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Air Temperature Bias Correction

Issue Resolution: Air Temperature Data Bias

Air Temperature Bias Correction

Beginning in the 1990s, the YSI 44019A thermistor with the 44211A thermilinear package (the extended-range YSI temperature sensor) replaced the YSI 44230 temperature sensor as the default temperature sensor used at SNOTEL sites because of its ability to measure a broader temperature range. Air temperature is calculated from sensor output voltage using a linear least-squares regression algorithm created by the Snow Survey and Water Supply Forecasting Program*. Use of the linear least-squares regression algorithm results in a data bias.

* Air temperature data from Alaska automated sites also contains a similar but different bias.  Use of a different equation is the source of Alaska’s temperature data bias.  Correction of Alaska air temperature data will require the use of a separate debiasing equation.  The Alaska debiasing equation is also under development.

Bias correction:

  • Resolves bias introduced by inadequate sensor output conversion equation.  The inadequate equation was erroneously used in place of the manufacturer suggested equation.
  • Will not take into account or correct for environmental factors (e.g., sensor shield, shading, or radiative heating).
  • Applies to all erroneous data across the SNOTEL network for which the sensor and equation were applied.

Starting in 2023, as the YSI sensors are phased out and replaced with an upgraded temperature sensor, historic YSI data will be de-biased and available to data users. This effort will take 4-5 years to complete.

Full map of Continental US Unbias locations


The Snow Survey and Water Supply Forecasting Program has created a map illustrating which sites have been bias reduced and related machine readable source data files (see legend).


Correction Equations

Air Temperature Bias Correction Equation for sensors installed in the CONUS are provided in the image below. 

Information on the equation and derivation, including the correction equation for Alaska sites, are detailed in the  publication Evaluation of NOAA vs SNOW YSI Temperature Correction Equations, provided in the Additional Documents section of this page.

Equation for removing bias from air temperature data at affected sites

The Historical Air Temperature Bias Correction Metadata document, linked below, provides established dates on which known biased air temperature data were collected.  The biased data are currently available in the SNOTEL database.