- Troposphere (clear air) -Super-refraction and ducting

• Standard atmosphere

  • -40 N units/km (median)
  • temperate climates

• Super-refractive atmosphere

  • < -40 N units/km
  • warm maritime regions e.g. west Africa

• Ducting

  • < -157 N units/km

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• Summary of previous diagram

• With super-refractivity or ducting conditions, radio signals can travel to far greater distances than is normally the case and so the potential for interference arises when frequencies are shared.