rain gage vs rain gauge

rain gage and rain gauge both are nouns.

Word	Noun	Adjective	Verb	Adverb
rain gage	Yes	No	No	No
rain gauge	Yes	No	No	No

As nouns, rain gage and rain gauge are synonyms defined as:

rain gage and rain gauge: gauge consisting of an instrument to measure the quantity of precipitation

Other synonyms of rain gage include pluviometer, udometer.

rain gage (noun)	rain gauge (noun)
gauge consisting of an instrument to measure the quantity of precipitation	gauge consisting of an instrument to measure the quantity of precipitation