Why do signal of satellite TV disappear when it rain?

Satellite signals are transmitted from a satellite above the earth's atmosphere. To reach the satellite dish, they need to pass through the atmosphere. Normally, the signals pass without a problem but they can be affected by clouds and rain. Most satellite receivers receive enough signal even in times of atmospheric disturbance but extreme weather may disrupt the signal enough to prevent reception.

If the problem is happening on a regular basis, it might be worth checking the alignment of the dish and the LNB (the receiver unit on the arm in front of the dish). If the dish is mis-aligned even by a very small amount, it will not receive a full strength signal so poor weather will affect the quality of the signal more than normal. Checking the alignment is a very quick job for a satellite installer (and much slower for most other people). If alignment doesn't help, it's time to check the cable from the dish to the receiver inside.