@jasondegraw Cool, I would prefer that we handle all the "magic" numbers for the user somehow, I think your defaults are one way to do this.  It looks to me like we want a single default constructor with no arguments that sets all these variables to their magic numbers.  Then you just call setFoo for each of the fields.

I like what you do in EpwDataPoint::dryBulbTemperature where a 99.9 is returned as an empty optional and I like what you do in EpwDataPoint::setDryBulbTemperature.