APPLICATION OF FORECASTING TECHNIQUES AND CLIMATOLOGY 



119 



^^^ Radio-Meteorology 



Evidenc;es of Nonstandard PiiorAGATiON 



Since the start of the war, cases of very loug radio 

 ranges and radar coverages, together with extreme 

 variations of these quantities, have become well Ivnown 

 to personnel working at microwave frequencies. Such 

 phenomena, when due to influences acting ou the 

 propagated elcctroniaguetic waves and not to freak 

 behavior in set performance, have been classed under 

 the term nonstandard propagation. It has been found 

 that nonstandard propagation (such as is illustrated 

 by the behavior of microwaves when they are con- 

 strained to follow a path of such curvature that the 

 rays remain close to the surface of the earth and hence 

 reach otherwise inaccessible targets — a phenomenon 

 frequently referred to as trapping) is directly associ- 

 ated with certain conditions that occur in the lower 

 levels of the atmosphere (normally below 5,000 ft) 

 which have been given the name of ducts. Detailed 

 analyses of the structure of ducts have been presented 

 adequately in the previously mentioned reports. Hence 

 the paragraphs immediately following give only a brief 

 and somewhat simplified description of the meteor- 

 ological elements associated with ducts. 



Meteokological Conditions Associated 

 AviTH Ducts 



Remarks on Pressure, Temperature, and HumidUij. 

 The meteorological situations in which trapping of 

 microwaves occurs involve certain types of stratifica- 

 tion in the lower levels of the atmosphere. The amount 

 of stratification is dependent on the vertical distribu- 

 tions of pressure, temperature, and humidity. 



Although the atmospheric pressure at any particular 

 elevation and, to a lesser extent, the rate at which 

 pressure decreases with altitude may vary from one 

 time to another, these variations are relatively unim- 

 portant as far as their direct infliience on propagation 

 is concerned and so may be neglected in practical 

 considerations. 



On the other hand, temperature and its change with 

 altitude do have an immediate bearing on duct forma- 

 tion. Under more or less average conditions through- 

 out the troposphere, the temperature decreases with 

 increasing altitude and the term "temperature lapse 

 rate" is defined as the rate of decrease of temperature 

 with height (and consequently is usually expressed in 

 degrees Fahrenheit per 1,000 ft or degrees centigrade 

 per kilometer). For reference purposes a "standard" 

 lapse rate has been taken as 3.47 F per 1,000 ft. 



(Further details of the National Advisory Committee 

 on Aeronautics standard atmosphere are given in 

 the Appendix on page 130.) Under certain conditions 

 the temperature throughout a layer of the atmosphere 

 may increase with height, in which case a temperature 

 inversion (Figure 10) is said to exist. 



TEMPERATURE - 



Figure 10. Vertical variation of temperature showing 

 a ground inversion EF and an elevated inversion BC. 

 The slopes of the portions FG, AB, and CD denote 

 standard conditions, a decrease of temperature with 

 elevation. 



In general the lapse rate of temperature is im- 

 portant in meteorology because of its relationship to 

 the vertical stability of the atmosphere, that is, to the 

 feasibility with which vertical air currents can de- 

 velop. It turns out that, except within clouds or regions 

 of active precipitation, the stability conditions can be 

 closely evaluated from a knowledge of the actual lapse 

 rate relative to the "dry adiabatic" lapse rate (approxi- 

 mately 6.5 F per 1,000 ft) . If the actual lapse rate is 

 larger than the dry adiabatic, i.e., if the temperature 

 decreases at a rate greater than 5.5 F per 1,000 ft in 

 elevation, any vertical currents which develop will 

 tend to exaggerate in intensity, and a condition of 

 unstable equilibrium (Figure llA) will exist. Con- 

 versely, if the actual lapse rate is less than the dry 

 adiabatic or, especially, if a temperature inversion is 

 present, the deA^elopment of vertical currents will be 

 hindered and the air will tend to become horizontally 

 stratified. This is the case of stable equilibrium (Fig- 

 ure IIC). The in-between case, in which the actual 

 temperature lapse rate is the same as the dry adiabatic, 

 is that of neutral equilibrium (Figure IIB). 



