You may wish to reserve this adjective for a rainy day, when you can enliven the inevitable discussion about the weather by dropping it into the conversation. Do not, however, expect it to be understood, even though you’re merely referring to rain.
One of those hyetal days
The Oxford English Dictionary, my first stop in investigating our language, has no examples of it at all in its entry (written about 110 years ago), noting only that it is recorded in the 1864 edition of Webster’s Dictionary. A further century of opportunities for the word to be used, coupled with the magic of searchable electronic databases, allows me to do better:
The hydrologic cycle has undergone an atmospheric mutation here. They don’t measure the rain in inches but in feet. A waterproofing contractor could definitely find happiness here, while rainmakers and dousers would quickly go out of business. This is the kind of place where words like pluviose, hyetal, and affusion actually belong in conversation.
Washington Post, 4 March 1990. Affusion means the pouring of water on the body, as in one form of baptism; pluviose is another adjective meaning “rainy”.
Hyetal comes from Greek huetos, rain, and is related to Greek hyei, it is raining. It means “relating to rain”. A hyetal chart is a rain chart; an isohyet or isohyetal is a line on a map connecting places of equal rainfall; a hyetograph is a self-registering rain gauge; hyetology is the study of the geographical variation and distribution of rainfall. Meteorologists, the principal users of the word and its compounds, have extended the meaning to include all forms of precipitation.