Need and yield are making cat bonds roar. Low interest rates and a hunger for uncorrelated returns are fueling investor demand for bonds insuring against hurricanes, pandemics and other catastrophes, pleasing insurers eager to lay off risk. Yields on these instruments have fallen even as issuance has soared. Disaster, if it strikes, may only increase their popularity.In a typical cat bond, an issuing insurer or government agency pays a healthy interest-rate premium to investors over about three years. If a pre-specified event happens, the buyer forfeits some or all of the principle. So if, say, an earthquake above a certain