Input Parameters
Uses standard meteorological units: pressure in hectopascals, temperature in Celsius, humidity in percent. Internally converts to Kelvin for the refractivity equations.
Results
| Quantity | Value | Unit |
|---|---|---|
| Air temperature | 15.00 | °C |
| Temperature | 288.15 | K |
| Total pressure | 1,013.25 | hPa |
| Relative humidity | 50.0 | % |
| Saturation vapor pressure | 17.017 | hPa |
| Water vapor pressure | 8.508 | hPa |
| Dry term (Ndry) | 272.872 | N-units |
| Wet term (Nwet) | 38.222 | N-units |
| Total refractivity (N) | 311.095 | N-units |
| Refractive index (n) | 1.000311095 | — |
| Modified refractivity (M) | 311.095 | M-units |
Example Scenarios
Typical inputs and their computed refractivity. Use them to validate your workflow or to compare environmental regimes.
| # | Scenario | P (hPa) | T (°C) | RH (%) | Alt (m) | N (N-units) | n (—) |
|---|---|---|---|---|---|---|---|
| 1 | Standard sea level (moderate) | 1,013.25 | 15.00 | 50.0 | 0 | 311.095 | 1.000311095 |
| 2 | Hot and humid | 1,000.00 | 30.00 | 80.0 | 50 | 393.448 | 1.000393448 |
| 3 | Cold and dry | 900.00 | -10.00 | 20.0 | 1,500 | 268.492 | 1.000268492 |
| 4 | High altitude cool | 800.00 | 5.00 | 40.0 | 2,500 | 240.000 | 1.000240000 |
Formula Used
The calculator uses the standard radio refractivity definition N = (n − 1) × 106 and separates dry and wet components:
Ndry = 77.6 · (P / T)Nwet = 3.73 × 105 · (e / T²)
Here P is total air pressure in hPa, T is absolute temperature (K), and e is water vapor partial pressure (hPa).
The partial pressure is estimated from relative humidity using the Tetens saturation vapor pressure relation:
e = RH · es(Tc) / 100 with
es(Tc) = 6.112 · exp(17.62·Tc / (243.12 + Tc)) and Tc in °C.
The refractive index is then n = 1 + N × 10−6.
If altitude is provided, the modified refractivity is M = N + 0.157 · h (with h in meters).
These expressions are commonly used in atmospheric propagation studies and provide accurate first-order estimates across typical tropospheric conditions.
How to Use This Calculator
- Enter the total pressure, air temperature, and relative humidity. Optionally add altitude for the modified refractivity.
- Click Compute to generate dry, wet, and total refractivity along with the refractive index and optional modified refractivity.
- Review the results table. Use the Download CSV or Download PDF buttons to save the outputs.
- Compare with the provided Example Scenarios to understand how environmental changes affect refractivity.
- Use consistent units: pressure in hPa, temperature in °C, humidity in percent, altitude in meters.
- For batch analyses, repeat with different inputs and export each set as needed.
Frequently Asked Questions
1) What is atmospheric radio refractivity?
It quantifies how much the atmosphere bends electromagnetic waves. It is defined by N in N-units, where N = (n − 1) × 106 and n is the refractive index.
2) Which units should I use?
Pressure in hPa, temperature in degrees Celsius, relative humidity in percent, and altitude in meters. The tool converts temperature to Kelvin internally.
3) How is water vapor pressure determined?
From relative humidity and saturation vapor pressure via the Tetens relation. The product gives the partial pressure of water vapor e in hPa.
4) What is modified refractivity M?
It incorporates height to better describe ducting and propagation. The calculator uses M = N + 0.157 · h with height h in meters.
5) How accurate are these equations?
They are widely accepted first-order approximations for tropospheric conditions. For extreme environments or high precision work, consult advanced models and radiosonde data.
6) Can I export and share the results?
Yes. Use the CSV or PDF buttons for the results and example tables. CSV opens in spreadsheets; the PDF is formatted using an embedded client-side generator.