January in Dumbarton is the cloudiest and the coldest month of the year. In January, the weather in Dumbarton is typically cold. Average daily high temperature in January is about 5 degrees above zero. On average, Dumbarton has 17 rainy, 8 cloudy, 5 partly cloudy, and 1 sunny day in January. The rains are usually heavy. January as well as February, March, April, and May is considered the most uncomfortable time to visit Dumbarton.