Conditional Instability of the Second Kind (CISK)
It takes a large force to cause convection in unsaturated air in a state of conditional instability. The atmosphere above the tropics, where typhoons occur is generally unsaturated.
A typhoon gathers moist air to its center inside the boundary layer, and at the same time that gathering of moist air pushes the air around the center to saturation point. When the air reaches saturation point, it fulfills the conditions of conditional instability, so buoyant force causes it to rise, causing the air pressure to fall, which in turn makes the vortex of the typhoon stronger, and the gathering of air to the center stronger again.
Conditions which would not become unstable on a small scale, but become unstable when combined with the effect of a large scale vortex, are called Conditional Instability of the Second Kind.