Abstract :
On the mixing-in-gradient hypothesis of incoherent scattering of radio waves in a dry atmosphere, the intensity of the irregularities in dielectric constant depends on the excess of the temperature gradient above that appropriate to an adiabatic atmosphere. In going from the upper troposphere to the stratosphere, there is a significant increase in this gradient excess and consequently, a significant increase in the intensity of irregularities in dielectric constant. The decrease in intensity with increase of height measured by Crain in the troposphere does not, therefore, indicate reliably the intensity to be expected above the tropopause. Calculations have been made concerning the effect of stratospheric, as distinct from tropospheric, scattering. Stratospheric scattering is expected to predominate over tropospheric scattering at ranges greater than about 600 km. At a range of 1000 km, the calculated transmission-loss due to stratospheric scattering is a few decibels greater than is indicated by observations. The effect of stratospheric scattering at a frequency of 108 mc is such that the minimum signal observed at this frequency over the path from Cedar Rapids, Iowa, to Sterling, Va., could conceivably have been stratospheric in origin, with ionospheric scattering being predominant at certain times, for example, during SID´s.
