260 TRANSHORIZON PARAMETERS 



The zero subscripts in (6.11) refer to conditions at the earth's surface. 

 Equations (6.11) and (6.12) are taken to hold as long as saturation does 

 not occur. It is further assumed that the mixing ratio 



e^eo-^ (6.13) 



-to 



is constant throughout the process. In (6.13) e is the partial pressure of 

 water vapor. 



A parcel of air which is caused to rise will cool to the dew point if forced 

 aloft far enough. Further rising will lead to condensation. Conversely, 

 a parcel of air which sinks down will have work done upon it adiabatically 

 and will become warmer than it was in its original elevated position. A 

 parcel of air following a condensation curve has a value of a, of — 6°C/km 

 denoted at a*. In addition, as water vapor condenses out of the parcel, 

 the vapor pressure will decrease. We here assume that the vapor pressure 

 will follow that of the saturation vapor pressure curve, Csz, as given by: 



esz = est exp [-^a*(z-l)], z > i, (6.14) 



where i is the height of the lifting condensation level and ^ = 0.064 (°C)~^. 

 The value of ( has been found to be given [44] with sufficient accuracy for 

 practical applications by 



^^0.125 (T-Trf)o (km) (6.15) 



where the zero subscript indicates that the difference between the tem- 

 perature and the dewpoint, Td, need be evaluated only at the earth's 

 surface. 



Consider now a parcel rising through some environmental distribution 

 of temperature. The dry adiabatic lapse of temperature with height is 

 greater than that observed on the average in the atmosphere (6 °C/km) 

 and thus the parcel becomes cooler, and more dense, than the environ- 

 mental air and will sink to its initial conditions with the removal of the 

 lifting force. If, on the contrary, anywhere in its trajectory it becomes 

 warmer, and thus less dense, it will become unstable and rise of its own 

 buoyancy through the environmental air. Past radio-meteorological 

 studies have taken the area between the adiabatic curves and the environ- 

 mental temperature distribution on a pressure ('^ height) versus tempera- 

 ture chart as a measure of atmospheric stability. On the general argu- 

 ment that the effects of atmospheric stability, or turbulence, influence 

 radio waves only through the radio refractive index, we shall extend the 

 above concepts to the radio refractive index. 



