Drier is the correct spelling when you're comparing how dry things are: if one towel is wetter than another, then the other towel is drier. You might say the air feels drier in winter than in summer, or that Arizona's climate is drier than Florida's.

The word can also describe something that makes things dry. A clothes dryer (usually spelled “dryer” in this case) removes moisture from wet laundry by tumbling it in heated air. A hair dryer blows warm air to dry your hair after a shower.

In painting, a drier (sometimes called a “drying agent”) is a substance artists mix into oil paints to make them dry faster. Oil paintings naturally take days or weeks to dry completely, so painters sometimes add driers when they need to work more quickly or add another layer of paint sooner.