The most pleasant months of the year for Ontario are June, September and August. In Ontario, there are 3 comfortable months with high temperatures in the range of 70-85°. July is the hottest month for Ontario with an average high temperature of 85.6°, which ranks it as about average compared to other places in South Dakota. February is the snowiest month in Ontario with 7.3 inches of snow, and 7 months of the year have significant snowfall. There are few days during the summer when the humidity becomes unpleasant.