212 



TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 



and humidity in the layer considered. If the point 

 is at the lower right relative to this straight line, M 

 decreases with height in the layer chosen; that is, a 

 duct exists. If the point is at the upper left of the 

 straight line, M increases with height and there is 

 no duct. 



The rate of change of M, (dM/dh), may be 

 obtained from the diagram by measuring the hori- 

 zontal distance from the point to the line and 

 multiplying by the function of the temperature f(T) 

 given in the table on Figure 27. The result is the 

 value of dM/dh, the rate of change of M , in M units 

 per 100 m. This quantity is negative when the point 

 is to the right of the line and positive when the point 

 is to the left of the line. 



It is seen at once from the diagram that for small 

 values of the moisture lapse an extremely steep 

 temperature gradient is required in order to produce 

 a duct (lower left part of the diagram). In cold air 

 such as is found in the arctic the total moisture is 

 small, and hence the moisture gradient will in general 

 be quite small. Ducts will then only occur when a 

 very strong temperature inversion exists. 



Strong temperature inversions occur only under 

 special meteorological conditions which will be 

 discussed below. Ordinarily the temperature of the 

 air decreases with height; and this will put our 

 representative point into the upper part of Figure 

 27. A duct can then exist only when the moisture 

 lapse is large enough, so that the representative 

 point falls to the right of the appropriate slanting 

 line. Such conditions are common in the lower 

 atmosphere. This leads to a wet duct, which is 

 determined almost completely by the moisture lapse. 



17 32 Physical Causes 



of Stratification — Turbulence 



There are three basic meteorological factors which 

 tend to modify the temperature and moisture distri- 

 butions in the lowest layers of the atmosphere. These 

 are: (1) advection, (2) nocturnal cooling (overland), 

 and (3) subsidence. 



Advection is a meteorological term used to desig- 

 nate the horizontal displacement of air having 

 particular properties. Advection is of great interest 

 in propagation problems particularly because it leads 

 to an exchange of heat and moisture between the air 

 and the underlying ground or sea surface and thus 

 affects the physical structure of the lowest layers. 



Nocturnal cooling over land is caused by a loss of 



heat from the ground by infrared (heat) radiation. 

 The cooling of the ground is communicated to the 

 lower layers of air and leads to the establishment of 

 a low-level temperature inversion. 



Subsidence means a slow vertical sinking of air 

 over a very large area. It is most likely to be found 

 in regions where barometric Highs are located. 

 Subsidence tends to produce a temperature inversion 

 and also produces very dry air which, spreading out 

 over a humid surface, creates a situation which is 

 favorable for the formation of a duct. 



The processes (1) and (2) change the physical 

 characteristics of the air through transfer of heat 

 or moisture between the air and the underlying 

 surface of the ground or sea. The operating factor 

 in this exchange is turbulence. The main features 

 of turbulence in the lower atmosphere are outlined 

 briefly below. 



Convection occurs spontaneously whenever the 

 decrease of temperature with height exceeds a value 

 of about 1 C per 100 m. This convective condition 

 is usually produced as a result of the heating of the 

 ground by the sun's rays. Even with a cloudy sky 

 the diffuse daylight often is strong enough to produce 

 moderate convection. On a hot summer day convec- 

 tion over land extends to great heights. Convection 

 mixes the air thoroughly and thus causes a uniform 

 distribution of moisture and a uniform decrease of 

 temperature with height of about 1 C per 100 m. 

 Hence even moderate convection tends to produce a 

 smooth M curve which varies linearly with height. 

 Standard conditions may therefore be assumed to 

 prevail on clear summer days (and not infrequently 

 on clear days in the cooler seasons) from the hours 

 of late morning until late afternoon, during which 

 time convection is most active. 



Frictional turbulence occurs frequently in the lower 

 atmosphere even in the absence of convective condi- 

 tions. It is caused by the wind and requires the 

 presence of at least light winds, but with moderate 

 or strong winds the effect is more pronounced. In 

 conditions of calm or with a gentle breeze, frictional 

 turbulence is confined to the lowest strata. Moderate 

 or strong winds develop a layer of intense turbu- 

 lence, caused by friction of the air at the irregularities 

 of the ground. This layer is usually quite well defined 

 in height and extends to an average elevation of 

 about 1,000 m over land. Over a relatively smooth 

 sea where friction is small the height of the layer is 

 much reduced. In this frictional layer the air becomes 

 thoroughly mixed; the vertical temperature gradient 



