Second case when considering propagation in the clear neutral atmosphere.
Diagram shows the behaviour of a ray in the atmosphere for:
standard atmosphere with ref. gradient –40 N-units/km, in which the ray is bent slightly towards Earth
with gradient less negative, where ray path is away from Earth (said to be sub-refractive conditions)
with gradient more negative, where ray is bent more strongly towards Earth until the gradient of –157 N-units/km is reached when ray travels parallel to Earth’s surface. At this point, the ray is ducted.
Between –40 and –157, the conditions are said to be super-refractive.
