Atmospheric Profile
Understanding Atmospheric Profiles
Radio waves do not travel in straight lines through the atmosphere. They bend, refract, and sometimes become trapped in atmospheric layers. The physics governing all of these effects is contained in the atmospheric profile — the vertical distribution of temperature, pressure, and humidity from the surface to the stratosphere.
Refractivity and Ray Bending
The atmosphere's radio refractivity N is defined as N = (n-1)×10⁶, where n is the refractive index. N depends on temperature, total pressure, and water vapor pressure. Under standard conditions, N decreases with altitude at approximately 40 N-units/km, causing radio waves to bend slightly downward (following the Earth's curvature), effectively extending the radio horizon by about 15% beyond the geometric line of sight.
Ducting and Anomalous Propagation
When the refractivity gradient exceeds approximately –157 N-units/km (a super-refractive condition typically caused by a temperature inversion or sharp humidity boundary), radio waves become trapped in a duct — a waveguide-like atmospheric layer that can propagate signals hundreds of kilometers beyond the normal horizon. Ducting is a major concern for radar systems, causing false targets from distant ground clutter that normally falls below the radar horizon.
Key Equations
N = (n−1)×106 = 77.6P/T + 3.73×105e/T²
P = pressure (hPa), T = temp (K), e = water vapor
Gradient:
dN/dh ≈ −40 N-units/km (standard)
K-factor:
K = 1/(1+Re(dN/dh)×10−6)
K = 4/3 standard atmosphere
Comparison
| Condition | dN/dh | K | Bending | Propagation |
|---|---|---|---|---|
| Standard | −40 N/km | 4/3 | Normal | Design reference |
| Sub-refractive | >−40 N/km | <4/3 | Less bending | Path loss higher |
| Super-refractive | −40 to −157 | >4/3 | More bending | Extended range |
| Trapping (duct) | <−157 N/km | Negative | Duct | Anomalous prop |
| No refraction | 0 | 1 | Straight line | Geometric |
Frequently Asked Questions
How are atmospheric profiles measured?
Radiosondes — instrument packages carried aloft by weather balloons — are the primary measurement tool. They transmit temperature, pressure, and humidity data in real time as the balloon ascends through the troposphere and lower stratosphere. Worldwide, approximately 800 radiosonde stations launch balloons twice daily at coordinated UTC times (00Z and 12Z). Between launches, numerical weather prediction models interpolate profiles for any location and time.
What is the modified refractivity M-profile?
Modified refractivity M(h) = N(h) + 0.157h accounts for the Earth's curvature by adding a linear term proportional to altitude. When plotted as M vs. height, a trapping duct appears as a region where M decreases with height — an easily visualized diagnostic. RF propagation tools use M-profiles extensively to identify and characterize ducting conditions that will affect radar and communication system performance.
Does the atmospheric profile affect 5G mmWave?
For terrestrial 5G mmWave links (short range, low elevation), atmospheric profile effects are minimal — the propagation path is too short for significant refraction. For 5G NTN (Non-Terrestrial Networks) using LEO satellites, the atmospheric profile is critical: the gaseous absorption depends on the integrated water vapor along the slant path, and tropospheric scintillation (caused by turbulence in the atmospheric profile) creates rapid signal fluctuations that affect link margin and fade statistics.