The flow around a tropical cyclone is often well approximated as parallel to circular isobars, such as in a vortex. A strong pressure gradient draws air toward the center of the cyclone, a centripetal force nearly balanced by Coriolis and centrifugal forces in gradient wind balance. The viscous secondary flow near the Earths surface converges toward the center of the cyclone, ascending in the eyewall to satisfy mass continuity. As the secondary flow is drawn upward the air cools as its pressure falls, causing extremely heavy rainfall and releasing latent heat which is an important driver of the storms energy budget.