132 



METEOROLOGY— FORECASTING 



;(, that of the second nietliuiii, oi.^ the angle wliieli the 

 ray in leaving the first medium makes Avith tlie boun- 

 dary, and a, the angle wliieli the ray in penetrating 

 the second medium makes with the boundary. In the 

 case of the atmosphere (one medium with a variable 

 refractive index) this expression can Ije modified to 

 relate the gradual bending of a ray to the manner in 

 which the refractive index varies. The value of the 

 refractive index, n, at any particular point in the 

 atmosphere can be determined from measurements of 

 pressure, temperature, and humidity by substitution 

 in the formula. 



n=l-\--rj^[v + 



4800A 



) 



10- 



or, expressed differently. 



{n - 1) 10« 



79 



r 



, 4S00c\ 

 is measured in °K, 



in which tlie temperature, 7', is measured in "K, and 

 the atmospheric pressure, [i. and vapor pressure, c, 

 in millibars. 



Modified licfractire litdc.r. It is considered more 

 convenient, in problems of radio propagation, to de- 

 fine a .slightly different quantity il/, which is related 

 to the index of refraction by 



M 



(-^0 



10^ 



in which /( is the index of refraction, a is the radius 

 of the earth (31 X 10" ft) and h is the height above 

 the surface of the earth (measured in the same units 

 as a). In terms of pressure, temperature, humidity, 

 and height, M is given by 



79 / , 4800c\ 

 Y 



M 



P + 



-,+-10" 



in which the nuits of measurement used are the same 

 as above. The rate at which M increases with altitude 

 is given by 





dh 



10" 



= -0.039-1-0.157 



dM 

 dh \(lli a/ 



which in the standard atmosphere is 

 dM 

 dh 



= 0.118 M unit per meter 



= 0.036 M unit per foot. 



Psijcliromeiric Noiitograiii. Eadiation Laboratory 

 (MIT) has developed a nomogram with which to 

 compute the modified index of refraction from the 



necessary meteorological i)aranieters. This chart is 

 known as the psychrometric nomogram. (See p. 131.) 



Meteorological Tekjis 



Ahsohiic Humidiiij. The mass of water vapor pres- 

 ent in a unit volume of air is known as the absolute 

 humidity of the air. It is another way of expressing 

 the water vapor density. 



Specific llumiditij. The specific humidity of moist 

 air is the ratio of the weight of water vapor mixed 

 with the air to the weight of the moist air. If p is the 

 barometric pressure and e is the partial pressure of 

 the water vapor, then the specific humidity is given by 



q = 622 — — g per kg . 



p — 0.37^e 



Mixing Ratio. The ratio of the mass of water vapor 

 mixed with unit mass of perfectly dry air is known 

 as the mixing ratio and may be expressed as 



e , 



w = 622 g per kg . 



p- e 



Helalive Humidiiij. The ratio of the actual water 

 vapor pressitre to the saturation vapor pressure at the 

 same temperature is kiKnvn as the relative humidity 

 of moist air. If e and e^ are the respective vapor pres- 

 sures, then (in per cent) the relative humidity is 

 expressed as 



RH = - X 100 . 



Cs 



^yet Bulb Temperature. The lowest temperature 

 to which a wetted ventilated thermometer can be 

 brought by evaporation is called the wet bulb tempera- 

 ture. It is not strictly an air temperature. 



Air Mass. An extensive body of air which approxi- 

 mates horizontal homogeneity is known as an air 

 mass. The four principal types are illustrated by the 

 accompanying table. 



Front. The surface of separation between dissimilar 

 air masses is known as a frontal surface. On a surface 

 weather map a "front" is the intersection of this sur- 

 face with the surface of the earth. 



Dry Adiahatic Lapse Rate. When dry air ascendS 

 so as to expand adiabatically, it is said to cool at the 



