Atmosphere of Earth the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around . Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition. The study of Earth's atmosphere and its processes is called atmospheric science (aerology). Early pioneers in the field include Léon Teisserenc de Bort and Richard Assmann. The three major constituents of Earth's atmosphere are nitrogen, oxygen, and argon. Water vapor accounts for roughly 0.25% of the atmosphere by mass. The concentration of water vapor (a greenhouse gas) varies significantly from around
Most common words in Spanish includes samples from all Spanish-speaking countries. The list of "1000 most frequent word forms" comes from an analysis of CREA version 3.2. Plurals, verb conjugations, and other inflections are ranked separately. Homonyms, however, are not distinguished from one another. CREA 3.2 was published in June 2008. In 2006, Mark Davies, an associate professor of linguistics at Brigham Young University, published his estimate of the 5000 most common words in Modern Spanish. To make this list, he compiled samples only from 20th-century sources—especially from the years 1970 to 2000. Most of the sources are from the 1990s. Of the 20 million