The ionisation of the atmosphere near the ground in fair weather

In Part 1 observations of the positive and negative small ions in the lowest 2 m of the atmosphere near Cambridge are described, showing that the conductivity decreases with height in this region under most conditions. Simultaneous observations of the space‐charge density are incompatible with the theory that the convection current is small compared with the conduction current, and are supported by observations of the vertical electric field. Estimates of the size of the convection current and the vertical distribution of space charge have been made. Observations at the time of the ‘sunrise effect’ show that the increase in the field is associated with an increase in the positive space‐charge density and the breakdown of the nocturnal inversion, while the conductivity shows no reduction. In Part 2 the equation relating the concentration of atmospheric small ions to their rate of production and lifetime, the electric field and the atmospheric diffusivity has been solved on a computer. The solutions obtained for various sets of conditions are compared with experimental results. Good agreement is found with observations of the space charge, and moderate agreement with those of the field and the diffusivity, but the theoretical prediction that the nucleus concentration is the most important factor influencing the smallion concentration is not supported by the observations. Some possible reasons for this are discussed.