Rainbands exist on the edges of tropical cyclones, and point towards the cyclone's center of low pressure. Rainbands within tropical cyclones need moisture and a low level pool of cooler air. Bands located to from a cyclone's center move outward. They can cause heavy rains and squalls of wind, as well as tornadoes. Some rainbands move closer to the center, forming a secondary, or outer, eyewall within intense hurricanes. Spiral rainbands are such a basic part of a tropical cyclone that in most tropical cyclone basins, use of the satellite-based Dvorak technique is the main method used to determine a tropical cyclone's maximum sustained winds. Using this method, the area of spiral banding and difference in temperature between the eye and eyewall is used to estimate the maximum sustained wind and a central pressure. 