During light rain or mist, this water is evaporated instantly when it reaches the high temperature combustion chamber. Its sort of like throwing tiny bits of water onto an extremely big fire, the water will have little effect as long as there is enough fuel burning. Planes normally avoid extremely high rain and snow simply because it provides a smoother, safer, and more efficient flight. Think about it this way, the period of extremely heavy rain is never too long, so making a few loops around the airport above the clouds is not too difficult an endeavor. Lastly, note that the design of the jet engine has an automatic water bypass system. The shape of the first level fan blades direct water, snow, and ice through a bypass chamber, preventing high amounts from ever reaching the center of the engine. They are instead expelled without hurting power or fuel consumption. Watch this video, it's an Air France Boeing 747 landing in heavy rain. Notice what happens to the engines when the rain hits them. Not to mention this has the incredible 747 engine sound. http://www.flightlevel350.com/Aircraft_Boeing_747-400-Airline_Air_France_Aviation_Video-8581.html