Although much more common than hurricanes and tornadoes, thunderstorms can be equally dangerous. The structure of the selection above can best be described as

Contrast Structure using conjunction "although"

The conjunction "although" in this sentence is used to describe contrasting ideas present in two clauses.

Explanation:

"Although" is normally used as subordinating conjunctions i. e. to connect subordinate clause to the main clause. The sentence describes two contrasting or different ideas. From the idea, that thunderstorms are more common than hurricanes and tornadoes, one expects they would be less dangerous. But in contrast it is not the case (according to the author) - they are equally dangerous. Hence "although" is used to connect two contrasting ideas of two clauses.