Experiments on the attenuation-rain-rate relation for short millimeter waves

The effects of the rain accumulation rate on radio-wave attenuation at 137.5 GHz are studied. Measurements are performed along a 1025 m terrestrial path equipped with two weighting rain gauges of high temporal resolution. It is found that with many short-term rains, the attenuation-rain-rate curve has a hysteresis shape: the attenuation factor at the progressive phase of rain is less than that at the regressive phase for the same rain rate. The difference can be 2-3 times as large. In many cases, the experimental range of the attenuation factor obtained for the same rain rate with long-term rains completely covers the calculated range obtained from various empirical distributions of raindrops in size.