on of the troposphere is relatively frequent. By postulating layers of varying thickness, horizontal area and surface roughness, and of varying lifetime it is possible in principle 7.5 gm/m³ of water to interpret many of the features of tropospheric propagation. In- vapour at surface deed, some experimental results (e.g. the small distance-depend- ence of v.h.f. fields at times of anomalous propagation) can be explained by calculating the reflection coefficient of model layers of constant height and with an idealised height-variation of refrac- tive index such as half-period sinusoidal, exponential, etc. The correlation between field strength and layer height has also been examined and some results can be explained qualitatively in terms of double-hop reflection from extended layers. Progress in ray-trac- ing techniques has also been made. Nevertheless, the problems, of calculating the field strength variations on particular links remain formidable, and for many practical purposes statistical and empiri- f (GHz) cal techniques for predicting link performance remain the only Figure 9.9 Zenith attenuation (dB) in clear air solution (see CCIR Report 238). 9/16 The lonosphere and the Troposphere shows the zenith attenuation from sea level for an average clear Because precipitation (and to a smaller extent atmospheric gases) atmosphere as a function of frequency. It illustrates the 'window' absorb microwaves, they also radiate thermal-type noise. It is often regions mentioned in the survey of propagation modes. From an convenient to specify this in terms of an equivalent blackbody altitude of 4km, the values would be about one-third of those shown. temperature or simply noise temperature for an antenna pointing in This indicates the potential application of frequencies above 40GHz a given direction. With radiometers and low-noise receivers it is for communication on paths located above the lower layers of the now possible to measure this tropospheric noise and asses its im- troposphere. portance as a factor in limitin