Predicting air moisture content at high altitude

Goo day,

I am modelling a humidifier for a PEM fuel cell that will be used in high altitude condition. I need to know the moisture content in the air at high altitude in order to make the control system of the humidifier. The himdifier needs to humidify the inlet air using an ultrasonic humidification system. However, I can't seem to find a way to calculate the air moisture content at high altitude in order to set it is right level.

In addition, I tried to predict the mositure content from relative humidity which turned to be irrelevent, since relative humidity is 100% at high altitude becuase the air is cold and dry.

Can some please guide me here please.

Many thanks in advance.