Ballistics programs use pressure, temperature and humidity to calculate the density of the air, because density obviously affects air resistance. You can enter air density directly into the program, usually we use Density Altitude (DA).
DA is a fake height, based on the air density. So if the air pressure at your house drops, your DA increases, even though your house isn't really moving up. Temperature and humidity affect DA too. So pilots use Density Altitude to say, "The air is behaving as though I am at 500 feet".
So the Nokia App and the BlackBerry app support Density Altitude, you type in one number instead of pressure, temperature and humidity. Many weather meters, such as Kestrel, will tell you the DA.
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