First off I am not 100% sure I am in the correct forum, so I apologize in advance if I am not...

The problem I have is in reference to the psychometric chart used in HVAC applications. On the psychometric chart, the saturation curve (100% humidity) is a parabolic shaped curve, so I would think it can be defined in an equation. I also know a point in space which is not at 100% humidity, so this point sits off the parabolic saturation curve, let’s call this point "A".

What I want to find out is from point "A", how can I find the tangent where point "A" and the tangent point would form a straight line tangent to the parabolic curve?

I am sure I am not explaining this well and I can provide more detail as/if questions come up but I am not sure what more is needed...

Thanks in advance for any help!

Brian