Sunshine is really two separate measurements that often get confused. Solar radiation is the energy arriving on a horizontal surface, measured in watts per square metre, and it peaks at roughly 1,000 W/m² on a clear UK summer day. Sunshine duration is the number of hours during which that radiation exceeds a defined threshold of 120 W/m² — the figure the Met Office quotes as sunshine hours. Ultraviolet is a small, high-energy slice of the same spectrum and is reported separately as the UV index.
If you grow anything, own solar panels, plan irrigation, or need to understand why your temperature sensor reads high in the afternoon, solar radiation is the measurement you want. If you are concerned with skin exposure, it is UV.
How solar radiation is measured
The reference instrument is a pyranometer. A precision version uses a thermopile: a black absorbing surface heats up under sunlight, a stack of thermocouples measures the temperature difference between that surface and the shaded body, and the resulting voltage is proportional to incoming energy. A glass dome gives it a full 180-degree hemispherical view and keeps weather off the sensor.
The version fitted to weather stations uses a silicon photodiode behind a diffuser instead. It is far cheaper, responds instantly and is perfectly adequate for agriculture, evapotranspiration and solar yield work, though it is slightly less accurate at low sun angles because a photodiode does not respond equally across the whole spectrum. Either way, the sensor must be mounted level, facing straight up, with nothing shading it at any time of day — which in practice usually means the top of the mast, above the rain gauge and away from the anemometer.
Sunshine hours
Sunshine duration is not measured directly by most stations; it is derived. The software counts the minutes during which measured radiation exceeded the 120 W/m² threshold and reports the total. The historic instrument was the Campbell-Stokes recorder, a glass sphere that focused the sun onto a card and burned a trace along it, which is why old sunshine records exist at all.
How UV is measured
A UV sensor uses a photodiode with an optical filter that admits only the ultraviolet band, mostly UVA and UVB. The output is converted into the UV index, an open-ended scale where 0 to 2 is low, 3 to 5 moderate, 6 to 7 high, 8 to 10 very high and 11 upwards extreme. In the UK the index peaks around 7 to 8 in high summer at midday. It is worth knowing that UV passes through thin cloud far better than visible light does, so a bright overcast day can carry a surprisingly high index.
Why solar radiation is worth logging
Three groups need it. Growers use it to calculate evapotranspiration, the combined loss of water from soil and leaf, which is the proper basis for irrigation scheduling rather than guessing from rainfall alone. Solar panel owners use it to separate a genuine array fault from a simply dull week. And anyone running a professional temperature record needs it to understand and correct for radiation error, because it is direct sunlight on the housing that makes an unshielded sensor read several degrees too high.
What we recommend
Solar and UV are almost always bought as part of a sensor suite rather than as standalone instruments. The Davis Vantage Pro2 Plus is the standard choice: the Davis Wireless Vantage Pro2 Plus with UV & Solar adds both sensors to the full station, and the Davis Vantage Pro2 Plus Cabled is the wired equivalent. For horticultural and plant-growth work the Spectrum WatchDog 3230 Wireless Plant Growth Station is built specifically around light and growing conditions. If you want a UV reading on a domestic budget, the ACCUR8 DWS7100 7-in-1 Solar-Powered WiFi Weather Station includes light and UV alongside the standard five elements.
Related measurements
Solar radiation is the reason correct temperature shielding matters, so see Temperature and the radiation shields in Sensors, Spares & Accessories. Full suites carrying solar and UV sensors are in Davis Weather Stations and Professional Weather Stations, live streaming and logging is covered by the Davis WeatherLink range and Data Loggers, and the other elements are indexed under Elements.