The specific definition for "partly cloudy" is 30-70 percent sky covered by clouds. Partly sunny is similar to "partly cloudy" but is used more frequently during the day to emphasize daytime sunshine. I find that when people hear "partly cloudy" they think more clouds than sun and when they hear "partly sunny" more sun than clouds. "Mostly sunny" is less than 30 percent covered by clouds and "mostly cloudy" more than 70 percent covered by clouds. When the sky is "overcast" more than 90 percent of the sky is covered by clouds.
Plane, train, or automobile?