When Typhoon Dujuan swept past Hong Kong, it possessed a rare and feared characteristic of the most severe of storms - double eye walls surrounding its centre. The Observatory has not seen such a phenomenon since Typhoon Elsie in 1975 and Typhoon Hope in 1979 - both intense typhoons with wind speeds exceeding 200km/h that forced the hoisting of signal No10. The eye is the centre of a typhoon, usually tens of kilometres in diameter. The clouds surrounding the eye form so-called walls. Most typhoons have a single eye wall. The most intense typhoons sometimes have a double-eye-walled structure. Such a structure often occurs as a typhoon intensifies and usually dissipates in one to two days, leaving one larger, more powerful eye wall. Dujuan's inner eye was 20km in diameter, with the outer wall beginning 100km from the centre, the observatory said. By late last night, the outer wall had already passed Hong Kong, bringing average wind speeds of more than 100km/h in some exposed areas.