How do I calculate the rain rate exceeded for a given percentage of time at a specific location?
Rain Rate Statistics
Rain rate is measured in mm/hr and represents the vertical depth of water accumulated per hour if the rain were collected in a flat container. The instantaneous rain rate varies dramatically: most of the time it is zero (not raining), and during storms it can peak at 50-150 mm/hr for brief periods. The statistics describe the cumulative distribution of rain rate: the percentage of time that a given rate is exceeded.
| Parameter | Free Space | Urban | Indoor |
|---|---|---|---|
| Path Loss Model | Friis (1/r²) | Okumura-Hata | IEEE 802.11 |
| Fading Margin | 0 dB | 10-30 dB | 5-15 dB |
| Multipath | None | Severe | Moderate-severe |
| Typical Range | Line of sight | 1-30 km | 10-100 m |
| Shadow Fading (σ) | 0 dB | 6-12 dB | 3-8 dB |
Frequently Asked Questions
Why 0.01% of the time?
0.01% of a year = 52.6 minutes. This is the standard design threshold for high-reliability communications links (99.99% availability). Lower reliability requirements (99.9% = 8.7 hours) use the rain rate exceeded at 0.1%, which is typically 2-3× lower than R0.01.
How do I use this for satellite links?
For satellite links: the rain column extends from the ground to the rain height (hr, typically 2-5 km depending on latitude). The slant-path attenuation is: A = γR × (hr - hs) / sin(El), where hs is the station altitude and El is the elevation angle. The effective path through the rain is the rain height divided by the sine of the elevation angle.
Are 1-minute rain rates different from hourly?
Yes. 1-minute integration captures the peak rain rates during storms, which are much higher than hourly averages. A storm average might be 10 mm/hr, but the 1-minute peak during the same storm could be 50-100 mm/hr. Link design must use 1-minute statistics because the link outage occurs during these brief intense periods.