Drier - dryer

Drier and dryer are permissible variant spellings of the same words, derived from the adjective and adverb dry.

dryly.

• The Concise A Dictionary of Modern English Usage (ed. Butterfield, 2016) observes, s.v. 'dryly', "As the adverb formed from dry, this spelling is nowadays more common than drily throughout the English-speaking world. The spelling drily conforms to the rule that words ending in –y change it to –i before a suffix. Some people prefer it for that reason, especially in Br[itish] E[nglish], where it is still slightly more frequent than dryly."