HITRAN (an acronym for High Resolution Transmission) molecular spectroscopic database is a compilation of spectroscopic parameters used to simulate and analyze the transmission and emission of light in gaseous media, with an emphasis on planetary atmospheres. The knowledge of spectroscopic parameters for transitions between energy levels in molecules (and atoms) is essential for interpreting and modeling the interaction of radiation (light) within different media.
For half a century, HITRAN has been considered to be an international standard which provides the user a recommended value of parameters for millions of transitions for different molecules. HITRAN includes both experimental and theoretical data which are gathered from a worldwide network of contributors as well as from articles, books, proceedings, databases, theses, reports, presentations, unpublished data, papers in-preparation and private communications. A major effort is then dedicated to evaluating and processing the spectroscopic data. A single transition in HITRAN has many parameters, including a default 160-byte fixed-width format used since HITRAN2004. Wherever possible, the retrieved data are validated against accurate laboratory data.
The original version of HITRAN was compiled by the US Air Force Cambridge Research Laboratories (1960s) in order to enable surveillance of military aircraft detected through the terrestrial atmosphere.  One of the early applications of HITRAN was a program called Atmospheric Radiation Measurement (ARM) for the US Department of Energy. In this program spectral atmospheric measurements were made around the globe in order to better understand the balance between the radiant energy that reaches Earth from the sun and the energy that flows from Earth back out to space. The US Department of Transportation also utilized HITRAN in its early days for monitoring the gas emissions (NO, SO, NO) of super-sonic transports flying at high altitude. HITRAN was first made publicly available in 1973 and today there are a multitude of ongoing and future NASA satellite missions which incorporate HITRAN. One of the NASA missions currently utilizing HITRAN is the Orbiting Carbon Observatory (OCO) which measures the sources and sinks of CO in the global atmosphere. HITRAN is a free resource and is currently maintained and developed at the Center for Astrophysics  Harvard & Smithsonian, Cambridge MA, USA ([https://www.cfa.harvard.edu/research/hitran-and-hitemp-database CFA/HITRAN]).
HITRAN is the worldwide standard for calculating or simulating atmospheric molecular transmission and radiance from the microwave through ultraviolet region of the spectrum. The HITRAN database is officially released on a quadrennial basis, with updates posted in the intervening years on [https://hitran.org HITRANonline]. There is a new journal article published in conjunction with the most recent release of the HITRAN database, and users are strongly encouraged to use the most recent edition. Throughout HITRANs history, there have been around 50,000 unique users of the database and in recent years there are over 24,000 users registered on [https://hitran.org HITRANonline]. There are YouTube tutorials on the [https://hitran.org HITRANonline'] webpage to answer frequently asked questions by users.