So I've been piddling around with the equation bocijn found, using 10% relative humidity
Come on Kirk... You can't just go around plucking numbers out of your backside. You are supposed to be a proper scientist, remember?
The average daily relative humidity in Hokkaido in May is 79%
http://www.myweather2.com/City-Town/Japan/NayoroOnHokkaido/climate-profile.aspx?month=3