Using Airmagnet to determine wireless signal strength.
Answers:
0 dbm is plentifully stronger that -100dbm
-10 dbm is something like 10 times weaker than 0dbm
-20 dbm is roughly 100 times weaker than 0dbm
-30 dbm is almost 1,000 times weaker than 0dbm
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-100 dbm is in the region of 10,000,000,000 times weaker than 0dbm