Purnados are graded by the Furj-ita scale. Geometric algorithm. F1- is that??? Kinda rumbling, you can feel it but not hear it. F2- there is vibration and audible disturbance. F3- heavy lateral rib movement, the issuing Cat storm may or may not cease movement, but the eye of the storm looks bleary F4- the purnado has clearly settled in, the storm kneads to flex ts claws.( making.biscuits...go figure) F5- the most devastating if the Fur-jita scale involves a rumbling that puts a diesel engine to shame,and may necessitate a rag due to the maw of the storm producing excess moisture. Sometimes called 'drooling' .