Mosjøen: are you wondering about the weather you might find during your holiday? Befor organizing a holiday we always think about the wheather we might find when we get there. In Mosjøen the average yearly temperature is 3°C. During the hottest month, which in this case is July, the temperature can get up to 14°C and the temperature of the sea reaches 13°C. Instead the coldest month is January, reaching a minimum temperature of -4°C. Here instead the water temperature drops to 4°C.