Altitude affects firearm ballistics more then any of the other atmospheric conditions. The main reason is that air density decreases dramatically with altitude. With air that is less dense the bullet as it travels encounters less resistance and is able to conserve its velocity longer. The affects of altitude are apparent on this ballistic trajectory graph.

If you do not check the 'Correct for Atmosphere' checkbox then the International Standard Atmosphere altitude of 0 will be used as altitude. Here is a map that shows roughly your altitude if you live in the United States.

The Weather Underground is a good website to use to find out the altitude, pressure, temperature, and humidity for where you live. Plug those numbers into the ballistic trajectory calculator in order to have more accurate results.