Kipp & Zonen CGR 4 Instruction Manual

Kipp & Zonen CGR 4 Instruction Manual Manual pdf 19 page image

4. Pyrgeometer physical properties4.1. Spectral rangeThe spectral properties of the pyrgeometer are mainly determined by the filter characteristics of the siliconwindow and the coatings. The application is primarily to measure long-wave downward atmosphericradiation. The spectral range is from 4.5 to 42 μm, where most of this radiation is present.Figure 11: Pyrgeometer spectral window propertiesThe atmosphere is transparent to long-wave radiation emitted by the Earth’s surface in certain wavelengthintervals, particularly within a spectral range of approximately 8 to 14 μm. This is called the ‘atmosphericwindow’. Within this spectral range the Earth is able to maintain an equilibrium temperature by losing acertain quantity of heat gained each day from the sun.The sun radiates approximately as a blackbody at an equivalent temperature of 5770K. Almost 99% of itsemitted energy is contained in wavelengths less than 4μm, called short-wave radiation. The equivalentradiant temperature of the Earth’s surface is about 275K. More than 99% of this energy is emitted atwavelengths greater than 3 μm and is called long-wave, thermal, or infrared radiation.Downward long-wave radiation is a result of atmosphericre-emission. Re-emission is the reversible effect ofabsorption of long-wave radiation emitted by the Earthand by chemical elements such as water (H₂O), Oxygen(O₂), Ozone (O₃), Carbon dioxide (CO₂), etc. Theseelements are the main emitters of long-wave radiation inthe atmosphere. The remaining unabsorbed portion of theEarth’s radiation escapes into outer space. Under clear skiesan object can be cooled below ambient air temperature byradiative heat loss to the sky.Observing the earth from outer space, a blackbody is seenin a range of 8 to 14 μm with a temperature of 14°C andoutside this wavelength range a blackbody of -60°C.Under clear sky conditions in a reverse direction, outer Figure 12: Atmospheric radiationspace can be observed in the same spectral range.Page 19CGR 4 Manual