na-adjective, noun which may take the genitive case particle 'no', noun
mainly cloudy; tending to be cloudy
Describes weather that is often or mostly cloudy, or a period when cloudy conditions predominate. Often used in weather forecasts.
明日は曇りがちの天気でしょう。
Tomorrow will be mainly cloudy.
今週は曇りがちで、なかなか晴れません。
This week has been tending to be cloudy, and it hasn't cleared up much.
曇り simply means 'cloudy weather', while 曇りがち emphasizes a tendency or predominance of cloudiness.
Compound of 曇り (cloudiness) + がち (tending to). The suffix がち attaches to nouns or verb stems to indicate a tendency or frequent occurrence.