Air is pumped into a spherical balloon so that its volume increases at a constant $36\pi$ cubic centimeters per second. At the instant when the balloon's surface area is $144\pi$ square centimeters, at what rate is the surface area increasing, in square centimeters per second? (For a sphere, $V=\frac{4}{3}\pi r^3$ and $S=4\pi r^2$.)