Tropical cyclones are like giant engines that use warm, moist air as fuel. That is why they form only over warm ocean waters near the equator. The warm, moist air over the ocean upsurges upward from near the surface. Because this air travels up and away from the surface, there is fewer air left near the surface. Another way to say the similar thing is that the warm air rises, causing an area of lower air pressure below.
|A cumulonimbus cloud. A tropical cyclone has many of these, they form huge, circular bands. |