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We average around 5 days per summer season with 0 sunshine hours.
Additionally, we average around 16 days in summer with < 3 hours (what I consider a "cloudy" day). This is around 18% of the total days in summer.
Summertime here is generally the cloudiest time of the year with dull patches not uncommon (our long term average for 0 sunshine hour days across the whole year is 15 days).
It's rare here, we can get a couple days a summer that are have less than 1 hour sunshine, but a completely sunless day in summer might only occur once every 2-3 years on average and would generally be associated with an ex-Tropical Cyclone system.
Completely sunless days are quite rare here at anytime of year actually, even in winter there is usually some sunny spells between showers and this year there has only been one fully overcast day recorded on the 11th of April.
^^^I find it had to get a clear picture for NZ, as statistics for this aren't good.
The only thing I can say with certainty is that early summer is the cloudiest time of the year, and late summer the sunniest.
Summer does seem to have quite a few totally cloudy days, but they're almost never associated with rain. Periods of totally clear skies (5-10 days), don't seem to be confined to seasons, but I think would still be just a little more common in autumn and winter.
In your region the seasonal variation in sunshine % is small anyway. For all records at Nelson the range is only from 55% in Dec to 61.5% in Feb. The last 3 months of the year are slightly lower than late winter or spring proper, and Jan-April the sunniest.
Sydney sees far more overcast days in summer than spring which is a blessing indeed with the former recording about three to four days of cloudy skies per week compared to the latter which sees about one and if we're lucky two.
Overcast days are exceptionally rare in June and are much more common in July and August. I haven't seen an overcast day in June since 2008, and I've at least seen three overcast days in every July and August since then.
To be fair, totally overcast days are quite rare during summer here. In fact the last time I think it happened was in Summer 2012 where the whole summer average was around 380-680 hours across the whole south coast of England (Cornwall to Kent), compare that to the average of around 600-760 hours across the whole south coast. This year so far, I think the most overcast (and coolest day) was in early July when we had an afternoon of totally overcast skies, but the morning was partly cloudy. However, some might prove me wrong and remember a totally overcast day in summer 2013 and so far this year.
On average we probably get, maybe, 1-5 full overcast day a summer. Usually if it rains, it will brighten up later, or visa versa... But that's the British weather for you!
There is maybe one or two completely overcast days where the sun doesn't come out at all.
Most cloudy summer days have the sun come out in the late afternoon/evening though.
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