is the ninja

Ninja

A or was a covert agent or mercenary of feudal Japan specializing in unorthodox arts of war. The functions of the ninja included espionage, sabotage, infiltration, and assassination, as well as open combat in certain situations...

 art of understanding and using meteorology

Meteorology

Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

as a strategic weapon. It allowed ninja to foresee weather changes and to use them as an advantage. By knowing nature's changes, animal behaviour, or atmospheric signs, one could use rainy weather or a hot sunny period as strategic elements to weaken and defeat the enemy.