Capturing sunshine

Posted 3 July, 2009 13:04 by Karla Baker

Contrary to popular belief, the gorgeous, sweltering, sunshiny weather that we have been hearing about ad nauseam recently was not universally enjoyed. Images of beaches ripe to overflowing and the sad laments of city commuters were the stuff of dreams for those of us suffering torrential rain, blankets of cloud and “nothing to write home about” temperatures. But bitterness aside, it is of course not to be expected that all regions will enjoy the same weather at the same time, this very fact being the source of employment for the bunch of ne’er-do-wells that call themselves weather forecasters. The mapping of rainfall and temperature trends, or to get fancy about it, maps which employ isohyets and isotherms, is the stock and trade of most weather maps that you will see on the news but, as recent events have brought to mind, there is a lesser known type of map which attempts to show trends in sunshine.

Bartholomew printed 2175 copies of this map on the 27 July, 1893. It is called an “Approximate Sketch Map of Mean Annual Sunshine of the British Isles” and was produced by H. N. Dickson (1866-1922) and J. G. Bartholomew (1860-1920) for the Scottish Geographical Magazine. It is self-referentially “a first attempt to map sunshine”.

John George Bartholomew is a man that has been encountered on numerous occasions before in this blog. As head of John Bartholomew & Co. at this time he was not only innovative in his approach to cartography, but also happened to print all of the maps which appeared in the Scottish Geographical Magazine, a publication of the Society which he had helped to found in 1884. As such, I will turn my particular attention to H. N. Dickson instead.

Henry Newton Dickson was an Edinburgh man through and through. The final decades of the Nineteenth Century seem to have been particularly good ones for the city in terms of the commerce, industry and thinkers that were seemingly overflowing. John George himself was a prominent part of this mini Enlightenment, as was Henry Newton Dickson. He studied at the University of Edinburgh and soaked up the radical teachings in experimental science that was influencing physics, meteorology and oceanography at this time. Men such as Peter Guthrie Tait (1831–1901), physicist, mathematician and former Cambridge Senior Wrangler, and the mathematician George Chrystal (1851–1911) were amongst those that inspired Dickson whilst at university. Post university, Dickson continued to move in the company of such giants as Sir John Murray (1841-1914), the legendary oceanographer, through his voluntary work with the Challenger Commission and Alexander Buchan (1819-1907) known simply as the father of meteorology. It can hardly be surprising that Dickson thrived in this environment and company and went on to pursue a career largely concerned with meteorology, climate and the mapping thereof.

The accompanying article to the map is called “On Sunshine”. Mapping sunshine might seem like a pretty straightforward academic pursuit but Dickson reassures us that it is not and in fact, that it is full of complications. Consider, for example, how to judge if the sun is actually shining or not. In many respects this is subjective and as such could never fulfil the empirical need for fact. Also, sunshine is in itself quite a vague concept. What exactly does it mean? As Dickson argues, the light from the sun is comprised of many different types of wavelengths, which ones comprise light, which are to be recorded? When considering sunlight are we confusing light for heat? There are for example those winter days that are gloriously sunny but bitterly cold. Dickson wasn’t helped by a lack of precedence and therefore a lack of appropriate technology. But, after weighing up the possibilities of instruments such as the Jordan photographic recorder he settled on the Campbell-Stokes burning recorder as his objective tool of choice.

The Campbell-Stokes recorder is interesting in its own right being, in my opinion, an example of a genuine Elegant Experiment. So beautiful in its simplicity it is still made and indeed used, even by professional agencies, to this day. J. F. Campbell attached to his house one day a hollow sphere of glass filled with water. He popped this in a wooden bowl that was big enough to allow the sun to focus its rays through the glass and onto the surface of the bowl, leaving behind a scorch mark. As a result, a record was made not only of the voyage of the sun through the sky but crucially, the instances of its shining.

With recordings made in this way, Dickson was now in a position to produce his map. His methodology was to imagine the maximum sunlight a station could get if the skies remained cloudless all day and every day. He then took his recordings and, comparing the actual and hypothetical results, produced a percentage of actual sunlight against maximum sunlight. These were then plotted on the map with lines joining areas with equal value.

By no means was this an easy thing to do and indeed as a note on the map itself tells us:

So in fact, after all of that and all the effort, effectively, this map cannot actually be trusted and indeed may be detrimentally misleading. Indeed, I am not too sure how well the art of mapping sunlight has progressed subsequently. You can find such maps but they are comparatively rare and usually incredibly vague. Even in 2005, there were only 200 reliable stations capable of recording sunlight covering the whole of both Europe and Africa – not too great considering there are over thirty in the Dickson map of the British Isles alone. There is hope that satellite technology may provide the key but regardless, it just goes to show how something so simple sounding has in reality perplexed the human mind since the time of Dickson all the way to the present day.

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