electromagnetic wavefront. The path followed by electromagnetic energy in the atmosphere, whether
direct or reflected, usually is slightly curved; and the speed is affected by temperature, atmospheric
pressure, and the amount of water vapor present in the atmosphere, which all affect the refractive index.
As altitude increases, the combined effects of these influences, under normal atmospheric conditions,
cause a small, uniform increase in signal speed. This increase in speed causes the travel path to curve
slightly downward, as shown in figure 1-13. The downward curve extends the radar horizon beyond a line
tangent to the earth, as illustrated in figure 1-14.
Figure 1-13.Wavefront path.
Figure 1-14.Extension of the radar horizon.
The reason for the downward curve can be illustrated using line AB in figure 1-13. Line AB
represents the surface of a wavefront with point A higher in altitude than point B. As wavefront AB
moves to the point represented by AB, the speed at A and A is faster than the speed at B and B since A
and A are at a greater altitude. Therefore, in a given time, the upper part of the wavefront moves farther
than the lower part. The wavefront leans slightly forward as it moves. Since the direction of energy
propagation is always perpendicular to the surface of a wavefront, the tilted wavefront causes the energy
path to curve downward.