Tropospheric radio weather and extended range
Suddenly hearing repeaters hundreds of kilometres away on 2 m? That is tropo: weather with warm air above cold air guides VHF signals far beyond the horizon. The page explains how to recognise such conditions and calculates the refractivity of the air.
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What the page does
- Refractivity N from temperature, pressure and humidity
- Gradient and detection of ducting
- Effect on the radio horizon
- Typical weather patterns for extended range
- Links to the link and horizon calculators
Background
Air refractivity is given in N-units: N = 77.6 / T · (p + 4810 · e / T), with T in kelvin, p and water vapour pressure e in hPa. In the standard atmosphere N decreases by about 40 units per kilometre. If N drops faster than 157 per kilometre, waves bend more strongly than the earth – they are trapped in a layer (ducting).
In practice
How it works: enter temperature, pressure and humidity for several heights, for example from a radiosonde ascent. The calculator determines refractivity at each height and the gradient between them and classifies it: normal, super-refraction, ducting. When pressure rises over several days and nights become calm, it is worth listening for distant repeaters.
Frequently asked questions
When does tropo extended range occur?
Often during stable high pressure, especially late summer and autumn, frequently at night and early morning.
Does it affect HF?
No, HF is governed by the ionosphere. Tropo mainly matters from about 50 MHz.