 Following a pass through the storm by a United States Navy reconnaissance plane , Ethel was declared a " severe hurricane " with winds reaching 160 mph ( 260 km / h ) , equivalent to a modern @-@ day Category 5 hurricane on the Saffir – Simpson Hurricane Scale . At the time these winds were measured , a barometric pressure of 972 mbar ( hPa ; 28 @.@ 7 inHg ) was recorded , the lowest in relation to the hurricane . However , shortly thereafter , cool , dry air began to entrain the storm , causing it to rapidly weaken . In a six @-@ hour span , the storm suddenly weakened to a Category 1 hurricane , a decrease of 70 mph ( 110 km / h ) . 
