(3) Measuring Radiation

Physics of the Atmosphere II

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Measuring Radiation

Christoph Bichler

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Measuring Radiation

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Measuring Radiation

At state-of-the-art meteorological stations, equipped with a sun-tracker, solar radiation (irradiance) measure-ments comprise: Direct solar radiation (S), with a pyrheliometerGlobal solar radiation (G) with a pyranometer, Diffuse solar radiation (H) with a shaded pyranometer.

G = S cosθ + H

UF

S

GH

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Measuring Radiation at UoG

The University of Graz (UoG) performs “modern” radiation measurements – as part of the ARAD network (“Austrian radiation”) and “traditional ones” (TAWES = “Teilautomatische Wetterstation”) (credit: M. Mohr)

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Measuring Radiation at KSO

IGAM/IP solar radiation measurements are also performed at KSO (Kanzelhöhe Observatory for Solar and Environmental Research) (UF)

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Measuring Radiation at KSO

KSO is (usually) one of the places with maximum sunshine duration in Austria (above the notorious fog in the Klagenfurt basin (UF)

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Measuring Solar Radiation

Measurement devices for solar radiation employ thermocouples, which use the thermoelectric effect: Thermocouples contain two dissimilar metal conductors in contact, which produce a voltage when heated (Picture credit: M. Mohr).

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Measuring Solar Radiation

Pyranometers are used to measure global and diffuse solar radiation (from the halfspace). The thermopile is composed of several thermocouples, connected in series. The output is a voltage proportional to the temperature difference between the black surface of the sensor element and the housingas reference. Two quartz domes and a ventilation system (shall) minimize external influences (M. Mohr, after Kipp & Zonen)

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Measuring Radiation

Pyrgeometers are used to measure long wave radiation, using a silicon window with a “solar blind” filter, which only transmits the radiation with wavelengths from 4.5 µm up to 42 µm. Pyrheliometers (below) measure direct solar radiation (M. Mohr, after Kipp & Zonen)

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Measuring Radiation (“traditional”)

Most weather stations don’t have sun-trackers, they (may) have two (identical) Star pyranometers, one (left) measuring global solar radiation, the other one (shaded by a shadow ring) measuring (most of the) diffuse solar radiation. The direct solar radiation can only be derived. Star pyranometers measure the temperature difference between black and white sensors (credit: M. Mohr)

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Global Solar Radiation in Graz, Aug

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Global Solar Radiation on different summer days (August) (M. Mohr)

Global Solar Radiation - Eclipse

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Global Solar Radiation on sunny days with and without partial solar eclipse, and on a cloudy day in comparison (IGAM).

Global Solar Radiation in Styria, Jan

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Styrian Climate Atlas, ZAMG:http://www.umwelt.steiermark.at/cms/ziel/16178332/DE/

Global Solar Radiation in Styria, July

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Styrian Climate Atlas, ZAMG

Recording Sunshine (“traditional”)

In the past, recording of sunshine duration was – literally – done with a “crystal sphere” (Campbell-Stokes autograph) (UF, M. Mohr)

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Recording Sunshine Duration

The “Haenni Solar” device records differences between global and diffuse solar radiation (using rotating strip shades). Sunshine is – by WMO definition

(World Meteorological Organization) – when the direct solar radiation, perpendicular to the sun, exceeds 120 W/m2 (M. Mohr)

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Mean Sunshine Hours in Styria, Jan

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Styrian Climate Atlas, ZAMG

Sunshine in January?

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Sometimes, sunshine is a rare event during winter months .. (IGAM)

Sunshine in January!

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Above the low-level clouds this can be entirely different .. (UF)

Sunshine in November

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Under the low-level clouds you might not believe it .. (UF, IGAM)

Mean Sunshine Hours in Styria, July

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Styrian Climate Atlas, ZAMG

Increased Sunshine

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A part of the strong warming in Austria during the past ~35 years is due to increased sunshine (less clouds) in the same period (HISTALP)