Discussion
What is the refractivity of the atmosphere usually expressed in?
Answer: B
Because the refractive index differs from unity only in the sixth decimal place, it is quoted as N units, the excess over one multiplied by a million, giving values near three hundred at the surface. The gradient of N with height, typically minus forty N units per kilometre, is what bends the ray. Decibels, angles and percentages are not the convention. Scaling a quantity so that its interesting variation occupies convenient numbers is a small but useful piece of engineering practice.
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