This technical report summarizes the GLOBE Observer dataset from 1 April 2016 to 1 December 2019. GLOBE Observer is an ongoing NASA-sponsored international citizen science project that is part of the Global Learning & Observations to Benefit the Environment (GLOBE) Program. GLOBE Observer is the largest citizen science project in the Earth Science Division at NASA. Participants use the GLOBE Observer mobile app (launched in 2016) to collect atmospheric, hydrologic, and terrestrial observations. 38,000 participants have contributed 320,000 observations worldwide, including 1,000,000 geotagged photographs. It would take an individual more than 13 years to replicate this effort. Comparing the same data types between the GLOBE Observer app and the formal GLOBE Program, the app doubled data volume and substantially increased geographic coverage over the study period. GLOBE Observer data are publicly available at observer.globe.gov.