So this series (i think it was more than one book but not sure) probably dates from the 80s. It was centered on a system of magic in which the female protagonist could sense invisible creatures - sort of amorphous as I recall - that floated/flew through the atmosphere. I think she was able to ride them. And the higher in the atmosphere they came from, the bigger and more powerful they were. The largest may have been dreadnoughts, or behemoths. Driving me batty. Please help!