was resumed from 1998 to 2004, and I gather it had "exposure" problems later on as well. Meanwhile, Mangere has resumed as an EWS since mid-2002 or thereabouts.

You can gather from all of this that it's rather hard to draw many conclusions about trends!

There are few places where the same site has been used throughout, though in Wellington we have Kelburn records from 1928. Previous rainfall sites in the city numbered several, and there were 2 previous sun sites during 1906-1927.

My guess is that with a properly exposed site (ie minimal obstructions), Auckland would average close to 2100 hrs/annum these days, a little less than Christchurch (whose latest 30-year average is about 2130), and similar to Wellington - but W'gton's 30-year average has been increasing for quite a few years now - it was 2089 to end-2006, and may be close to 2100 for the 30 years ending 2007.



That's probably enough for a first go....
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Re: Cloudless skies

Unread post by spwill »

It would be interesting to know the impact of the seabreeze convergence zone on sunshine across the Ak area. The convergence line varies location from day to day ( and is not there every day) but my guess is Owairaka would get more than it's fair share of cloud cover from it.
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Re: Cloudless skies

Unread post by RWood »

Probably so, but actually the only sunshine site currently being used for Auckland is at Mangere, so that's where the numbers come from.
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David
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Re: Cloudless skies

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Convergence zone has been over the east a few times in the last week, however the atmosphere must be too stable for anything to happen, as only a trace of rain has come of it. The convergence line stretched NW to SE over here this afternoon, the darkest area moved from NW to SE, and then came back over in the opposite direction :o but there was only a brief light shower. About sunshine variation across Auckland - quite often during windy southwesterly episodes with a few showers about, the first hour or so of the day can be sunny in the east whereas the west can be cloudy. Not quite sure why but it happens enough to be noticed.
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Re: Cloudless skies

Unread post by spwill »

but actually the only sunshine site currently being used for Auckland is at Mangere, so that's where the numbers come from.
Mangere and Whenuapai are on a similar line to Owairaka. All generally through the center of the Isthmus.
the first hour or so of the day can be sunny in the east whereas the west can be cloudy. Not quite sure why but it happens enough to be noticed.
The warming of the land helps bring the surface SouthWester across and helps generate cloud.
RWood
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Re: Cloudless skies

Unread post by RWood »

Then is not also the airport on a similar line? While measurements were being taken there it slightly outscored the city site and was probably about comparable to the earlier Mechanics Bay one for average values. And of course even Michael must concede that there are flows other than SW... 8)

To settle the matter I think you would need to have about 3 sites with good open exposure all operating, say one in the zones of southwest vulnerability, another in the E/NE reaches and a 3rd one somewhere else.
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