Sunshine Today — Your Live Recorder
The instrument above is a working model of the Campbell-Stokes sunshine recorder, tracing the hours of bright sunshine at your location. As the page finds you — or you search a city — it reads how much genuine, card-scorching sunshine there has been today and burns the trace along the curved card to match. When the sun is bright enough right now, the glass sphere focuses it to a glowing point and the burn spot flares; when cloud rolls over, the point fades and the card stops charring, exactly as the real instrument behaves. Below, the readouts give the sunshine hours so far, the current solar radiation, cloud cover, the length of daylight and the percentage of possible sunshine you are actually getting.
There is a lovely honesty to this instrument: it cannot be fooled by a bright-looking sky. Only sunshine strong enough to burn counts — which is precisely why its records have been trusted for over a century and a half.
The Campbell-Stokes Sunshine Recorder
The design is one of the most elegant in all of meteorology. A solid glass sphere, about ten centimetres across, sits in a curved metal bowl. The sphere behaves as a lens with no single focal direction — it focuses the sun to an intense point no matter where the sun sits in the sky. Behind the sphere, a specially treated card is held in the bowl along the sphere's focal curve. When direct sunlight is strong enough, the focused point burns a char mark into the card, and as the sun tracks across the sky through the day, that point moves along the card and scorches a continuous line. At the end of the day the card is removed and the total length of burn, read against printed hour lines, gives the hours of bright sunshine.
It was devised by John Francis Campbell in 1853 and refined by Sir George Gabriel Stokes in 1879, and the combination has carried both their names ever since. For more than a hundred years it was the world standard, and its beautiful brass-and-glass form is still a familiar sight on the roofs of older weather stations.
What Counts as "Bright Sunshine"
The recorder measures a specific thing: bright sunshine, not daylight. The card only chars when the direct beam from the sun exceeds a threshold of roughly 120 watts per square metre — the level the World Meteorological Organization adopted, in part because it matches where the Campbell-Stokes card begins to burn. Diffuse light scattered by cloud, however bright the overcast may seem to the eye, is far too weak to focus into a burning point, so it leaves no mark. This is the instrument's defining virtue: it distinguishes genuine sunshine, the kind that warms and powers and casts sharp shadows, from mere daylight.
Reading the Burn Trace
The card is printed with hour lines, and the burn trace reads directly against them. An unbroken scorch from one hour mark to another means the sun shone continuously through that period; gaps in the burn show when cloud passed over and the sun was hidden. A day of clear skies leaves a long, solid line; a showery day of sunshine and cloud leaves a broken, dotted trace that tells the story of the day's weather at a glance. The live version here builds the trace to match the sunshine recorded so far, and flares the burning point whenever the sun is bright enough to be scorching the card right now.
Sunshine Hours and Percentage of Possible Sunshine
Two figures describe a place's sunshine. The first is the raw hours of bright sunshine — a resort advertising "over 3,000 hours of sunshine a year" is quoting exactly this. The second, more revealing for comparison, is the percentage of possible sunshine: the hours actually recorded divided by the maximum possible for that day's length. Because summer days are long and winter days short, raw hours favour summer and low latitudes; the percentage levels the field, showing how often the sun broke through relative to how long it was up. The recorder shows both, so you can see not just how much sun you got but how much of the available daylight was genuinely sunny.
Why Sunshine Duration Matters
Sunshine is measured because so much depends on it. In solar energy, hours of bright sunshine directly predict the output of photovoltaic panels and solar-thermal systems, guiding where they are worth installing. In agriculture, sunshine drives photosynthesis, ripening and drying, and features in models of crop yield and disease. In tourism and property, sunshine hours are a headline selling point for resorts and regions. In health, sunlight governs vitamin D, mood and seasonal affective patterns. And in climatology, long, consistent sunshine records — many of them made with Campbell-Stokes recorders — are among the most valuable datasets for tracking how climate is changing.
Sunshine, Cloud and Solar Radiation
Sunshine duration is closely tied to two related measurements the recorder also displays. Cloud cover is the enemy of sunshine: the more of the sky is covered, the fewer hours the card burns, though a sky of broken cloud can still deliver long spells of sunshine between the gaps. Solar radiation — measured in watts per square metre by an instrument called a pyranometer — is the continuous cousin of sunshine hours, capturing the full intensity of sunlight including the diffuse part that the recorder ignores. Reading the three together gives a complete picture of the sun's contribution: how intense it is now, how much of the sky obstructs it, and how many genuine sunshine hours have accumulated.
Sunshine Terms Around the World
Sunshine duration is recorded globally, under names such as durée d'ensoleillement in French, Sonnenscheindauer in German, duración de la insolación in Spanish, durata dell'insolazione in Italian and duração da insolação in Portuguese. The instrument itself is the héliographe, Sonnenscheinautograph or heliógrafo — the "sun-writer" — a fitting name for a device that lets the sun record its own presence in fire.
From Burning Glass to Photodiode
The Campbell-Stokes recorder is still in use, valued for its unbroken continuity with historical records, but it is increasingly joined by electronic instruments. Modern automatic sunshine sensors use arrays of photodiodes to detect when direct sunlight crosses the same brightness threshold, logging sunshine digitally with no card to change and no daily visit required. Pyranometers measure the full solar radiation continuously. The live recorder on this page draws on modern solar data to reproduce, in an old and beautiful form, a measurement that humans have found worth making for a very long time: how long the sun truly shone.