為何下雨時手機通話差了?

Such phenomenon is called "rain fade".

"Rain fade" is an interruption of wireless communication signals as a result of rain (or snow) droplets whose separation is comparable to the electromagnetic signal wavelengths. Severe attenuation of the signal can occur, leading to a degradation or loss of communication.

Rain fade will affect, not only mobile phone communication, but also satellite internet connections as well as satellite television and other systems.

Rain fade usually does not last long. Once a heavy shower or squall has passed, normal communications returns.

You may refer to the web-page below:

http://searchmobilecomputing.techtarget.com/definition/rain-fade





2015-05-21 14:11:34 補充：
Q:空氣中多塵是否手機通話好些?
A: No. There is publication that wireless signal transmission after rainfall is far better than that before rainfall. This is known as the "clear sky sffect". The reason is explained as due to washing out of atmospheric aerosol (dust) by rain water.

你手機灌水了，抽濕冇事。