Pyrgeometer
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A pyrgeometer () is a device that measures near-surface infra-red (IR)
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...
, approximately from 4.5 μm to 100 μm on the
electromagnetic spectrum The electromagnetic spectrum is the full range of electromagnetic radiation, organized by frequency or wavelength. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band. From low to high ...
(thereby excluding
solar radiation Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared (typically p ...
). It measures the resistance/
voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
changes in a material that is sensitive to the net energy transfer by
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...
that occurs between itself and its surroundings (which can be either in or out). By also measuring its own
temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
and making some assumptions about the nature of its surroundings it can infer a temperature of the local
atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
with which it is exchanging radiation. Since the
mean free path In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...
of IR radiation in the atmosphere is ~25 meters, this device typically measures IR flux in the nearest 25 meter layer.


Pyrgeometer components

A pyrgeometer consists of the following major components: *A
thermopile A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel. Such a device works on the principle of the thermoel ...
sensor which is sensitive to radiation in a broad range from 200 nm to 100 μm *A silicon dome or window with a solar blind filter coating. It has a transmittance between 4.5 μm and 50 μm that eliminates solar shortwave radiation. *A temperature sensor to measure the body temperature of the instrument. *A sun shield to minimize heating of the instrument due to solar radiation.


Measurement of long wave downward radiation

The atmosphere and the pyrgeometer (in effect its sensor surface) exchange long wave IR radiation. This results in a net radiation balance according to: :E_\mathrm = E_\mathrm - E_\mathrm Where (in
SI unit The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of units of measurement, system of measurement. It is the only system ...
s): * = net radiation at sensor surface /m2* = long-wave radiation received from the atmosphere /m2* = long-wave radiation emitted by the sensor surface /m2 The pyrgeometer's
thermopile A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel. Such a device works on the principle of the thermoel ...
detects the net radiation balance between the incoming and outgoing long wave radiation flux and converts it to a voltage according to the equation below. : E_\mathrm = \frac Where (in SI units): * =net radiation at sensor surface /m2* =
thermopile A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel. Such a device works on the principle of the thermoel ...
output voltage * = sensitivity/calibration factor of instrument /W/m2 The value for is determined during calibration of the instrument. The calibration is performed at the production factory with a reference instrument traceable to a regional calibration center. To derive the absolute downward long wave flux, the temperature of the pyrgeometer has to be taken into account. It is measured using a temperature sensor inside the instrument, near the cold junctions of the
thermopile A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel. Such a device works on the principle of the thermoel ...
. The pyrgeometer is considered to approximate a black body. Due to this it emits long wave radiation according to: :E_\mathrm = \sigma T^4 Where (in SI units): * = long-wave radiation emitted by the earth surface /m2* = Stefan–Boltzmann constant /(m2·K4)* = Absolute temperature of pyrgeometer detector From the calculations above the incoming long wave radiation can be derived. This is usually done by rearranging the equations above to yield the so-called pyrgeometer equation by Albrecht and Cox. :E_\mathrm = \frac+ \sigma T^4 Where all the variables have the same meaning as before. As a result, the detected voltage and instrument temperature yield the total global long wave downward radiation.


Usage

Pyrgeometers are frequently used in
meteorology Meteorology is the scientific study of the Earth's atmosphere and short-term atmospheric phenomena (i.e. weather), with a focus on weather forecasting. It has applications in the military, aviation, energy production, transport, agricultur ...
,
climatology Climatology (from Greek , ''klima'', "slope"; and , '' -logia'') or climate science is the scientific study of Earth's climate, typically defined as weather conditions averaged over a period of at least 30 years. Climate concerns the atmospher ...
studies. The atmospheric long-wave downward radiation is of interest for research into long term climate changes. The signals are generally detected using a data logging system, capable of taking high resolution samples in the millivolt range.


See also

*
Pyranometer A pyranometer () is a type of actinometer used for measuring solar irradiance on a planar surface and it is designed to measure the solar radiation flux density (W/m2) from the hemisphere above within a wavelength range 0.3 μm to 3 μm. A typ ...
*
Radiometer A radiometer or roentgenometer is a device for measuring the radiant flux (power) of electromagnetic radiation. Generally, a radiometer is an infrared radiation detector or an ultraviolet detector. Microwave radiometers operate in the micro ...


References

{{Meteorological equipment Electromagnetic radiation meters Radiometry