Why do frogs croak more before a storm?

Noises all become louder and more vibrant just before rain because the sound is reflected and magnified by the low clouds. The croaking of frogs will echo loudly if rain is imminent.

Actually, a storm is preceded by humid air. Frogs have to keep their skins wet to be comfortable, and moist air allows them to stay out of the water and croak longer.

ACTUALY... the frogs DO croak more before a storm because it means it is going to rain and they ponds will fill more and there will be more ponds made etc... and so they croak to atract their mates so because now that there is more water there is more oxygen in the water and more space for their little tadpoles to develop in.
in my animal science class, the teacher had a treefrog and before a storm the little frog would croak

Seems to be related to detecting air humidity, and understanding that this makes a good environment for mating

When male frogs croak, they are either sending out a signal to search for possible mating partners or they are trying to protect their territory from other male frogs. Mating and territorial protection are more likely to happen after it rains because the ground tends to be wet making it a comfortable environment for frogs. They prefer wet, dark areas because they need moisture to keep their amphibian skin healthy. If put in a hot environment, in direct sunlight, a frog is much more likely to dry out.

https://www.reference.com/pets-animals/frogs-croak-after-rains-ae1ec9148c42d4d3

4. At the tropics, some species have a rain croak, as they perceive air humidity before a rainfall.

http://news.softpedia.com/news/10-Thing-You-Did-Not-Know-About-Croaking-67126.shtml