GEOS-FP Relative Humidity

Weather models begin their forecasts by integrating observations from ground stations, aircraft, balloons, and a growing constellation of Earth observing satellites to estimate the most realistic atmospheric state. The Goddard Earth Observing System Forward Processing (GEOS-FP) system is a research model maintained by NASA’s Global Modeling and Assimilation Office to demonstrate innovative new ways to use satellite data to improve predictions of weather, air quality, and greenhouse gases and to help NASA collect new observations of our home planet.

Relative humidity is the ratio of water vapor in the atmosphere relative to the amount of water vapor that would be present if the air were saturated. It is calculated by using a combination of near surface temperature and water vapor fields. Variations reflect differences in daytime and nighttime relative humidity and the influence of large scale weather systems. Near surface humidity is an indicator of the likelihood of rain or fog and also has implications for human health. Humidity influences the evolution of air pollution events and high relative humidity can increase the risk of developing heat-related illnesses on very hot days.

See https://gmao.gsfc.nasa.gov/GMAO_products/NRT_products.php