Measuring sunshine as a percentage

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Measuring sunshine as a percentage

Postby greg_gruner » 2012 Jul 10, 10:36

The fact that June has been one of the dullest on record brought to my attention the usefulness of measuring sunshine as a percentage of possible, rather than just a total of hours. This is done as a matter of course in the USA, and I think it is a useful additional measurement.
For example, comparing sunshine in June and December would be far easier: roughly, 50 hours in December is the same percentage as 100 hours in June. It would also make it easier to compare sunshine in different locations, for example, Sydney in August compared with London in May.
I am not saying that sunshine hours should not be recorded - they represent the real amount of sunshine received at the site. However, the sunshine figure is a combination of astronomy (day length) and meteorology. I think a percentage measurement (which is easy to calculate) would be a useful additional measure.
Greg Gruner
Farnborough, Hampshire
greg_gruner
 
Posts: 224
Joined: 2012 Feb 04, 17:28

Re: Measuring sunshine as a percentage

Postby Stephen Burt » 2012 Jul 10, 17:10

As a matter of interest, I've covered this subject in some detail in my new book, The Weather Observer's Handbook, which is published tomorrow by Cambridge University Press. Here's part of that topic
(p 245):

A very useful calculator providing precise sunrise and sunset times and hours of daylight for any location (enter latitude, longitude and time zone, and year) can be downloaded from http://www.esrl.noaa.gov/gmd/grad/solca ... tails.html – choose ‘NOAA_Solar_Calculations_Year’ in the spreadsheet format preferred.

Because sunshine recorders are insensitive to low solar angles, sunshine will generally not be registered until the Sun rises to about 3° above the horizon after dawn, or when it sets below 3° near sunset. The length of time taken for the Sun to reach 3° after dawn, or to sink from 3° to the horizon at sunset, varies with latitude and season; on the equator it is as little as 13 minutes, but at 60°N it varies between 44 minutes near the summer solstice and 53 minutes at the winter solstice. (At higher latitudes the Sun does not reach 3° elevation in midwinter.) In mid-latitudes, 20–30 minutes is typical. A 20 minute cutoff after sunrise and before sunset equates to roughly 7 per cent of the maximum possible daily duration averaged over the year. It is therefore unlikely that even the sunniest days will exceed about 95 per cent of the ‘maximum possible duration’ – the limits of current sensor sensitivity dictate that 100 per cent cannot quite be attained.


Many countries - including the US - are abandoning 'sunshine' measurements in favour of the more scientifically useful 'total global solar radiation' measure, but I think we'd all concur that '11 hours of sunshine' means more to the man in the street than '25.4 MJ/m2 total global solar radiation' ! Ideally of course, both should be recorded, site permitting.

More details on www.measuringtheweather.com

SB
-----
Stratfield Mortimer, Berkshire - central southern England
51.4°N, 1.0°W, 60 m AMSL, station grade A - AAAA47R
Records commenced here 1987 - local records available back to 1862
The Weather Observer's Handbook: www.measuringtheweather.com
Stephen Burt
 
Posts: 247
Joined: 2011 Dec 02, 19:36
Location: Stratfield Mortimer, Berkshire; a well-exposed rural site, 10 km SW of Reading

Re: Measuring sunshine as a percentage

Postby greg_gruner » 2012 Jul 11, 15:12

Stephen, many thanks for that. Yes, it is a complex business! I notice that some weather sites linked to an AWS are recording sunshine based on a threshold amount of radiation, I think something like 200 or 250 w/sq meter. But that is a crude measure and will not distinguish between a high sun shining through a layer of cirrus and a low sun with a clear sky.
I will certainly buy your book and I'm looking forward to reading it.
Greg Gruner
Farnborough, Hampshire
greg_gruner
 
Posts: 224
Joined: 2012 Feb 04, 17:28

Re: Measuring sunshine as a percentage

Postby Stephen Burt » 2012 Jul 11, 17:32

I notice that some weather sites linked to an AWS are recording sunshine based on a threshold amount of radiation, I think something like 200 or 250 w/sq meter. But that is a crude measure and will not distinguish between a high sun shining through a layer of cirrus and a low sun with a clear sky.


Ah, yes - that's an even more vexed question! At the risk of over-plugging here, the topic is considered in some detail in my book. Frankly - the words 'bargepole' and 'don't touch' are probably sufficient of a clue to my view on the matter, and I explain my reasoning. I also covered this in slightly less detail in a note in COL a couple of months back.

SB
-----
Stratfield Mortimer, Berkshire - central southern England
51.4°N, 1.0°W, 60 m AMSL, station grade A - AAAA47R
Records commenced here 1987 - local records available back to 1862
The Weather Observer's Handbook: www.measuringtheweather.com
Stephen Burt
 
Posts: 247
Joined: 2011 Dec 02, 19:36
Location: Stratfield Mortimer, Berkshire; a well-exposed rural site, 10 km SW of Reading


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