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EO·Atlas / Topics / Methodologies / Hyperspectral infrared nadir sounding
Methodology ยท Atmospheric spectroscopy

Hyperspectral infrared nadir sounding

High-spectral-resolution thermal infrared spectrometry of the atmosphere viewed at or near nadir, using Fourier-transform or grating spectrometers with more than a thousand channels to retrieve vertical profiles of temperature and humidity and columns of trace gases such as ozone, carbon monoxide and methane. The sounding backbone of numerical weather prediction. Orbit (polar or geostationary) and scan geometry are attributes, not separate methods.

How it works

Hyperspectral infrared nadir sounding is used to retrieve vertical profiles of atmospheric temperature and humidity, and columns of trace gases such as ozone, carbon monoxide, ammonia and methane, from the thermal radiation the atmosphere emits.[1] The instruments are passive and measure emitted thermal radiation, so they work by day and by night.[2] The physics is summarised in Wikipedia: Atmospheric sounding.

Requirements. The method depends on three things. First, spectral coverage in the thermal infrared. The class as a whole spans roughly 3.6 to 15.5 micrometres, but individual instruments cover narrower and sometimes discontinuous ranges: IASI (Infrared Atmospheric Sounding Interferometer) lists 3620 to 15500 nanometres,[3] IKFS-2 begins near 5 micrometres,[4] and GIIRS (Geostationary Interferometric Infrared Sounder) and the Meteosat Third Generation Infrared Sounder (MTG-IRS) begin near 4.4 micrometres.[5][6] Second, more than a thousand spectral channels; CrIS (Cross-track Infrared Sounder) has 1305 channels in normal mode and 2211 in full spectral resolution mode.[7] Third, acceptance of footprints of kilometres to several kilometres rather than fine imaging resolution; a 2023 conference presentation gives 4 km nadir sampling for MTG-IRS.[8]

Limits. Cloud is the main constraint on an optical thermal infrared measurement. AIRS (Atmospheric Infrared Sounder) was designed to retrieve through partial cloud by cloud clearing with co-located microwave data,[9] and the EUMETSAT IASI Level 2 temperature and humidity product likewise combines IASI with the AMSU-A and MHS microwave sounders.[10] Membership of the WMO OSCAR cross-nadir infrared sounder class does not imply a hyperspectral instrument: that class has 54 instrument records,[11] and the INSAT Sounder in it uses a filter wheel with 18 infrared channels and one visible channel.[12]

Implementation. AIRS is an echelle grating spectrometer,[13] while IASI and CrIS are Fourier transform instruments.[3][7] Channel counts range from 2378 on AIRS[13] and 2670 on IKFS-2[4] to 8461 on IASI[3] and 16921 on IASI-NG (IASI New Generation).[14] Geostationary members step and stare at a fixed disc: GIIRS covers 700 to 1130 and 1650 to 2250 reciprocal centimetres,[15] and a 2023 presentation on instability products gives a 30 minute repeat over Europe for MTG-IRS.[8]

Status as of 21 October 2026. OSCAR lists IASI on Metop-B and Metop-C, with Metop-A ended in November 2021;[3] CrIS on Suomi NPP, NOAA-20 and NOAA-21;[7] AIRS on Aqua;[13] HIRAS on FY-3D;[16] GIIRS on FY-4A;[5] GIIRS-2 on FY-4B, in service since September 2021;[17] and IKFS-2 on Meteor-M N2-3 and N2-4.[4] Metop-SG-A1, carrying IASI-NG, launched on 13 August 2025,[18] and CNES reports first light on 30 September 2025.[19] ESA reports that MTG-S1 commissioning closed on 2 July 2026,[20] and a repost of a EUMETSAT release states that pre-operational IRS data went to all users on 30 July 2026.[21]

Applications. Published work covers temperature profile validation for IKFS-2 against radiosondes,[22] height-resolved methane retrievals from IASI,[23] global ammonia distributions from IASI,[24] near-real-time outgoing longwave radiation estimated from CrIS radiances,[25] comparison of AIRS and MODIS cloud retrievals,[26] and an accuracy assessment of IASI sea surface temperature against AVHRR (Advanced Very High Resolution Radiometer).[27] A study of FY-4A GIIRS assimilates its radiances in a forecast of Typhoon Bavi.[28]

Topics this serves
Topic
Fit
Atmospheric profilingfirst choice

Primary optical route to temperature and humidity vertical profiles, with finer vertical detail than microwave sounders in clear or cleared scenes.

Air qualitysuitable

Daily global columns of CO, NH3, O3, HNO3 and SO2 from thermal IR emission, day and night.

Cloud propertiessuitable

Delivers cloud flag, cloud top pressure, cloud fraction and cloud phase per sounder footprint as part of the operational IASI Level 2 chain.

Weather nowcastingsuitable

Geostationary members give frequent thermodynamic profiles and instability information before convection; an environmental supporting layer for nowcasting, not a hazard detector.

CO2adequate

Mid-tropospheric CO2 columns for global, coarse-scale carbon cycle context.

Earth radiation budgetadequate

Supplies outgoing longwave radiation estimates by regression on sounder radiances, complementing broadband radiometers.

Greenhouse gas inventoryadequate

Contributes mid-tropospheric greenhouse gas constraints at global scale for inversion systems.

Methane (CH4)adequate

Global mid-tropospheric methane columns and two-layer retrievals for screening and trend context.

Sea surface temperatureadequate

Skin SST from window channels under clear sky; complements imager SST.

Stratospheric ozoneadequate

Total and tropospheric ozone columns from thermal IR emission, day and night.

Existing implementations
Demonstrated
Capable, undemonstrated
Related methodologies
  • Atmospheric limb sounding

    Retrieves vertical profiles of atmospheric composition (O3, H2O, temperature) from limb or solar/stellar occultation geometry; high vertical resolution makes it suited for stratospheric and upper-tropospheric science.

  • Tropospheric column mapping (NO2/SO2)

    Maps tropospheric NO2 and SO2 column density from UV-VIS backscatter; good for urban air quality, shipping emissions, and volcanic SO2 tracking.

Sources
Methodology

Edited from public sources. Last reviewed 2026-10-21 by SpectraWorks editorial. See the data dictionary for field definitions.

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Plain text

Hyperspectral infrared nadir sounding. EO-Atlas, SpectraWorks B.V., last reviewed 2026-10-21. https://eo-atlas.org/topics/methodologies/hyperspectral-ir-nadir-sounding (CC BY 4.0)

BibTeX

@misc{eoatlas:topics:methodologies:hyperspectral-ir-nadir-sounding,
  title = {Hyperspectral infrared nadir sounding},
  author = {{EO-Atlas}},
  publisher = {SpectraWorks B.V.},
  year = {2026},
  note = {Last reviewed 2026-10-21. CC BY 4.0},
  url = {https://eo-atlas.org/topics/methodologies/hyperspectral-ir-nadir-sounding}
}