You use partly to indicate that something happens or exists to some extent, but not completely. It's partly my fault He let out a long sigh, mainly of relief, partly of sadness I feel partly responsible for the problems we're in. = partially. to some degree, but not completely
The state of the weather when the clouds are conspicuously present, but do not completely dull the sky or the day at any moment The National Weather Service does not have an amount of sky cover for this condition