AtmosphereZero_img1.gif Atmosphere Zero
These are the weather conditions for when you zero.
Temperature
This is important. 
Pressure
This is important. 
Weather forecast "Barometric" pressure is the seal level value, so type in your  current altitude to correct the pressure.
If you are measuring actual pressure at your location, just leave the altitude as zero,  no correction is needed.
Barometric (Weather Report, Sea Level), Enter your Altitude to get the Actual Pressure
Station (Actual), set Altitude to Zero
Humidity
0(dry)-100(damp)%. It doesn't make much difference.
Altitude
If you are using "adjusted for sea level" pressure that they use in weather reports,  type in the altitude.
If you are measuring the actual press at you location, leave this as zero.
Adjusts Sea Level Pressure (Barometric, Weather Report) to your Altitude
If using a meter, type in your altitude to get Sea Level pressure, Sea level on the meter, sea level in the program. If you don't type in Altitude into your meter, leave the Altitude at Zero.
AtmosphereZero_img2.gif Density Altitude
A single number instead of Temp/Pressure/Humidity/Altitude