From AMS Glossary
(Symbol H.) A colorless and odorless gaseous element, atomic number 1, atomic weight 1.008; the lightest and apparently the most abundant chemical element in the universe.
However, it is found only in trace quantities in the observable portion of our atmosphere, in the amount of only about 0.00005% by volume of dry air. Hydrogen H2 has a molecular weight of 2.0160 and specific gravity referred to air of 0.0695. The low escape velocity of hydrogen, combined with absence of active generating mechanisms on the earth, holds its concentration to negligible amounts. Its concentration increases above altitudes of several hundred kilometers as a consequence of diffusive separation. At one time, hydrogen was commonly used for inflating meteorological balloons, but because of its dangerous combustibility, it has been replaced by helium.