. Compendium of meteorology. Meteorology. 122 ATMOSPHERIC ELECTRICITY length, by breaking drops, and by drifting snow and dust. Under particular circumstances one or more of the above-mentioned processes may add appreciably to the ion population of the air. None of these, however, are normally important as ionizers of the atmosphere. The chief ionizer in the lower stratosphere and the tropo- sphere, except in the lower atmosphere over land, is the cosmic rays. In the lowest kilometer or so over land, ionization of the air is due chiefly to radiations from radioactive matter in the earth and in
. Compendium of meteorology. Meteorology. 122 ATMOSPHERIC ELECTRICITY length, by breaking drops, and by drifting snow and dust. Under particular circumstances one or more of the above-mentioned processes may add appreciably to the ion population of the air. None of these, however, are normally important as ionizers of the atmosphere. The chief ionizer in the lower stratosphere and the tropo- sphere, except in the lower atmosphere over land, is the cosmic rays. In the lowest kilometer or so over land, ionization of the air is due chiefly to radiations from radioactive matter in the earth and in the air. The average rate of ionization of the atmosphere q, over land and near the earth's surface, has been esti- mated by Hess [10, pp. 167-171] at about 10 pairs per cc per sec. This value is based on the average amount of radioactive matter in the earth and in the air and is summarized in Table II. According to this estimate, approximately half of the total ionization is due to radioactive matter in the air, one third is due to radio- active matter in the soil, and one sixth due to cosmic rays. The ionization due to cosmic rays and radioactive matter in the soil is probably more or less constant with time at a given station. That due to radioactive matter in the air, however, is subject to variations, since the Table II. Ionization ok the Air Near the Earth's Surface Over Land in Per Cent of Total Ionizing ray Total Ionizer a /3 7 Cosmic rays Radium \- „• Thoriumr ^'' Radium \- •, Thorium)'" ^°'^ Cosmic ra3^s 30 18 1 1 1 1 32 16 33 16 Total 48 3 33 16 100 quantity of radioactive matter in the air varies with time. The amount of radioactive matter in the air depends upon two factors: (1) the rate at which it is dissipated in the atmosphere, and (2) the rate of exhala- tion from the soil. The rate of exhalation of radioactive gas from the soil is subject to considerable variation, being affected by such factors as temperature of the soil, wind force, dryness of
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