Sporadic E openings on the higher frequency bands may often be predicted by observing the characteristics of the 28-MHz band. The geometry of propagation is such that as the skip distance decreases on the 28-MHz band, the highest frequency that will be reflected by a sporadic E cloud is increasing. Tropospheric Ducting Tropospheric ducting of vhf signals is quite common and is the result of change in the refractive index of the atmosphere at the boundary between air masses of dif- fering temperatures and humidities. Using a simplified analogy, it can be said that the denser air at ground level slows the wave front a little more than does the rarer upper air, imparting a downward curve to the wave travel. Ducting can occur on a very large scale when a large mass of cold air is overrun by warm air.
Temperature inversions occur most frequently along coastal areas bordering large bodies of water.
This is the result of natural onshore movement of cool, humid air shortly after sunset when the ground air cools more quickly than the upper air layers. The same action may take place in the morning when the rising sun warms the upper air layers. Less spectacular communica- tions are possible as a result of simple temperature inversion, where ducting is not believed possible.