From Carolyn's Corner:
isohyetal I couldn't find a definition to this, but all the sites were about weather. So I searched for "hyetal" and found:
hyetal

SYLLABICATION: hy·e·tal
PRONUNCIATION: h-tl
ADJECTIVE: Of or relating to rain or rainy regions.
ETYMOLOGY: From Greek hetos, rain. See seu-2 in Appendix I.

So "isohyetal" refers to weather maps with areas of equal
rainfall outlined.