Abstract 



This atlas presents world maps and graphs of upper-air radio refractivity, N(z) 

 (where z is the altitude), mean monthly AN (the difference between the refractivity 

 at the surface and at 1 km above the surface) , extreme values of gradients of refrac- 

 tivity observed in the lowest layer of the atmosphere (including maps of minimum 

 trapped frequency ducting gradients) , and monthly mean tropopause heights. All 

 refractivity values were derived from radiosonde data. The N(z) maps are presented 

 in terms of a three-part exponential model, with separate exponentials for the water 

 vapor and air density terms of N(z), with the latter separated into tropospheric and 

 stratospheric terms. The AN data were derived by interpolation from fixed pressure- 

 level data. 



IX 



