index takes into account the cloud height of the storm, the radii of the eye, and other similar factors, using satellite imagery, and then produces a number, with the maximum value on the scale being 100. Any storm with a value of zero or lower are not annular, while storms with positive indices are considered to be annular. Tropical cyclones with higher indices have more annular characteristics than those with lower indices. Although tropical cyclones can achieve annular characteristics across a wide spectrum of intensities, annular storms are typically strong tropical cyclones, with average maximum sustained windspeeds of . In addition, storms attaining annular characteristics are less prone to weakening as a result of negative environmental factors. Annular cyclones can maintain their respective peak intensities for extended periods of time unlike their asymmetric counterparts. Following peak intensity, such systems will tend to gradually taper off. This unusual intensity persistence makes their future intensities difficult to forecast and often results in large forecast errors. In an analysis of hurricanes in the East Pacific and North Atlantic between 1995 and 1999, Knaff and Kossin observed that the National Hurricane Center underestimated the intensity of annular hurricanes 72 hours out by 18.9 kn (35.0 km/h, 21.7 mph). A survey of Pacific typhoons between 1990 and 2009 found only 12 with annular characteristics, representing an occurrence rate of 4 percent. Transition from asymmetric cyclones and necessary conditions Tropical cyclones can become annular as a result of eyewall mesovortices mixing the strong winds found in the eyewalls of storms with the weak winds of the eye, which helps to expand the eye. In addition, this process helps to make the equivalent potential temperature (often referred to as theta-e or ) within the eye relatively uniform. This transition takes roughly 24 hours to complete and can be considered a type of eyewall replacement cycle. Winds have also been found to decrease in a stairstep like fashion within the radius of maximum wind, which may indicate that more wind is mixed between the eye and eyewall as cyclones strengthen, which helps to explain why annular characteristics are generally exclusive to storms of higher intensities. The intensity of annular systems is typically greater than 83.5% of the maximum potential intensity, suggesting that the conditions in which storms gain annular characteristics are generally conducive for tropical cyclone persistence and intensification. Annular tropical cyclones also require low wind shear, and