Sunshine & Temperature

GHI is the sum of direct sunlight and diffuse skylight falling on a flat, horizontal surface. It is measured in watts per square metre (W/m²) at any instant, and accumulated into megajoules per square metre (MJ/m²) over a day or month. It is the single most useful quantity for characterising how much solar energy a location receives.

The inner dome blocks long-wave thermal radiation emitted by the outer dome itself, so only shortwave solar radiation reaches the absorber. The outer dome shields the inner dome from wind, rain and direct convective cooling. Together they make the instrument's response depend almost entirely on incoming sunlight rather than on wind speed or ambient temperature.

A thermopile is a stack of thermocouple junctions — pairs of dissimilar metals that generate a small voltage whenever the two junctions are at different temperatures. The black absorber disk heats up in sunlight while the instrument body stays cooler; the temperature difference drives a voltage that is calibrated against a reference standard and expressed directly in W/m².

Moisture inside the sealed dome space would condense on the inner surface and scatter or absorb some of the incoming light, creating a reading error. A small desiccant cartridge — visible as the ribbed cylinder on the side of the instrument body — absorbs water vapour to keep the dome interior dry.

Instantaneous readings are in W/m². This lab shows the daily total in MJ/m² (megajoules per square metre), obtained by integrating the instantaneous reading over each day. Monthly and annual values in the charts are the mean of the available daily totals for that period. 1 MJ/m²/day ≈ 0.278 kWh/m²/day.

The sharp rise to around 100 stations in the late 1970s was driven by two large US-funded field campaigns: NOAA's Regional Energy/Climate Studies programme and the WEST Associates solar resource surveys, which temporarily deployed dense networks of pyranometers across the American Southwest and Great Plains to assess solar energy potential. Most of these campaign stations were decommissioned once the surveys ended, which explains the sharp drop back down in the early 1980s.

The Baseline Surface Radiation Network, established in 1992 by the World Climate Research Programme, operates around 60 stations with tightly standardised instruments, calibration schedules, and data-quality procedures. BSRN stations report at 1-minute resolution and are considered the global reference for surface radiation measurements. The steady growth from the early 1990s onward largely reflects BSRN expansion and the formalisation of national radiation networks to BSRN standards.

Knut Ångström made the first systematic solar radiation measurements in Stockholm from 1893 using an instrument of his own design — the Ångström pyrheliometer. Routine monitoring elsewhere only followed once international instrument standards were established, first by the International Pyrheliometric Scale (IPS) in 1924 and later refined by the World Radiometric Reference (WRR) in 1979. The early count reflects only stations with confirmed, uninterrupted measurement records that survive in modern catalogs.

All stations shown measure global horizontal irradiance (GHI) — total downwelling shortwave radiation on a horizontal surface. Instrument types vary (thermopile pyranometers, silicon-cell sensors, rotating shadowband radiometers) and calibration standards have changed over time, which is one reason long-term consistency requires careful quality control. The WRR, maintained by the Physical Meteorological Observatory Davos (PMOD/WRC) in Switzerland, is the current international reference against which all pyranometers are calibrated.