hot air rises, this is what generally causes wind. When the conditions are just right, they can form a typhoon, for this to happen the air needs flat terrain and a good heat source, warm water can store a lot of heat, and the water-surface is relatively flat. When the ocean temperatures rise, water stores more heat, and more places can form the conditions to form typhoons. Once typhoons leave the area where the conditions allow their formation, they start to loose energy and because the areas where they can form are growing, they both have more time to grow AND loose less energy before hitting land
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