Chemical and physical processes which may give rise to isotope fractionation of molecular species in the atmospheres of both Earth and other planets are reviewed, along with observations of isotopically substituted molecules in planetary atmospheres. Mechanisms for production of isotope fractionation considered include escape and effect of isotope substitution on equilibrium constants (including those of phase changes), photolysis rates, and chemical reaction rates. The isotopes considered for compounds in

The purpose of the International Global Precipitation Measurement (GPM) Program is to develop a next-generation space-based measuring system which can fulfill the requirements for frequent, global, and accurate precipitation measurements. The associated GPM Mission is being developed as an international collaboration of space agencies, weather and hydrometeorological forecast services, research institutions, and individual scientists. The design and development of the GPM Mission is an outgrowth of valuable knowledge and published findings enabled by the Tropical Rainfall Measurement Mission (TRMM). From the TRMM experience, it was recognized that the GPM Mission must consist of a mixed nonsunsynchronous and sunsynchronous orbiting satellite constellation in order to have the capability to provide physically based retrievals on a global basis, with ~3-h sampling assured at any given Earth coordinate ~90% of the time. The heart of the GPM constellation is the Core satellite, under joint development by NASA and the Japan Aerospace Exploration Agency (JAXA), which will carry a dual frequency Ku/Kaband precipitation radar (PR) and a high-resolution, multichannel passive microwave (PMW) rain radiometer. The core is required to serve as the calibration reference system and the fundamental microphysics probe to enable an integrated measuring system made up of additional constellationsupport satellites, each carrying at a minimum some type of PMW radiometer. In this article the background, planning, design, and implementation of the GPM is described.

The possible enhancement of isotopically heavy ozone (50O3) in the earth's stratosphere and mesosphere produced by the preferential photodissociation of heavy oxygen (34O2) is considered. On the basis of a realistic oxygen‐only model, we show that the rapidity of the exchange reaction 18O + 32O2 ⇄ 16O + 34O2 prevents any enhancement at stratospheric temperatures. Since observations of Mauersberger (1981) suggest that such an enhancement exists, one is left with a paradox. Possible modifications to the chemical model, which include isotopically specific photochemistry of ozone, are shown to be non‐competitive with the exchange reaction.

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