Much attention has been devoted to strontium-90 and to its availability to man by deposit on plants and soils. Because strontium bears a close chemical relation to calcium, a unit expressing this relation, the _strontium unit_ (one picocurie[15] [1 × 10^-12 curie of strontium-90 per gram of calcium]) is used in following strontium-90 through food chains. Soils, however, present confusing factors. Experiments and fallout observations show that strontium-90 does not penetrate soils deeply. In typical instances it remains in the upper inch or two of the soil surface, where its availability to root systems is as variable as the conditions of mixing, leaching, and plant growth. Experiments have shown that plant uptake of strontium from soils can be reduced by introduction of calcium in available form into the soil. Radiobiological developments on land result from combinations of environmental influences. Studies in the Rocky Mountains show that ecological conditions above the timberline, particularly in areas where snowbanks accumulate, are efficient in concentrating fallout radionuclides. Concentrations thus take place in the snow-packed heights that are the sources of mountain streams flowing to the plains far below. Atmosphere The environment of the earth is a product of “weather”—of the transport of moisture, of the actions between winds and oceans, of the cycling of energy through biotic systems. Understanding of biological potentials of atmospheric factors involves understanding of atmospheric motions affecting transport and mixing of contaminants and the processes of deposition of radionuclides from atmosphere to earth. [Illustration: _Network of towers on the Atomic Energy Commission reservation near Richland, Washington, used by atmospheric physicists in measuring quantity, concentration, and dilution of radioactive materials in the atmosphere._] At some thousands of feet above the earth’s surface—at 30,000 to 40,000 feet in the middle and polar latitudes and at 50,000 to 60,000 feet in the tropics—there is a level, the tropopause, at which air temperature, rather than decreasing, becomes constant or increases with height. Below this level is the troposphere, the turbulent zone of clouds, rain, and fog. Above it is the stratosphere, where there is no turbulence and only a slow mixing of dry and cloudless air. The stratosphere continues to a height of about 100,000 feet. Investigators have noted the importance of rain or snow in washing fallout particles from the air in the troposphere. There is disagreement on the precise modes of distribution of radioactive materials projected into the stratosphere.