Explanation: Waveguides and rotary joints are critical components in microwave systems that must be hermetically sealed to prevent moisture ingress. If a leak is present, water from rain or high humidity will enter these transmission lines. Water is a highly absorptive material for microwave frequencies. When water accumulates inside a waveguide or rotary joint, it causes significant attenuation of the RF signal, both transmitted and received. This substantial loss of power directly leads to a reduction in system sensitivity, resulting in the inability to detect weaker signals from distant targets. The effect is tied to wet weather because that's when moisture enters, and it persists until the components dry out. Other options are less likely: B) High atmospheric absorption affects all signals but is a general environmental factor, not a system fault leading to sudden specific loss. C) Dirt on the exterior is unlikely to cause such a drastic, wet-weather-dependent signal loss. D) Humidity in the transmitter might cause arcing or failure, not typically a selective loss of distant targets due to RF path attenuation.